sound travel air vs water

How far does sound travel in the ocean?

The distance that sound travels in the ocean varies greatly, depending primarily upon water temperature and pressure..

Water temperature and pressure determine how far sound travels in the ocean.

While sound moves at a much faster speed in the water than in air , the distance that sound waves travel is primarily dependent upon ocean temperature and pressure. While pressure continues to increase as ocean depth increases, the temperature of the ocean only decreases up to a certain point, after which it remains relatively stable. These factors have a curious effect on how (and how far) sound waves travel.

Imagine a whale is swimming through the ocean and calls out to its pod. The whale produces sound waves that move like ripples in the water. As the whale’s sound waves travel through the water, their speed decreases with increasing depth (as the temperature drops), causing the sound waves to refract downward . Once the sound waves reach the bottom of what is known as the thermocline layer, the speed of sound reaches its minimum. The thermocline is a region characterized by rapid change in temperature and pressure which occurs at different depths around the world. Below the thermocline "layer," the temperature remains constant, but pressure continues to increase. This causes the speed of sound to increase and makes the sound waves refract upward .  

The area in the ocean where sound waves refract up and down is known as the "sound channel." The channeling of sound waves allows sound to travel thousands of miles without the signal losing considerable energy.  In fact, hydrophones, or underwater microphones, if placed at the proper depth, can pick up whale songs and manmade noises from many kilometers away.

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• Noise in the Ocean: A National Issue (National Marine Sanctuaries)
• Just how noisy is the ocean? Learn about a NOAA Effort to Monitor Underwater Sound
• Sound in the Sea Gallery
• Acoustic Monitoring

Last updated: 06/16/24 Author: NOAA How to cite this article

Does Sound Travel Faster in Water or Air?

Most people, whether they are students or workers, have a pretty clear idea of how sound works. After all, who hasn’t heard about sound waves, vibrations, and other similar concepts? Yet, there are a few notions that still baffle people to this day, particularly regarding the way sound propagates in water and air. And, with so much contradictory information online, it’s easy to see why.

So, if you’re one of the many who want to know if sound travels faster in water or air, this article has got you covered. But first, let’s start with the basics!

Temperature and Pressure

What is sound.

Generally speaking, sound is a type of longitudinal mechanical wave that travels through a medium. However, there are two definitions regarding how sound is produced.

For starters, in physiology, sound is created when an object’s vibrations travel through a medium until they reach the human eardrum . In physics, sound is produced in the form of a pressure wave . More specifically, when objects vibrate, they cause the nearby molecules to also vibrate, triggering a chain reaction of sound wave vibrations in the specific medium.

But no matter which definition you prefer, you’ll notice a similarity — sound needs a medium to propagate and will not travel through a vacuum. As a matter of fact, sound travels at different speeds depending on the medium. In other words, the medium’s density and compressibility directly affect the speed of sound. For instance, sound waves will travel slower in a less dense and more compressible medium .

How Fast Does Sound Travel in Water?

When it comes to water, sound can travel as fast as 1,498 meters per second, or approximately 3,350 miles per hour . However, as mentioned earlier, the physical characteristics of the medium highly affect the speed.

As a result of its high salinity, seawater, such as oceans, allows sound to travel up to 33 meters per second faster than the freshwater found in lakes . That’s because salt molecules respond quickly to the disturbances of neighboring molecules, propagating sound waves faster and at longer distances.

The speed of sound is also dependent on density. As you might already know, water has an impressive density due to its unique molecular arrangement. Thus, sound waves can travel much faster underwater as the wave bumps and vibrates with more molecules.

You need to understand that, as the ocean gets deeper, its temperature decreases and its pressure increases. These affect the particle arrangement and, by extension, the speed of sound. To put it simply, sound travels slower at the surface level than at lower depths.

How Fast Does Sound Travel in Air?

Sound is able to travel through the air at an average of 332 meters per second, or 742 miles per hour. Although that might seem fast, it is not nearly as fast as light , which travels at 186,411.358 miles per hour. But as with water, there are also many factors that affect how sound propagates in the air:

Temperature

Air molecules tend to have more energy at higher temperatures, meaning that they will vibrate faster. That allows sound waves to also travel faster and farther , as they are propelled by molecule collisions. Yet, as the sound moves through the atmosphere, some parts of its wave will travel faster than others due to temperature differences.

What’s interesting about sound is that, at a constant temperature, its speed is not dependent on the pressure of the medium. That’s because these two properties are tied to one another. So, increasing temperature will also increase pressure and, consequently, the speed of sound.

Air Direction

The wind direction can impact the speed of sound and the distance it can travel . In fact, you might notice that sound levels are higher when the wind is blowing down, such as from a highway towards the ground level.

Water vapors are less dense than dry air at a constant temperature. Naturally, the presence of moisture will decrease the air’s density and increase the speed of sound. Therefore, humid environments experience much faster sound propagation than dry and cold areas.

Why Does Sound Travel Faster in Water Than Air?

By now, you might have noticed that sound travels about four times faster in water than in air. The main reason behind this is that water is denser than air. Sure, not all water has identical properties , as salinity and temperature vary and affect its density. But even so, molecules in the water are closer together, causing more vibrations to be transmitted at a faster speed of sound.

Furthermore, water is an incompressible environment . Actually, it’s better to imagine water as being similar to a solid object, as they tend to behave similarly when it comes to compression. More specifically, when water encounters a force, it will immediately transfer its energy to nearby molecules, just like solids. This characteristic is partially offset by the water’s high density, creating the perfect environment for sound to travel through.

And lastly, it’s important to mention that sound travels faster in harder materials . It’s true that water as a unit is not necessarily hard; however, it has a strong bond between its molecules. Hence, the propagation of sound is faster as it passes more quickly from one particle to the next.

But Why Is It Harder to Talk to Someone Underwater?

Naturally, you might assume that, since sound travels faster in water, it would be incredibly easy to chat with someone while swimming or diving . But that couldn’t be further from the truth.

When someone talks, they do so by emitting air and sending compression waves through it. That’s thanks to your lungs, vocal cords, and mouth, which work together to imprint a sound waveform on the burst of air that comes from your body. So, in order for someone that’s in the water to hear you, the sound will need to travel from your mouth into the surrounding water.

However, sound couples very poorly from air to water. As a matter of fact, water tends to reflect external sound waves instead of allowing them to penetrate its surface. That’s also the reason why phenomenons like echos occur when you scream or talk near a well, as the water at its bottom reflects the sound waves back to you.

What About Sound Travel Distance?

When it comes to sound travel, water is again the clear winner, as it allows sound to propagate to distances of almost 15,500 miles. To understand why that’s the case, imagine a whale that is swimming through the ocean and calls out to its peers. The sound waves it produces move similarly to ripples in the water.

As the sound travels and reaches increasing depths , it begins to slow down and eventually refracts downward. Once the sound reaches a region called the thermocline layer, its speed further decreases to a minimum. That’s because the thermocline layer features rapid changes in pressure and temperature.

After breaking through the layer, sound waves encounter another area where the temperature remains constant. However, the pressure continues to rise, which causes a boost in the sound speed, making it refract upward. This channeling of waves allows the sound to travel thousands of miles with little to no energy loss. It’s thanks to this process that scientists can pick up whale songs from many miles away.

Key Takeaways

Understanding how sound works and travels is extremely important. Sure, you might not deal with mediums like water every day. However, air is all around you, and learning the way it affects sound speed can help you figure out the perfect way to soundproof your environment and enjoy a noise-free life!

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How fast does sound travel through water?

Sounds travel faster through water than in air, but it takes more energy to get it going.

Sound is a wave of alternating compression and expansion, so its speed depends on how fast it bounces back from each compression – the less compressible the medium it’s travelling through, the faster it bounces back. Water is about 15,000 times less compressible than air, but it is also 800 times denser. The extra density means that the molecules accelerate more slowly for a given force, which slows the compression wave down. So water’s high density partly offsets its extreme incompressibility and sound travels at 1,493m/s, about four times faster than through air. The speed of sound in diamond is so high because it is extremely incompressible and yet relatively light.

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June 27, 2019

What Do You Hear Underwater?

A submerged science activity from Science Buddies

By Science Buddies & Sabine De Brabandere

Make waves--underwater! Learn how sound travels differently in water than it does in the air. 

George Retseck

Key Concepts Physics Sound Waves Biology

Introduction Have you ever listened to noises underwater? Sound travels differently in the water than it does in the air. To learn more, try making your own underwater noises—and listening carefully. 

