sound waves travel faster in

• Sound Waves
• Speed Of Sound Propagation

Speed of Sound

A sound wave is fundamentally a pressure disturbance that propagates through a medium by particle interaction. In other words, sound waves move through a physical medium by alternately contracting and expanding the section of the medium in which it propagates. The rate at which the sound waves propagate through the medium is known as the speed of sound. In this article, you will discover the definition and factors affecting the speed of sound.

Speed of Sound Definition

The speed of sound is defined as the distance through which a sound wave’s point, such as a compression or a rarefaction, travels per unit of time. The speed of sound remains the same for all frequencies in a given medium under the same physical conditions.

Speed of Sound Formula

Since the speed of sound is the distance travelled by the sound wave in a given time, the speed of sound can be determined by the following formula:

v = λ f

Where v is the velocity, λ is the wavelength of the sound wave, and f is the frequency.

The relationship between the speed of sound, its frequency, and wavelength is the same as for all waves. The wavelength of a sound is the distance between adjacent compressions or rarefactions . The frequency is the same as the source’s and is the number of waves that pass a point per unit time.

Solved Example:

How long does it take for a sound wave of frequency 2 kHz and a wavelength of 35 cm to travel a distance of 1.5 km?

We know that the speed of sound is given by the formula:

v = λ ν

Substituting the values in the equation, we get

v = 0.35 m × 2000 Hz = 700 m/s

The time taken by the sound wave to travel a distance of 1.5 km can be calculated as follows:

Time = Distance Travelled/ Velocity

Time = 1500 m/ 700 m/s = 2.1 s

Factors Affecting the Speed of Sound

Density and temperature of the medium in which the sound wave travels affect the speed of sound.

Density of the Medium

When the medium is dense, the molecules in the medium are closely packed, which means that the sound travels faster. Therefore, the speed of sound increases as the density of the medium increases.

Temperature of the Medium

The speed of sound is directly proportional to the temperature. Therefore, as the temperature increases, the speed of sound increases.

Speed of Sound in Different Media

The speed of the sound depends on the density and the elasticity of the medium through which it travels. In general, sound travels faster in liquids than in gases and quicker in solids than in liquids. The greater the elasticity and the lower the density, the faster sound travels in a medium.

Speed of Sound in Solid

Sound is nothing more than a disturbance propagated by the collisions between the particles, one molecule hitting the next and so forth. Solids are significantly denser than liquids or gases, and this means that the molecules are closer to each other in solids than in liquids and liquids than in gases. This closeness due to density means that they can collide very quickly. Effectively it takes less time for a molecule of a solid to bump into its neighbouring molecule. Due to this advantage, the velocity of sound in a solid is faster than in a gas.

The speed of sound in solid is 6000 metres per second, while the speed of sound in steel is equal to 5100 metres per second. Another interesting fact about the speed of sound is that sound travels 35 times faster in diamonds than in the air.

Speed of Sound in Liquid

Speed of Sound in Water

The speed of sound in water is more than that of the air, and sound travels faster in water than in the air. The speed of sound in water is 1480 metres per second. It is also interesting that the speed may vary between 1450 to 1498 metres per second in distilled water. In contrast, seawater’s speed is 1531 metres per second when the temperature is between 20 o C to 25 o C.

Speed of Sound in Gas

We should remember that the speed of sound is independent of the density of the medium when it enters a liquid or solid. Since gases expand to fill the given space, density is relatively uniform irrespective of gas type, which isn’t the case with solids and liquids. The velocity of sound in gases is proportional to the square root of the absolute temperature (measured in Kelvin). Still, it is independent of the frequency of the sound wave or the pressure and the density of the medium. But none of the gases we find in real life is ideal gases , and this causes the properties to change slightly. The velocity of sound in air at 20 o C is 343.2 m/s which translates to 1,236 km/h.

Speed of Sound in Vacuum

The speed of sound in a vacuum is zero metres per second, as there are no particles present in the vacuum. The sound waves travel in a medium when there are particles for the propagation of these sound waves. Since the vacuum is an empty space, there is no propagation of sound waves.

Table of Speed of Sound in Various Mediums

Another very curious fact is that in solids, sound waves can be created either by compression or by tearing of the solid, also known as Shearing. Such waves exhibit different properties from each other and also travel at different speeds. This effect is seen clearly in Earthquakes. Earthquakes are created due to the movement of the earth’s plates, which then send these disturbances in the form of waves similar to sound waves through the earth and to the surface, causing an Earthquake. Typically compression waves travel faster than tearing waves, so Earthquakes always start with an up-and-down motion, followed after some time by a side-to-side motion. In seismic terms, the compression waves are called P-waves, and the tearing waves are called S-waves . They are the more destructive of the two, causing most of the damage in an earthquake.