Background Sound is a wave created by vibrations. These vibrations create areas of more and less densely packed particles. So sound needs a medium to travel, such as air, water—or even solids. 

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Sound waves travel faster in denser substances because neighboring particles will more easily bump into one another. Take water, for example. There are about 800 times more particles in a bottle of water than there are in the same bottle filled with air. Thus sound waves travel much faster in water than they do in air. In freshwater at room temperature, for example, sound travels about 4.3 times faster than it does in air at the same temperature.

Sound traveling through air soon becomes less loud as you get farther from the source. This is because the waves’ energy quickly gets lost along the way. Sound keeps its energy longer when traveling through water because the particles can carry the sound waves better. In the ocean, for example, the sound of a humpback whale can travel thousands of miles!

Underwater sound waves reaching us at a faster pace and keeping their intensity longer seem like they should make us perceive those sounds as louder when we are also underwater. The human ear, however, evolved to hear sound in the air and is not as useful when submerged in water. Our head itself is full of tissues that contain water and can transmit sound waves when we are underwater. When this happens, the vibrations bypass the eardrum, the part of the ear that evolved to pick up sound waves in the air. 

Sound also interacts with boundaries between two different mediums, such as the surface of water. This boundary between water and air, for example, reflects almost all sounds back into the water. How will all these dynamics influence how we perceive underwater sounds? Try the activity to find out! 

Bathtub or swimming pool (a very large bucket can work, too)

Two stainless steel utensils (for example, spoons or tongs)

Two plastic utensils

Small ball 

Adult helper

An area that can get wet (if not performing the activity at a pool)

Floor cloth to cleanup spills (if not performing the activity at a pool)

Other materials to make underwater sounds (optional)

Access to a swimming pool (optional)

Internet access (optional)

Preparation

Fill the bathtub with lukewarm water—or head to the pool—and bring your helper and other materials.

Ask your helper to click one stainless steel utensil against another. Listen. How would you describe the sound? 

In a moment, your helper will click one utensil against the other underwater . Do you think you will hear the same sound? 

Ask your helper to click one utensil against the other underwater. Listen. Does the sound appear to be louder or softer? Is what you hear different in other ways, too?

Submerge one ear in the water. Ask your helper to click one utensil against the other underwater. Listen. How would you describe this sound? 

Ask your helper to click one utensil against the other underwater soon after you submerge your head. Take a deep breath, close your eyes and submerge your head completely or as much as you feel comfortable doing. Listen while you hold your breath underwater (come up for air when you need to!). Does the sound appear to be louder or softer? Does it appear to be different in other ways? 

Repeat this sequence but have your helper use two plastic utensils banging against each other instead.

Repeat the sequence again, but this time listen to a small ball being dropped into the water. Does the sound of a ball falling into the water change when you listen above or below water? Does your perception of this sound change? Why would this happen? 

Switch roles. Have your helper listen while you make the sounds. 

Discuss the findings you gathered. Do patterns appear? Can you conclude something about how humans perceive sounds when submerged in water? 

Extra : Test with more types of sounds: soft as well as loud sounds, high- as well as low-pitched sounds. Can you find more patterns?

Extra: To investigate what picks up the sound wave when you are submerged, use your fingers to close your ears or use earbuds when submerging your head. How does the sound change when you close off your ear canal underwater? Does the same happen when you close off your ear canal when you are above water? If not, why would this be different? 

Extra: Go to the swimming pool and listen to the sound of someone jumping into the water. Compare your perception of the sound when you are submerged with when your head is above the water. How does your perception change? Close your eyes. Can you tell where the person jumped into the water when submerged? Can you tell when you have your head above the water?

Extra: Research ocean sounds and how sounds caused by human activity impact aquatic animals.  

Observations and Results Was the sound softer when it was created underwater and you listened above the water? Did it sound muffled when you had only your ear submerged? Was it fuller when you had your head submerged? 

Sound travels faster in water compared with air because water particles are packed in more densely. Thus, the energy the sound waves carry is transported faster. This should make the sound appear louder. You probably perceived it as softer when you were not submerged, however, because the water surface is almost like a mirror for the sound you created. The sound most likely almost completely reflected back into the water as soon as it reached the surface. 

When you submerged only your ear, the sound probably still appeared muffled. This happens because the human ear is not good at picking up sound in water—after all, it evolved to pick up sound in air. 

When you submerged your head, the sound probably sounded fuller. That is because our head contains a lot of water, which allows the tissue to pick up underwater sound—without relying on the eardrum. It also explains why closing your ear canal makes almost no difference in the sound you pick up while you are underwater. 

If you tried to detect where the sound came from when submerged, you probably had a hard time. Our brain uses the difference in loudness and timing of the sound detected by each ear as a clue to infer where the sound came from. Because sound travels faster underwater and because you pick up sound with your entire head when you are submerged, your brain loses the cues that normally help you determine where the sound is coming from. 

More to Explore Discovery of Sound in the Sea , from the University of Rhode Island and the Inner Space Center Can You Hear Sounds in Outer Space? , from Science Buddies Talk through a String Telephone , from Scientific American Sound Localization , from Science Buddies  Ears: Do Their Design, Size and Shape Matter? , from Scientific American STEM Activities for Kids , from Science Buddies 

This activity brought to you in partnership with Science Buddies

The basic components of a sound wave are frequency, wavelength and amplitude. In this example of a sound wave, the period of one cycle of this wave is 0.5 seconds, and the frequency of this wave is 2 cycles per second or 2 Hertz (Hz). Click image for larger view.

Click image to hear a scale of various frequencies (576 K, QuickTime). Click image for larger view.

Understanding Ocean Acoustics

Sharon Nieukirk, Research Assistant Acoustic Monitoring Project NOAA Pacific Marine Environmental Laboratory

What is Sound?

Ocean acoustics is the study of sound and its behavior in the sea. When underwater objects vibrate, they create sound-pressure waves that alternately compress and decompress the water molecules as the sound wave travels through the sea. Sound waves radiate in all directions away from the source like ripples on the surface of a pond. The compressions and decompressions associated with sound waves are detected as changes in pressure by the structures in our ears and most man-made sound receptors such as a hydrophone, or underwater microphone.

The basic components of a sound wave are frequency, wavelength and amplitude.

Frequency is the number of pressure waves that pass by a reference point per unit time and is measured in Hertz (Hz) or cycles per second. To the human ear, an increase in frequency is perceived as a higher pitched sound, while a decrease in frequency is perceived as a lower pitched sound. Humans generally hear sound waves whose frequencies are between 20 and 20,000 Hz. Below 20 Hz, sounds are referred to as infrasonic, and above 20,000 Hz as ultrasonic. The frequency of middle “C” on a piano is 246 Hz.

Wavelength is the distance between two peaks of a sound wave. It is related to frequency because the lower the frequency of the wave, the longer the wavelength.

Amplitude describes the height of the sound pressure wave or the “loudness” of a sound and is often measured using the decibel (dB) scale. Small variations in amplitude (“short” pressure waves) produce weak or quiet sounds, while large variations (“tall” pressure waves) produce strong or loud sounds.

The two examples below show sound waves that vary in frequency and amplitude.

These two waves have the same frequency but different amplitudes. Click image for larger view .

These two waves have the same amplitude but different frequencies. Click image for larger view.

The decibel scale is a logarithmic scale used to measure the amplitude of a sound. If the amplitude of a sound is increased in a series of equal steps, the loudness of the sound will increase in steps which are perceived as successively smaller. A decibel doesn’t really represent a unit of measure like a yard or meter, but instead a pressure value in decibels expresses a ratio between the measured pressure and a reference pressure. On the decibel scale, everything refers to power, which is amplitude squared. And just to confuse things, the reference pressure in air differs from that in water. Therefore a 150 dB sound in water is not the same as a 150 dB sound in air. So when you are describing sound waves and how they behave it is very important to know whether you are describing sound in the sea or in air.

Note on Acoustic Noise Level Units: Hydrophones measure sound pressure, normally expressed in units of micropascals (µPa). Early acousticians working with sound in air, realized that human ears perceive differences in sound on a logarithmic scale, so the convention of using a relative logarithmic scale (dB) was adopted. In order to be useful, the sound levels need to be referenced to some standard pressure at a standard distance. The reference level used in air (20µPa @ 1m) was selected to match human hearing sensitivity. A different reference level is used for underwater sound (1µPa @ 1m). Because of these differences in reference standards, noise levels cited in air do NOT equal underwater levels. To compare noise levels in water to noise levels in air, one must subtract 26 dB from the noise level referenced in water. For example, a supertanker radiating noise at 190 dB (re 1µPa @ 1m) has an equivalent noise level in air of about 128 dB (re 20µPa @ 1m). These numbers are approximate, and amplitude often varies with frequency.

Faster than the Speed of Sound...

The speed of a wave is the rate at which vibrations move through the medium. Sound moves at a faster speed in water (1500 meters/sec) than in air (about 340 meters/sec) because the mechanical properties of water differ from air. Temperature also affects the speed of sound (e.g. sound travels faster in warm water than in cold water) and is very influential in some parts of the ocean. Remember that wavelength and frequency are related because the lower the frequency the longer the wavelength. More specifically, the wavelength of a sound equals the speed of sound in either air or water divided by the frequency of the wave. Therefore, a 20 Hz sound wave is 75 m long in the water (1500/20 = 75) whereas a 20 Hz sound wave in air is only 17 m long (340/20 = 17) in air.

As we descend below the surface of the sea, the speed of sound decreases with decreasing temperature. At the bottom of the thermocline, the speed of sound reaches its minimum; this is also the axis of the sound channel. Below the thermocline the temperature remains constant, but pressure increases which causes the speed of sound to increase again. Sound waves bend, or refract, towards the area of minimum sound speed. Therefore, a sound wave traveling in the sound channel bends up and down and up and down and can travel thousands of meters. Click image for larger view.

The SOFAR Channel

Sound in the sea can often be “trapped” and effectively carried very long distances by the “deep sound channel ” that exists in the ocean. This SOFAR or SOund Fixing And Ranging channel is so named because it was discovered that there was a "channel" in the deep ocean within which the acoustic energy from a small explosive charge (deployed in the water by a downed aviator) could travel over long distances. An array of hydrophones could be used to roughly locate the source of the charge thereby allowing rescue of downed pilots far out to sea. Sound, and especially low-frequency sound, can travel thousands of meters with very little loss of signal. Read more information on the SOFAR channel.

The field of ocean acoustics provides scientists with the tools needed to quantitatively describe sound in the sea. By measuring the frequency, amplitude, location and seasonality of sounds in the sea, a great deal can be learned about our oceanic environment and its inhabitants. Hydroacoustic monitoring (listening to underwater sounds) has allowed scientists to measure global warming, listen to earthquakes and the movement of magma through the sea floor during major volcanic eruptions, and to record low-frequency calls of large whales the world over. As our oceans become more noisy each year, the field of ocean acoustics will grow and only become more essential. For more information and a tutorial on ocean acoustics .

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by Chris Woodford . Last updated: July 23, 2023.

Photo: Sound is energy we hear made by things that vibrate. Photo by William R. Goodwin courtesy of US Navy and Wikimedia Commons .

What is sound?

Photo: Sensing with sound: Light doesn't travel well through ocean water: over half the light falling on the sea surface is absorbed within the first meter of water; 100m down and only 1 percent of the surface light remains. That's largely why mighty creatures of the deep rely on sound for communication and navigation. Whales, famously, "talk" to one another across entire ocean basins, while dolphins use sound, like bats, for echolocation. Photo by Bill Thompson courtesy of US Fish and Wildlife Service .

Robert Boyle's classic experiment

Artwork: Robert Boyle's famous experiment with an alarm clock.

How sound travels

Artwork: Sound waves and ocean waves compared. Top: Sound waves are longitudinal waves: the air moves back and forth along the same line as the wave travels, making alternate patterns of compressions and rarefactions. Bottom: Ocean waves are transverse waves: the water moves back and forth at right angles to the line in which the wave travels.

The science of sound waves

Picture: Reflected sound is extremely useful for "seeing" underwater where light doesn't really travel—that's the basic idea behind sonar. Here's a side-scan sonar (reflected sound) image of a World War II boat wrecked on the seabed. Photo courtesy of U.S. National Oceanographic and Atmospheric Administration, US Navy, and Wikimedia Commons .

Whispering galleries and amphitheaters

Photos by Carol M. Highsmith: 1) The Capitol in Washington, DC has a whispering gallery inside its dome. Photo credit: The George F. Landegger Collection of District of Columbia Photographs in Carol M. Highsmith's America, Library of Congress , Prints and Photographs Division. 2) It's easy to hear people talking in the curved memorial amphitheater building at Arlington National Cemetery, Arlington, Virginia. Photo credit: Photographs in the Carol M. Highsmith Archive, Library of Congress , Prints and Photographs Division.

Measuring waves

Understanding amplitude and frequency, why instruments sound different, the speed of sound.

Photo: Breaking through the sound barrier creates a sonic boom. The mist you can see, which is called a condensation cloud, isn't necessarily caused by an aircraft flying supersonic: it can occur at lower speeds too. It happens because moist air condenses due to the shock waves created by the plane. You might expect the plane to compress the air as it slices through. But the shock waves it generates alternately expand and contract the air, producing both compressions and rarefactions. The rarefactions cause very low pressure and it's these that make moisture in the air condense, producing the cloud you see here. Photo by John Gay courtesy of US Navy and Wikimedia Commons .

Why does sound go faster in some things than in others?

Chart: Generally, sound travels faster in solids (right) than in liquids (middle) or gases (left)... but there are exceptions!

How to measure the speed of sound

Sound in practice, if you liked this article..., don't want to read our articles try listening instead, find out more, on this website.

• Electric guitars
• Speech synthesis
• Synthesizers

On other sites

• Explore Sound : A comprehensive educational site from the Acoustical Society of America, with activities for students of all ages.
• Sound Waves : A great collection of interactive science lessons from the University of Salford, which explains what sound waves are and the different ways in which they behave.

Educational books for younger readers

• Sound (Science in a Flash) by Georgia Amson-Bradshaw. Franklin Watts/Hachette, 2020. Simple facts, experiments, and quizzes fill this book; the visually exciting design will appeal to reluctant readers. Also for ages 7–9.
• Sound by Angela Royston. Raintree, 2017. A basic introduction to sound and musical sounds, including simple activities. Ages 7–9.
• Experimenting with Sound Science Projects by Robert Gardner. Enslow Publishers, 2013. A comprehensive 120-page introduction, running through the science of sound in some detail, with plenty of hands-on projects and activities (including welcome coverage of how to run controlled experiments using the scientific method). Ages 9–12.
• Cool Science: Experiments with Sound and Hearing by Chris Woodford. Gareth Stevens Inc, 2010. One of my own books, this is a short introduction to sound through practical activities, for ages 9–12.
• Adventures in Sound with Max Axiom, Super Scientist by Emily Sohn. Capstone, 2007. The original, graphic novel (comic book) format should appeal to reluctant readers. Ages 8–10.

Popular science

• The Sound Book: The Science of the Sonic Wonders of the World by Trevor Cox. W. W. Norton, 2014. An entertaining tour through everyday sound science.

Academic books

• Master Handbook of Acoustics by F. Alton Everest and Ken Pohlmann. McGraw-Hill Education, 2015. A comprehensive reference for undergraduates and sound-design professionals.
• The Science of Sound by Thomas D. Rossing, Paul A. Wheeler, and F. Richard Moore. Pearson, 2013. One of the most popular general undergraduate texts.

Text copyright © Chris Woodford 2009, 2021. All rights reserved. Full copyright notice and terms of use .

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Understanding Sound in the Ocean

Levels of underwater noise from human activities—including from ships, sonar, and drilling—have increased dramatically. Those growing levels of ocean noise affect marine animals and habitats in complex ways.

Table of Contents

What types of sounds occur in the ocean, why is sound important to marine animals, does sound behave differently underwater than in air, what kinds of underwater sounds do people produce, how does the sound we produce affect marine species, what is noaa fisheries doing about sound in the ocean, how does noaa fisheries help protect marine life from the harmful impacts of ocean noise.

Both natural and human-made sounds occur in the ocean. Natural sounds come from marine life and naturally occurring events like underwater earthquakes. Human-made sounds come from many sources, such as ships, underwater energy exploration, military sonar, and underwater construction, among others.

Learn more about sound in the ocean .

Sound is essential to many types of marine animals and is one of the main tools they use to survive in the ocean. Light can only penetrate a few hundred feet underwater, but sound can travel much farther. As a result, cetaceans (whales, dolphins, and porpoises) have evolved over millions of years to send and receive a variety of complex sounds. They rely on sound to communicate with each other, navigate, find mates and food, defend their territories and resources, and avoid predators. Fish and invertebrates also use sound for basic life functions.