Visualise sound waves like never before with the help of animations provided in the video

Frequently Asked Questions – FAQs

What is the speed of sound in vacuum, name the property used for distinguishing a sharp sound from a dull sound., define the intensity of sound., how does the speed of sound depend on the elasticity of the medium, why is the speed of sound maximum in solids, name the factors on which the speed of sound in a gas depends., what is a sonic boom, the below video helps to completely revise the chapter sound class 9.

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Since the speed of a wave is defined as the distance that a point on a wave (such as a compression or a rarefaction) travels per unit of time, it is often expressed in units of meters/second (abbreviated m/s). In equation form, this is

The faster a sound wave travels, the more distance it will cover in the same period of time. If a sound wave were observed to travel a distance of 700 meters in 2 seconds, then the speed of the wave would be 350 m/s. A slower wave would cover less distance - perhaps 660 meters - in the same time period of 2 seconds and thus have a speed of 330 m/s. Faster waves cover more distance in the same period of time.

Factors Affecting Wave Speed

The speed of any wave depends upon the properties of the medium through which the wave is traveling. Typically there are two essential types of properties that affect wave speed - inertial properties and elastic properties. Elastic properties are those properties related to the tendency of a material to maintain its shape and not deform whenever a force or stress is applied to it. A material such as steel will experience a very small deformation of shape (and dimension) when a stress is applied to it. Steel is a rigid material with a high elasticity. On the other hand, a material such as a rubber band is highly flexible; when a force is applied to stretch the rubber band, it deforms or changes its shape readily. A small stress on the rubber band causes a large deformation. Steel is considered to be a stiff or rigid material, whereas a rubber band is considered a flexible material. At the particle level, a stiff or rigid material is characterized by atoms and/or molecules with strong attractions for each other. When a force is applied in an attempt to stretch or deform the material, its strong particle interactions prevent this deformation and help the material maintain its shape. Rigid materials such as steel are considered to have a high elasticity. (Elastic modulus is the technical term). The phase of matter has a tremendous impact upon the elastic properties of the medium. In general, solids have the strongest interactions between particles, followed by liquids and then gases. For this reason, longitudinal sound waves travel faster in solids than they do in liquids than they do in gases. Even though the inertial factor may favor gases, the elastic factor has a greater influence on the speed ( v ) of a wave, thus yielding this general pattern:

Inertial properties are those properties related to the material's tendency to be sluggish to changes in its state of motion. The density of a medium is an example of an inertial property . The greater the inertia (i.e., mass density) of individual particles of the medium, the less responsive they will be to the interactions between neighboring particles and the slower that the wave will be. As stated above, sound waves travel faster in solids than they do in liquids than they do in gases. However, within a single phase of matter, the inertial property of density tends to be the property that has a greatest impact upon the speed of sound. A sound wave will travel faster in a less dense material than a more dense material. Thus, a sound wave will travel nearly three times faster in Helium than it will in air. This is mostly due to the lower mass of Helium particles as compared to air particles.  

The Speed of Sound in Air

The speed of a sound wave in air depends upon the properties of the air, mostly the temperature, and to a lesser degree, the humidity. Humidity is the result of water vapor being present in air. Like any liquid, water has a tendency to evaporate. As it does, particles of gaseous water become mixed in the air. This additional matter will affect the mass density of the air (an inertial property). The temperature will affect the strength of the particle interactions (an elastic property). At normal atmospheric pressure, the temperature dependence of the speed of a sound wave through dry air is approximated by the following equation:

where T is the temperature of the air in degrees Celsius. Using this equation to determine the speed of a sound wave in air at a temperature of 20 degrees Celsius yields the following solution.

v = 331 m/s + (0.6 m/s/C)•(20 C)

v = 331 m/s + 12 m/s

v = 343 m/s

(The above equation relating the speed of a sound wave in air to the temperature provides reasonably accurate speed values for temperatures between 0 and 100 Celsius. The equation itself does not have any theoretical basis; it is simply the result of inspecting temperature-speed data for this temperature range. Other equations do exist that are based upon theoretical reasoning and provide accurate data for all temperatures. Nonetheless, the equation above will be sufficient for our use as introductory Physics students.)

Look It Up!

Using wave speed to determine distances.

At normal atmospheric pressure and a temperature of 20 degrees Celsius, a sound wave will travel at approximately 343 m/s; this is approximately equal to 750 miles/hour. While this speed may seem fast by human standards (the fastest humans can sprint at approximately 11 m/s and highway speeds are approximately 30 m/s), the speed of a sound wave is slow in comparison to the speed of a light wave. Light travels through air at a speed of approximately 300 000 000 m/s; this is nearly 900 000 times the speed of sound. For this reason, humans can observe a detectable time delay between the thunder and the lightning during a storm. The arrival of the light wave from the location of the lightning strike occurs in so little time that it is essentially negligible. Yet the arrival of the sound wave from the location of the lightning strike occurs much later. The time delay between the arrival of the light wave (lightning) and the arrival of the sound wave (thunder) allows a person to approximate his/her distance from the storm location. For instance if the thunder is heard 3 seconds after the lightning is seen, then sound (whose speed is approximated as 345 m/s) has traveled a distance of

If this value is converted to miles (divide by 1600 m/1 mi), then the storm is a distance of 0.65 miles away.