Because water is denser than air, sound travels faster and farther in the ocean. Its speed and distance depends on the density of the water (determined by its temperature, salinity, and depth) and the frequency of the sound, measured in hertz (Hz). Some sounds, particularly low-frequency ones, can cover vast distances, even across ocean basins.

People produce some sounds intentionally, such as military sonar and seismic tests for oil and gas exploration. Other sounds are an unintentional by-product of an activity, such as shipping and underwater construction. Many human-produced sounds in the ocean are intermittent, whereas shipping creates an almost constant rumble in the ocean. Even the motor of a fishing boat creates extra sound underwater.

All of these sounds add to overall ocean noise and contribute to the “soundscape,” which scientists define as the combined sounds made by humans, natural events, and marine animals. Because sound travels so well underwater, many of these sounds can be heard miles from their sources.

Depending on the sound source, duration, and location, human-caused sound has the potential to affect animals by:

• Causing temporary or permanent hearing loss.
• Causing a stress response.
• Forcing animals to move from their preferred habitat.
• Disrupting feeding, breeding/spawning, nursing, and communication behaviors.

The impacts may be immediate and severe, or they may accumulate over time.

We are engaged on several fronts to better understand and manage ocean sounds, specifically in regard to cetaceans and other types of marine life. Most recently, we published the Ocean Noise Strategy Roadmap which defines a 10-year plan for the agency to address ocean noise.

In 2011, we started the CetSound mapping project . CetSound provides two mapping tools: SoundMap and CetMap . SoundMap allows us to map the time and location of noise, and CetMap shows how many cetaceans are in a given area at a specific time. This information is used to determine where marine animals go to breed and find food, what routes they use to migrate, and where small or fixed populations are concentrated. We then have a better understanding of how ocean noise affects them.

NOAA Fisheries is also part of an interagency partnership that established a set of undersea listening stations around the United States to measure levels of background noise in the ocean.

Among many efforts to protect marine species, NOAA Fisheries administers the Endangered Species Act to recover threatened and endangered species and prevent their extinction. Through consultations under the ESA , we develop biological opinions  to determine how the actions of federal agencies may affect ESA-listed species and critical habitat .

We also are responsible for authorizing the “take” of marine mammals that can result from the sounds produced by human activities. These Incidental Take Authorizations are issued under the Marine Mammal Protection Act . The MMPA limits the numbers of animals that can be “taken” (disturbed or hurt) as a result of human activities and ensures that those activities result in a negligible impact on marine mammal species and stocks.

By knowing how much underwater noise humans produce around the world, scientists can develop ways to reduce or prevent it, as well as ways to protect marine animals from it.

Learn more about sound in the ocean and what NOAA is doing to reduce it to protect marine animals:

• NOAA's Ocean Noise Strategy  
• NOAA Fisheries' Ocean Acoustics Program  

Marine Mammal Acoustic Technical Guidance

How Far Does Sound Travel: The Science of Acoustics

Do you ever stop to think about how sound travels? It’s an interesting phenomenon that occurs everyday and yet we often take it for granted. In this blog post, we will explore the science of acoustics and how sound travels. We will answer the question of how far sound can travel and how it is affected by different factors. Stay tuned for an in-depth look at this fascinating topic!

Nature Of Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through some medium, such as air or water. Sound can propagate through solids and liquids better than gases because the density and stiffness are greater. So how far does sound travel? In this article we will answer how sound travels and how to calculate how far it travels in different scenarios.

Sound Transmits Conception

A common misconception with regard to how sound transmits itself between two points (for example from speaker to ear) is that the source creates waves of compression in the surrounding gas which then proceed on their way at a constant speed until they strike something else; either another solid object or our ears . This analogy might be okay for describing what goes on at low frequencies but once we go beyond around 1000 Hz, the propagation of sound becomes far more complex.

At low frequencies (below around 1000 Hz), sound waves tend to travel in all directions more or less equally and bounce off objects like a rubber ball would. As frequency increases however, the directivity of sound increases as well. So high-frequency sounds are more likely to travel in a straight line between two points than low frequencies. This is why we can often hear someone calling from some distance away when there is loud music playing – because the higher frequencies carry further than the lower ones.

How Far Can Sound Travel

There are three ways that sound can be transmitted: through air, through water, or through solids. The speed of sound through each medium is different and depends on the density and stiffness of the material.

The speed of sound through air is about 343 m/s (or 760 mph), and it travels faster in warmer air than colder air. The speed of sound through water is about 1500 m/s, and it travels faster in salt water than fresh water. The speed of sound through solids is much faster than through either gases or liquids – about 5000-15000 m/s. This is why we can often hear someone coming before we see them – the sound waves are travelling through the solid ground to our ears!

Now that we know how sound propagates and how its speed depends on the medium, let’s take a look at how to calculate how far it will travel between two points. We can use the equation

distance = speed x time

For example, if we want to know how far a sound will travel in one second, we have:

distance = 343 m/s x 0.001 s = 343 m

So sound travels 1 kilometer in roughly 3 seconds and 1 mile in roughly 5 seconds.

Does Вecibel Level affect the Sound Distance?

The surface area around a sound source’s location grows with the square of the distance from the source. This implies that the same amount of sound energy is dispersed over a larger surface, and that the energy intensity decreases as the square of the distance from the source (Inverse Square Law).

Experts of Acoustical control says, that

For every doubling of distance, the sound level reduces by 6  decibels  (dB), (e.g. moving from 10 to 20 metres away from a sound source). But the next 6dB reduction means moving from 20 to 40 metres, then from 40 to 80 metres for a further 6dB reduction.

How Far Can Sound Travel In Real World

In real world, there are many factors that can affect how far a sound travels. Factors such as air density, temperature and humidity have an impact on its propagation; obstacles like buildings or mountains could also block some frequencies from going through while letting others pass (this happens because at high frequencies they behave more like waves).

Sounds can propagate through solids better than they can propagate through air because their density/stiffness are greater (this means that sound travels faster). In addition to this, we also know that it takes less time for a high frequency wave to reach us from its source compared with low frequencies. For example if there’s some kind of obstacle blocking our path then it might take longer for waves at higher frequencies than those below 1000 Hz to past them.

Can Sound Waves Travel Infinitely?

No. The higher the frequency of a sound wave, the shorter its wavelength becomes. As wavelength decreases, the amount of energy in a sound wave also decreases and eventually it dissipates completely. This is why we often can’t hear someone calling from very far away when there’s loud music playing – because the high frequencies are being blocked out by all the noise!

Can Sound Travel 20 Miles?

The air may be permeable to these lower-frequency, sub-audible sound waves generated by elephants. Some whale species’ frequencies might travel through seawater for 1500 kilometers or 900 miles.

How Far Can a Human Scream Travel?

The normal intelligible outdoor range of the male human voice in still air is 180 m (590 ft 6.6 in).

The Guiness World Record of the Farthest distance travelled by a human voice belongs the Spanish-speaking inhabitants of the Canary Island of La Gomera, is intelligible under ideal conditions at 8 km (5 miles).

In Conclusion

At the end of this blog post, you should have a better understanding of how sound travel and what factors affect it. If you want to learn more about acoustics and sounds, you can check out our resources here.

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Sound waves in air and water.

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The molecules of air are much further apart than the molecules in a liquid. A sound wave therefore travels more slowly in the loosely packed air than it does in a much more tightly packed liquid. Sound waves also travel further in liquids and solids than they do in air.

The nature of the medium is a major factor in the speed of a wave. For example, if you make a wave on a string stretched loosely across a classroom, you will see the wave travel down the string. If you tighten the string the wave will move down the string faster. Tightness or stiffness of the string influences the speed.

Note: There is no sound on this video.

Sound – visualising sound waves

Sound is a form of energy that is caused by the vibration of matter. Sound is transmitted through waves, which travel through solids, liquids and gases. We are most used to the sound travelling ...

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Traveling waves

17 How sound moves

Speed of sound.

There’s a delay between when a sound is created and when it is heard. In everyday life, the delay is usually too short to notice. However, the delay can be noticeable if the distance between source and detector is large enough. You see lightning before you hear the thunder. If you’ve sat in the outfield seats in a baseball stadium, you’ve experienced the delay between seeing the player hit the ball and the sound of the “whack.” Life experiences tell us that sound travels fast, but not nearly as fast as light does. Careful experiments confirm this idea.

The speed of sound in air is roughly 340 m/s. The actual value depends somewhat on the temperature and humidity. In everyday terms, sound travels about the length of three and a half foot ball fields every second- about 50% faster than a Boeing 747 (roughly 250 m/s). This may seem fast, but it’s tiny compared to light, which travels roughly a million times faster than sound (roughly 300,000,000 m/s).

Sound requires some material in which to propagate (i.e. travel). This material sound travels through is called the medium . You can show that sound requires a medium by putting a cell phone inside a glass jar connected to a vacuum pump. As the air is removed from the jar, the cell phone’s ringer gets quieter and quieter. A youTube video (2:05 min) produced by the UNSW PhysClips project shows the demo with a drumming toy monkey [1] instead of a cell phone.

What affects the speed of sound?

Sound travels at different speeds though different materials. The physical properties of the medium are the only factors that affect the speed of sound- nothing else matters.

The speed of sound in a material is determined mainly by two properties- the stiffness of the material and the density of the material. Sound travels fastest through materials that are stiff and light. In general, sound travels fastest through solids, slower through liquids and slowest through gasses. (See the table on this page). This may seem backwards- after all, metals are quite dense. However, the high density of metals is more than offset by far greater stiffness (compared to liquids and solids).

The speed of sound in air depends mainly on temperature. The speed of sound is 331 m/s in dry air at 0 o Celsius and increases slightly with temperature- about 0.6 m/s for every 1 o Celsius for temperatures commonly found on Earth. Though speed of sound in air also depends on humidity, the effect is tiny- sound travels only about 1 m/s faster in air with 100% humidity air at 20 o C than it does in completely dry air at the same temperature.

Nothing else matters

The properties of the medium are the only factors that affect the speed of sound- nothing else matters.

Frequency of the sound does not matter- high frequency sounds travel at the same speed as low frequency sounds. If you’ve ever listened to music, you’ve witnessed this-  the low notes and the high notes that were made simultaneously reach you simultaneously, even if you are far from the stage. If you’ve heard someone shout from across a field, you’ve noticed that the entire shout sound (which contains many different frequencies at once) reaches you at the same time. If different frequencies traveled at different rates, some frequencies would arrive before others.

The amplitude of the sound does not matter- loud sounds and quiet ones travel at the same speed. Whisper or yell- it doesn’t matter. The sound still takes the same amount of time to reach the listener.  You’ve probably heard that you can figure out how far away the lightning by counting the seconds between when you see lightning and hear thunder. If the speed of sound depended on loudness, this rule of thumb would have to account for loudness- yet there is nothing in the rule about loud vs. quiet thunder. The rule of thumb works the same for all thunder- regardless of loudness . That’s because the speed of sound doesn’t depend on amplitude.

Stop to thinks

• Which takes longer to cross a football field: the sound of a high pitched chirp emitted by a fruit bat or the (relatively) low pitched sound emitted by a trumpet?
• Which sound takes longer to travel 100 meters: the sound of a snapping twig in the forest or the sound of a gunshot?
• Which takes longer to travel the distance of a football field: the low pitched sound of a whale or the somewhat higher pitched sound of a human being?

Constant speed

Sound travels at a constant speed. Sound does not speed up or slow down as it travels (unless the properties of the material the sound is going through changes). I know what you’re thinking- how is that possible? Sounds die out as they travel, right? True. That means sounds must slow down and come to a stop, right? Wrong. As sound travels, its amplitude decreases- but that’s not the same thing as slowing down. Sound (in air) covers roughly 340 meters each and every second, even as its amplitude shrinks. Eventually, the amplitude gets small enough that the sound is undetectable. A sound’s amplitude shrinks as it travels, but its speed remains constant.

The basic equation for constant speed motion (shown below) applies to sound.

[latex]d=vt[/latex]

In this equation, [latex]d[/latex] represents the distance traveled by the sound, [latex]t[/latex] represents the amount of time it took to go that distance and [latex]v[/latex] represents the speed.

Rule of thumb for lightning example

Example: thunder and lightning.

The rule of thumb for figuring out how far away a lightning strike is from you is this:

Count the number of seconds between when you see the lightning and hear the thunder. Divide the number of seconds by five to find out how many miles away the lightning hit.

According to this rule, what is the speed of sound in air? How does the speed of sound implied by this rule compare to 340 m/s?

Identify important physics concept :   This question is about speed of sound.

List known and unknown quantities (with letter names and units):

At first glance, it doesn’t look like there’s enough information to solve the problem. We were asked to find speed, but not given either a time or a distance. However, the problem does allow us to figure out a distance if we know the time- “Divide the number of seconds by five to find out how many miles away the lightning hit.” So, let’s make up a time and see what happens; if the time is 10 seconds, the rule of thumb says that the distance should be 2 miles.

[latex]v= \: ?[/latex]

[latex]d=2 \: miles[/latex]

[latex]t=10 \: seconds[/latex]

You might ask “Is making stuff up OK here?” The answer is YES! If the rule of thumb is right, it should work no matter what time we choose. (To check if the rule is OK, we should probably test it with more than just one distance-time combination, but we’ll assume the rule is OK for now).

Do the algebra:  The equation is already solved for speed. Move on to the next step.

Do unit conversions (if needed) then plug in numbers:  If you just plug in the numbers, the speed comes out in miles per second:

[latex]v = \frac{2 \: miles} {10 \: seconds}=0.2 \: \frac{miles} {second}[/latex]

We are asked to compare this speed to 340 m/s, so a unit conversion is in order; since there are 1609 meters in a mile:

[latex]v =0.2 \: \frac{miles} {second}*\frac{1609 \: meters} {1 \:mile}=320 \frac{m}{s}[/latex]

Reflect on the answer:

• The answer for speed from the rule of thumb is less than 10% off the actual value of roughly 340 m/s- surprisingly close!
• At the beginning, we assumed a time of 10 seconds. Does the result hold up for other choices? A quick check shows that it does! For instance, if we use a time of 5 seconds, the rule of thumb gives a distance of 1 mile, and the math still gives a speed of 0.2 miles/second. The speed will be the same no matter what time we pick. The reason is this:  The more time it takes the thunder to arrive, the farther away the lightning strike is, but the speed remains the same. In the equation for speed, both time and distance change by the same factor and the overall fraction remains unchanged.

Stop to think answers

• Both sounds take the same amount of time. (High and low pitched sounds travel at the same speed).
• Both sounds take the same amount of time. (Quiet sounds and loud sounds travel at the same speed).
• The sound of the whale travels the distance in less time- assuming sound from the whale travels in water and sound from the human travels in air. Sound travels faster in water than in air. (The info about frequency was given just to throw you off- frequency doesn’t matter).
• Wolfe, J. (2014, February 20). Properties of Sound. Retrieved from https://www.youtube.com/watch?v=P8-govgAffY ↵

Understanding Sound Copyright © by dsa2gamba and abbottds is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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How does sound propagate from air into water?

In the simplest situations, sound travels through a medium in straight lines. However, at some point, the medium may have changes in temperature , salinity , or pressure that cause the speed of sound to change. This change in sound speed will bend or refract the sound. Sound will bend towards the region with a slower sound speed.

In other instances, the sound may encounter a medium with a different density . A particularly important case is when airborne sound meets water. A portion of the sound wave will reflect away from the water and into the air, while another part will transmit into the water. During reflection the direction of the incoming wave changes. The original wave moving towards the boundary is called the incident wave . It hits the boundary at an angle, and its path of movement is redirected. The resulting reflected wave  travels away from the boundary at an angle. A line perpendicular to the boundary, called a normal line , defines two angles: the angle of incidence , depicted in this figure as “∠I“, and the angle of reflection , depicted in this figure as “∠R“. The two angles are equal. A third angle often used in acoustics is the grazing angle , depicted in this figure as “∠G”, which is the complementary angle  to the angle of incidence.

The sound wave can also be transmitted into the water. The amount of energy transmitted depends on the angle of incidence and the acoustic properties of the two media. More transmission relative to reflection occurs if the acoustic properties of the two media are similar. Because the sound speed in water is greater than the sound speed in air , a special angle of incidence called the critical angle exists. When the sound arrives at an incident angle that is greater than the critical angle, the sound is almost perfectly reflected (the blue line in the figure below). When the incident angle of the sound is less than the critical angle, a portion of the wave enters the water (the yellow line in the figure below). For the specific case of sound entering water from the air, the critical angle (θ c ) is about 15° relative to the normal line.

The resulting pathways at four different angles of incidence at the air-water boundary: at 90 degrees, less than the critical angle (< θ c ), at the critical angle (θ c ), and greater than the critical angle (> θ c ). Image credit URI.

This science becomes important when discussing how certain anthropogenic activities, such as wind energy production , may affect marine animals. There is concern about the acoustic pathways in which sound from wind turbines might enter the water and affect marine life. When the blades of a wind turbine are spinning, they produce low frequency  sound that may enter the water through an air-to-water path. However, this transmission will only occur where sound energy reaches the air-water boundary at incident angles smaller than the critical angle. As a result, air-to-water transmission only takes place at distances close to the turbine.

Acoustic pathways for underwater noise from an offshore wind turbine under operation. Kikuchi, R. (2010). Risk formulation for the sonic effects of offshore wind farms on fish in the EU region. Marine Pollution Bulletin, 60(2), 172–177.

Air-to-water transmission of sound is also an important issue when understanding underwater sound transmission from manned aircraft (including sonic booms), rocket launches, explosions, and even autonomous aerial vehicles (drones).

Top left: SpaceX launch, March 18, 2020. The Falcon 9 lifted off from Launch Complex 39A (LC-39A) at NASA’s Kennedy Space Center in Florida. Photo credit: Space X, CC BY-NC 2.0 . Top right: A US Navy F/A-18 Hornet was photographed just as it broke the sound barrier. Photo Credit: Ensign John Gay, USS Constellation, US Navy, public domain. Bottom left: Sea Planes in Vancouver. Image Credit: Wikimedia Commons user FreebirdBiker, CC BY 3.0 . Bottom right: Drone flying over water. Photo Credit: Aaron Burden, public domain.

Additional Links on DOSITS

• Science > Sound Movement > How fast does sound travel?
• Science > How does sound move? > Refraction
• Science > How does sound move? > Reflection
• Science > How does sound in air differ from sound in water?
• Animals > Anthropogenic Sound Sources > Wind Turbines
• Audio Gallery > Wind Turbine Sounds
• Chapman, D. M. F., & Ward, P. D. (1990). The normal‐mode theory of air‐to‐water sound transmission in the ocean. The Journal of the Acoustical Society of America , 87 (2), 601–618. https://doi.org/10.1121/1.398929
• Chapman, D. M. F., Thomson, D. J., & Ellis, D. D. (1992). Modeling air‐to‐water sound transmission using standard numerical codes of underwater acoustics. The Journal of the Acoustical Society of America , 91 (4), 1904–1910. https://doi.org/10.1121/1.403701
• Medwin, H., & Clay , C. S. (1998). Fundamentals of acoustical oceanography. Academic Press.
• Rossing, T. D. (Ed.). (2007). Springer handbook of acoustics. Springer.
• Sparrow, V. W. (1993). Sonic boom wave propagation from air into water: Implications for marine mammals. The Journal of the Acoustical Society of America , 94 (3), 1850–1851. https://doi.org/10.1121/1.407693
• Urick, R. J. (1972). Noise Signature of an Aircraft in Level Flight over a Hydrophone in the Sea. The Journal of the Acoustical Society of America , 52 (3B), 993–999. https://doi.org/10.1121/1.1913206
• Urick, R. J. (1983). Principles of Underwater Sound, Third Edition (3rd edition, Reprint 2013). McGraw-Hill, Inc.

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Science Struck

How Does Sound Travel? Here’s the Science Behind This Concept

When sound waves travel through a medium, the particles of the medium vibrate. Vibrations reach the ear and then the brain which senses them and we recognize sound. Read on for an explanation of how sound travels.

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Sound is a series of compression and rarefraction waves that can travel long distances. It is produced by the vibration of the particles present in its medium; a medium is the material through which sound can travel. Presence of a medium is a must for the movement of sound waves. There are various types of medium through which sound waves can move like solids, liquids, gases, plasma, etc. Sound cannot travel through vacuum.

Characteristics of Sound Waves

The speed and the physical characteristics of sound largely varies with the change in its ambient conditions. The speed of sound depends on the density of the medium though which it is traveling. If its density is quite high, then sound would travel at a faster pace. When sound travels through gaseous medium, its speed varies with respect to changes in temperature.

The frequency of sound waves is nothing but the total number of vibrations that have been produced. The length of sound waves vary according to its frequency. Sound waves with long wavelengths have low frequency or low pitch; and those with short wavelengths have high frequency or high pitch. Our ears are capable of hearing only those sound waves which lie in the range between 20 and 20,000 vibrations per second.

How do Sound Waves Travel?

Basically, there are three things that are required for the transmission of sound. They are: a source that can transmit the sound, a medium through which sound can pass (like, water, air, etc.), and the receiver or the detector which receives the sound. The traveling process of sound has been explained below.

Creation of Sound

When a physical object moves in air, it causes vibrations which leads to formation of a series of compression waves in the air. These waves travel in the form of sound. For instance, when we strum the strings of a guitar or hit the head of a drum, the to-and-fro motion of the strings or the drum head creates compression waves of sound in the surrounding air. Similarly, when we speak, our vocal cords vibrate and the sound is created. This type of vibration occurs not just in atmospheric air but in other mediums like, solids and liquids as well. For instance, when a train is moving on a railroad made up of steel, the sound waves thus produced travel via these tracks.

At room temperature, sound travels through air with a speed of 343 m/s, through water at 1,482 m/s, and through steel at 5,960 m/s. As you can see, sound waves travel in a gaseous medium at a slow pace because its molecules are loosely bound and have to cover a long distance to collide with another molecule. In solid medium, the atoms are so closely packed that the vibration is readily transmitted to the neighboring atoms, and sound travels quite fast. In liquid medium, the bonding between the component particles are not as strong as in solids. Therefore, the sound waves move through it at a less speed as compared to solid.

Detection of Sound

When the sound waves hit the receiver, it causes some vibration in that object. The detector captures just a part of the energy from the moving sound wave. This energy of vibration is then converted to electrical signals. Thus, when the sound waves reach our ears, the eardrum present inside it vibrates. This vibration reaches our inner ear and is converted into nerve signals. As a result, we can hear the sound. Devices like microphone can detect sound. The sound waves create vibrations in its membrane which forms electrical signals that gets amplified and recorded.

So, how does sound travel? Vibration of an object causes vibrations of the same frequency in the surrounding medium. The vibrations are sent to the inner ear. After the auditory nerve picks up these vibrations, electrical signals are sent to the brain where the vibrations are recognized as sound.

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Acoustics and Vibration Animations

Daniel A. Russell , Graduate Program in Acoustics, The Pennsylvania State University

The content of this page was originally posted on December 2, 2013 . The interactive plot was updated on March 22, 2016 .

Absorption and Attenuation of Sound in Air

Overview of absorption.

I intend to write some text explaining the general process of absorption of sound in air (viscous effects, thermal conduction effects, and molecular relaxation processes). But, in the mean time, the interactive plot below works and allows for calculation of the absorption coefficient for sound in air.

As sound waves travel through the air, the amplitude of the sound wave decreases (attenuates) as some of the energy carried by the wave is lost to friction and relaxation processes in the gas (air). There are two main processes by which sound energy is absorbed by air:

• classical absorption --- viscous losses due to friction as molecules (nitrogen, oxygen, argon, carbon dioxide) collide with each other. This thermal-viscous classical absorption depends on the square root of temperature, and the square of the frequency.

NOTE: This plot is a Computable Document Format (CDF) object created by Mathematica and you will need to install the free Wolfram CDF Player to be able to see it and interact with it. Unfortunately, the CDF player is only available for Safari and Firefox on Mac OSX and Firefox and Internet Explorer for Windows. And the CDF player is not yet available for mobile devices.

Here is a link to a standalone version of the absorption calculator (you'll still need to download the Wolfram CDF Player to interact with it, but you won't have to use a web browser. [NOTE: for some reason the slider for pressure starts out at 0.5 atm].

How to Use this Interactive Plot

• relative humidity - from 0 to 100% (increments of 1%),
• frequency - from 10 Hz up to 60 kHz (increments of 5 Hz),
• temperature - from -10 o C to 45 o C (increments of 1 o C).
• pressure - from 0.5 P atm to 1.0 P atm (increments of 0.1 P atm ).
• The black dashed line represents the classical absorption due to viscous and thermal conduction effects. Viscous and thermal conduction absorption are both proportional to the square of frequency, so on a log-log plot the classical absorption looks like a straight line with a slope of 2.
• The Red curve represents the molecular relaxation due to the Nitrogen (N 2 ) molecules that comprise 78% of the air.
• The Blue curve represents the molecular relaxation due to the Oxygen (O 2 ) molecules that comprise 21% of the air.
• The solid Black curve is the sum total of all three absorption mechanisms. This is the attenuation number shown in the blue box at the top left in the plot
• The two Green lines are guides to show the value of absorption (given as a number in the blue box at the upper left of the plot) for a specified frequency (selected via the Frequency slider).
• H.E. Bass, L.C. Sutherland, A.J. Zuckerwar, D.T. Blackstock, and D.M. Hester, "Atmospheric absorption of sound: Further developments," J. Acoust. Soc. Am. , 97 (1), 680-683 (1995).
• ANSI Standard S1-26:1995, "Calculation of the Absorption of Sound by the Atmosphere" (ISO 9613-1:1996).

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Understanding Sound Waves and How They Work

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Sound. When a drum is struck, the drumhead vibrates and the vibrations are transmitted through the air in the form of sound waves . When they strike the ear, these waves produce the sensation of sound.

Technically, sound is defined as a mechanical disturbance traveling through an elastic medium — a material that tends to return to its original condition after being deformed. The medium doesn't have to be air. Metal, wood, stone, glass, water, and many other substances conduct sound — many of them even better than air.

The Basics of Sound

Sound waves, speed of sound, the behavior of a sound wave, sound quality, history of sound.

There are many sources of sound. Familiar kinds include the vibration of a person's vocal cords, vibrating strings (piano, violin), a vibrating column of air (trumpet, flute), and vibrating solids (a door when someone knocks). It's impossible to list them all because anything that imparts a disturbance to an elastic medium is a source of sound.

Sound can be described in terms of pitch — from the low rumble of distant thunder to the high-pitched buzzing of a mosquito — and loudness. Pitch and loudness , however, are subjective qualities; they depend in part on the hearer's sense of hearing. Objective, measurable qualities of sound include frequency and intensity, which are related to pitch and loudness. These terms, as well as others used in discussing sound, are best understood through an examination of sound waves and their behavior.

Speed of sound in various mediums

Air, like all matter, consists of molecules. Even a tiny region of air contains vast numbers of air molecules. The molecules are in constant motion, traveling randomly and at great speed. They constantly collide with and rebound from one another and strike and rebound from objects that are in contact with the air.

When an object vibrates it produces sound waves in the air. For example, when the head of a drum is hit with a mallet, the drumhead vibrates and produces sound waves. The vibrating drumhead produces sound waves because it moves alternately outward and inward, pushing against, then moving away from, the air next to it. The air particles that strike the drumhead while it is moving outward rebound from it with more than their normal energy and speed, having received a push from the drumhead.

These faster-moving molecules move into the surrounding air. For a moment, the region next to the drumhead has a greater-than-normal concentration of air molecules — it becomes a region of compression. As the faster-moving molecules overtake the air molecules in the surrounding air, they collide with them and pass on their extra energy. The region of compression moves outward as the energy from the vibrating drumhead is transferred to groups of molecules farther and farther away.

Air molecules that strike the drumhead while it's moving inward rebound from it with less than their normal energy and speed. For a moment, the region next to the drumhead has fewer air molecules than normal — it becomes a region of rarefaction. Molecules colliding with these slower-moving molecules also rebound with less speed than normal, and the region of rarefaction travels outward.

The nature of sound is captured through its fundamental characteristics : wavelength (the distance between wave peaks), amplitude (the height of the wave, corresponding to loudness), frequency (the number of waves passing a point per second, related to pitch), time period (the time it takes for one complete wave cycle to occur), and velocity (the speed at which the wave travels through a medium). These properties intertwine to craft the unique signature of every sound we hear.

The wave nature of sound becomes apparent when a graph is drawn to show the changes in the concentration of air molecules at some point as the alternating pulses of compression and rarefaction pass that point. The graph for a single pure tone, such as that produced by a vibrating tuning fork, would show a sine wave (illustrated here ). The curve shows the changes in concentration. It begins, arbitrarily, at some time when the concentration is normal and a compression pulse is just arriving. The distance of each point on the curve from the horizontal axis indicates how much the concentration varies from normal.

Each compression and the following rarefaction make up one cycle. (A cycle can also be measured from any point on the curve to the next corresponding point.) The frequency of a sound is measured in cycles per second or hertz (abbreviated Hz). The amplitude is the greatest amount by which the concentration of air molecules varies from the normal.

The wavelength of a sound is the distance the disturbance travels during one cycle. It's related to the sound's speed and frequency by the formula speed/frequency = wavelength. This means that high-frequency sounds have short wavelengths and low-frequency sounds have long wavelengths. The human ear can detect sounds with frequencies as low as 20 Hz and as high as 20,000 Hz. In still air at room temperature, sounds with these frequencies have wavelengths of 75 feet (23 m) and 0.68 inch (1.7 cm) respectively.

Intensity refers to the amount of energy transmitted by the disturbance. It's proportional to the square of the amplitude. Intensity is measured in watts per square centimeter or in decibels (db). The decibel scale is defined as follows: An intensity of 10-16 watts per square centimeter equals 0 db. (Written out in decimal form, 10-16 appears as 0.0000000000000001.) Each tenfold increase in watts per square centimeter means an increase of 10 db. Thus, an intensity of 10-15 watts per square centimeter can also be expressed as 10 db and an intensity of 10-4 (or 0.0001) watts per square centimeter as 120 db.

The intensity of sound drops rapidly with increasing distance from the source. For a small sound source radiating energy uniformly in all directions, intensity varies inversely with the square of the distance from the source. That is, at a distance of two feet from the source the intensity is one-fourth as great as it is at a distance of one foot; at three feet it is only one-ninth as great as at one foot, etc.

Pitch depends on the frequency ; in general, a rise in frequency causes a sensation of rising pitch. The ability to distinguish between two sounds that are close in frequency, however, decreases in the upper and lower parts of the audible frequency range. There is also variation from person to person in the ability to distinguish between two sounds of very nearly the same frequency. Some trained musicians can detect differences in frequency as small as 1 or 2 Hz.

Because of how the hearing mechanism functions, the perception of pitch is also affected by intensity. Thus, when a tuning fork vibrating at 440 Hz (the frequency of A above middle C on the piano) is brought closer to the ear, a slightly lower tone, as though the fork were vibrating more slowly, is heard.

When the source of a sound is moving at a relatively high speed, a stationary listener hears a sound higher in pitch when the source is moving toward him or her and a sound lower in pitch when the source is moving away. This phenomenon, known as the Doppler effect , is due to the wave nature of sound.

In general, an increase in intensity will cause a sensation of increased loudness. But loudness does not increase in direct proportion to intensity. A sound of 50 dB has ten times the intensity of a sound of 40 dB but is only twice as loud. Loudness doubles with each increase of 10 dB in intensity.

Loudness is also affected by frequency because the human ear is more sensitive to some frequencies than to others. The threshold of hearing — the lowest sound intensity that will produce the sensation of hearing for most people — is about 0 dB in the 2,000 to 5,000 Hz frequency range. For frequencies below and above this range, sounds must have greater intensity to be heard. Thus, for example, a sound of 100 Hz is barely audible at 30 dB; a sound of 10,000 Hz is barely audible at 20 dB. At 120 to 140 dB, most people experience physical discomfort or actual pain, and this level of intensity is referred to as the threshold of pain .

When we visualize waves, we often think of transverse waves — like the rolling waves on a beach — where the motion of the wave is perpendicular to the direction of energy transfer. However, sound waves are a different type altogether — a longitudinal wave. In longitudinal sound waves, such as sound waves produced by a vibrating drumhead or our vocal cords, the particles of the medium move parallel to the wave's direction of travel. This movement creates areas of compression and rarefaction in the medium — be it air, water, or a solid — which our ears interpret as sound. Understanding the difference between longitudinal and transverse waves is central to understanding sound.

The speed of sound depends on the elasticity and density of the medium through which it is traveling. In general, sound travels faster in liquids than in gases and faster in solids than in liquids. The greater the elasticity and the lower the density, the faster sound moves in a medium. The mathematical relationship is speed = (elasticity/density).

The effect of elasticity and density on the speed of sound can be seen by comparing the speed of sound in air, hydrogen, and iron. Air and hydrogen have nearly the same elastic properties, but the density of hydrogen is less than that of air. Sound travels faster (about 4 times as fast) in hydrogen than in air. Although the density of air is much less than that of iron, the elasticity of iron is very much greater than that of air. Sound travels faster (about 14 times as fast) in iron than in air.

The speed of sound in a material, particularly in a gas or liquid, varies with temperature because a change in temperature affects the material's density. In air, for example, the speed of sound increases with an increase in temperature . At 32 °F. (0 °C.), the speed of sound in air is 1,087 feet per second (331 m/s); at 68 °F. (20 °C.), it is 1,127 feet per second (343 m/s).

The terms subsonic and supersonic refer to the speed of an object, such as an airplane, in relation to the speed of sound in the surrounding air. A subsonic speed is below the speed of sound; a supersonic speed is above the speed of sound. An object traveling at supersonic speed produces shock waves rather than ordinary sound waves. A shock wave is a compression wave that, when produced in air, can usually be heard as a sonic boom .

The speeds of supersonic objects are often expressed in terms of Mach number — the ratio of the object's speed to the speed of sound in the surrounding air. Thus, an object traveling at Mach 1 is traveling at the speed of sound; at Mach 2, it is traveling at twice the speed of sound.

Like light waves and other waves, sound waves are reflected, refracted, and diffracted, and exhibit interference.

Sound is constantly being reflected off many different surfaces. Most of the time the reflected sound is not noticed, because two identical sounds that reach the human ear less than 1/15 of a second apart cannot be distinguished as separate sounds. When the reflected sound is heard separately, it's called an echo .

Sound is reflected from a surface at the same angle at which it strikes the surface. This fact makes it possible to focus sound by means of curved reflecting surfaces in the same way that curved mirrors can be used to focus light. It also accounts for the effects of so-called whispering galleries, rooms in which a word whispered at one point can be heard distinctly at some other point fairly far away, though it cannot be heard anywhere else in the room. (The National Statuary Hall of the United States Capitol is an example.) Reflection is also used to focus sound in a megaphone and when calling through cupped hands.

The reflection of sound can pose a serious problem in concert halls and auditoriums. In a poorly designed hall, a speaker's first word may reverberate (echo repeatedly) for several seconds, so that the listeners may hear all the words of a sentence echoing at the same time. Music can be similarly distorted. Such problems can usually be corrected by covering reflecting surfaces with sound-absorbing materials such as draperies or acoustical tiles. Clothing also absorbs sound; for this reason, reverberation is greater in an empty hall than in one filled with people. All these sound-absorbing materials are porous; sound waves entering the tiny air-filled spaces bounce around in them until their energy is spent. They are, in effect, trapped.

The reflection of sound is used by some animals, notably bats , for echolocation — locating, and in some cases identifying, objects through the sense of hearing rather than the sense of sight. Bats emit bursts of sound of frequencies far beyond the upper limits of human hearing. Sounds with short wavelengths are reflected even from very small objects. A bat can unerringly locate and catch even a mosquito in total darkness. Sonar is an artificial form of echolocation .

When a wave passes from one material to another at an angle, it usually changes speed, causing the wave front to bend. The refraction of sound can be demonstrated in a physics laboratory by using a lens-shaped balloon filled with carbon dioxide to bring sound waves to a focus.

Diffraction

When sound waves pass around an obstacle or through an opening in an obstacle, the edge of the obstacle or the opening acts as a secondary sound source, sending out waves of the same frequency and wavelength (but of lower intensity) as the original source. The spreading out of sound waves from the secondary source is called diffraction . Because of this phenomenon, sound can be heard around corners despite the fact that sound waves generally travel in a straight line.

Interference

Whenever waves interact, interference occurs. For sound waves, the phenomenon is perhaps best understood by thinking in terms of the compressions and rarefactions of the two waves as they arrive at some point. When the waves are in phase so that their compressions and rarefactions coincide, they reinforce each other ( constructive interference ). When they are out of phase, so that the compressions of one coincide with the rarefactions of the other, they tend to weaken or even cancel each other ( destructive interference ). The interaction between the two waves produces a resultant wave.

In auditoriums, destructive interference between sound from the stage and sound reflected from other parts of the hall can create dead spots in which both the volume and clarity of sound are poor. Such interference can be reduced by the use of sound-absorbing materials on reflecting surfaces. On the other hand, interference can improve an auditorium's acoustical qualities. This is done by arranging the reflecting surfaces in such a way that the level of sound is actually increased in the area in which the audience sits.

Interference between two waves of nearly but not quite equal frequencies produces a tone of alternately increasing and decreasing intensity because the two waves continually fall in and out of phase. The pulsations heard are called beats. Piano tuners make use of this effect, adjusting the tone of a string against that of a standard tuning fork until beats can no longer be heard.

Sound waves are fundamentally pressure waves, traveling through the compression and rarefaction of particles within a medium. Sound waves consist of areas where particles are bunched together, followed by areas where they're spread apart. These high-pressure and low-pressure regions propagate through environments such as air, water or solids, as the energy of the sound wave moves from particle to particle. It's the rapid variation in pressure that an ear drum detects and the brain decodes into the sounds we hear.

Sounds of a single pure frequency are produced only by tuning forks and electronic devices called oscillators ; most sounds are a mixture of tones of different frequencies and amplitudes. The tones produced by musical instruments have one important characteristic in common: they are periodic, that is, the vibrations occur in a repeating pattern. The oscilloscope trace of a trumpet's sound shows such a pattern. For most non-musical sounds, such as those of a bursting balloon or a person coughing, an oscilloscope trace would show a jagged, irregular pattern, indicating a jumble of frequencies and amplitudes.

A column of air, as that in a trumpet, and a piano string both have a fundamental frequency — the frequency at which they vibrate most readily when set in motion. For a vibrating column of air, that frequency is determined principally by the length of the column. (The trumpet's valves are used to change the effective length of the column.) For a vibrating string, the fundamental frequency depends on the string's length, its tension, and its mass per unit length.

In addition to its fundamental frequency, a string or vibrating column of air also produces overtones with frequencies that are whole-number multiples of the fundamental frequency. It is the number of overtones produced and their relative strength that gives a musical tone from a given source its distinctive quality or timbre . The addition of further overtones would produce a complicated pattern, such as that of the oscilloscope trace of the trumpet's sound.

How the fundamental frequency of a vibrating string depends on the string's length, tension, and mass per unit length is described by three laws:

1. The fundamental frequency of a vibrating string is inversely proportional to its length.

Reducing the length of a vibrating string by one-half will double its frequency, raising the pitch by one octave, if the tension remains the same.

2. The fundamental frequency of a vibrating string is directly proportional to the square root of the tension.

Increasing the tension of a vibrating string raises the frequency; if the tension is made four times as great, the frequency is doubled, and the pitch is raised by one octave.

3. The fundamental frequency of a vibrating string is inversely proportional to the square root of the mass per unit length.

This means that of two strings of the same material and with the same length and tension, the thicker string has the lower fundamental frequency. If the mass per unit length of one string is four times that of the other, the thicker string has a fundamental frequency one-half that of the thinner string and produces a tone one octave lower.

One of the first discoveries regarding sound was made in the sixth century B.C. by the Greek mathematician and philosopher Pythagoras . He noted the relationship between the length of a vibrating string and the tone it produces — what is now known as the first law of strings. Pythagoras may also have understood that the sensation of sound is caused by vibrations. Not long after his time it was recognized that this sensation depends on vibrations traveling through the air and striking the eardrum.

About 1640 the French mathematician Marin Mersenne conducted the first experiments to determine the speed of sound in air. Mersenne is also credited with discovering the second and third laws of strings. In 1660 the British scientist Robert Boyle demonstrated that the transmission of sound required a medium — by showing that the ringing of a bell in a jar from which the air had been pumped could not be heard.

Ernst Chladni , a German physicist, made extensive analyses of sound vibrations during the late 1700s and early 1800s. In the early 1800s, the French mathematician Fourier discovered that such complex waves as those produced by a vibrating string with all its overtones consist of a series of simple periodic waves.

An important contribution to the understanding of acoustics was made by Wallace Clement Sabine , a physicist at Harvard University, in the late 1890s. Sabine was asked to improve the acoustics of the main lecture hall in Harvard's Fogg Art Museum. He was first to measure reverberation time — which he found to be 5 1/2 seconds in the lecture hall. Experimenting first with seat cushions from a nearby theater, and later with other sound-absorbing materials and other methods, Sabine laid the foundation for architectural acoustics. He designed Boston Symphony Hall (opened in 1900), the first building with scientifically formulated acoustics.

In the second half of the 20th century, the rising level of noise in the modern world — particularly in urban areas — prompted a whole new series of investigations, dealing in large part with the physiological and psychological effects of noise on humans.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

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