Another phenomenon related to the perception of time delays between two events is an echo . A person can often perceive a time delay between the production of a sound and the arrival of a reflection of that sound off a distant barrier. If you have ever made a holler within a canyon, perhaps you have heard an echo of your holler off a distant canyon wall. The time delay between the holler and the echo corresponds to the time for the holler to travel the round-trip distance to the canyon wall and back. A measurement of this time would allow a person to estimate the one-way distance to the canyon wall. For instance if an echo is heard 1.40 seconds after making the holler , then the distance to the canyon wall can be found as follows:

The canyon wall is 242 meters away. You might have noticed that the time of 0.70 seconds is used in the equation. Since the time delay corresponds to the time for the holler to travel the round-trip distance to the canyon wall and back, the one-way distance to the canyon wall corresponds to one-half the time delay.

While an echo is of relatively minimal importance to humans, echolocation is an essential trick of the trade for bats. Being a nocturnal creature, bats must use sound waves to navigate and hunt. They produce short bursts of ultrasonic sound waves that reflect off objects in their surroundings and return. Their detection of the time delay between the sending and receiving of the pulses allows a bat to approximate the distance to surrounding objects. Some bats, known as Doppler bats, are capable of detecting the speed and direction of any moving objects by monitoring the changes in frequency of the reflected pulses. These bats are utilizing the physics of the Doppler effect discussed in an earlier unit (and also to be discussed later in Lesson 3 ). This method of echolocation enables a bat to navigate and to hunt.

The Wave Equation Revisited

Like any wave, a sound wave has a speed that is mathematically related to the frequency and the wavelength of the wave. As discussed in a previous unit , the mathematical relationship between speed, frequency and wavelength is given by the following equation.

Using the symbols v , λ , and f , the equation can be rewritten as

Check Your Understanding

1. An automatic focus camera is able to focus on objects by use of an ultrasonic sound wave. The camera sends out sound waves that reflect off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. If a sound wave (speed = 340 m/s) returns to the camera 0.150 seconds after leaving the camera, how far away is the object?

Answer = 25.5 m

The speed of the sound wave is 340 m/s. The distance can be found using d = v • t resulting in an answer of 25.5 m. Use 0.075 seconds for the time since 0.150 seconds refers to the round-trip distance.

2. On a hot summer day, a pesky little mosquito produced its warning sound near your ear. The sound is produced by the beating of its wings at a rate of about 600 wing beats per second.

a. What is the frequency in Hertz of the sound wave? b. Assuming the sound wave moves with a velocity of 350 m/s, what is the wavelength of the wave?

Part a Answer: 600 Hz (given)

Part b Answer: 0.583 meters

3. Doubling the frequency of a wave source doubles the speed of the waves.

a. True b. False

Doubling the frequency will halve the wavelength; speed is unaffected by the alteration in the frequency. The speed of a wave depends upon the properties of the medium.

4. Playing middle C on the piano keyboard produces a sound with a frequency of 256 Hz. Assuming the speed of sound in air is 345 m/s, determine the wavelength of the sound corresponding to the note of middle C.

 Answer: 1.35 meters (rounded)

Let λ = wavelength. Use v = f • λ where v = 345 m/s and f = 256 Hz. Rearrange the equation to the form of λ = v / f. Substitute and solve.

5. Most people can detect frequencies as high as 20 000 Hz. Assuming the speed of sound in air is 345 m/s, determine the wavelength of the sound corresponding to this upper range of audible hearing.

Answer: 0.0173 meters (rounded)

Let λ = wavelength. Use v = f • λ where v = 345 m/s and f = 20 000 Hz. Rearrange the equation to the form of λ = v / f. Substitute and solve.

6. An elephant produces a 10 Hz sound wave. Assuming the speed of sound in air is 345 m/s, determine the wavelength of this infrasonic sound wave.

Answer: 34.5 meters

Let λ = wavelength. Use v = f • λ where v = 345 m/s and f = 10 Hz. Rearrange the equation to the form of λ = v / f. Substitute and solve.

7. Determine the speed of sound on a cold winter day (T=3 degrees C).

Answer: 332.8 m/s

The speed of sound in air is dependent upon the temperature of air. The dependence is expressed by the equation:

v = 331 m/s + (0.6 m/s/C) • T

where T is the temperature in Celsius. Substitute and solve.

v = 331 m/s + (0.6 m/s/C) • 3 C v = 331 m/s + 1.8 m/s v = 332.8 m/s

8. Miles Tugo is camping in Glacier National Park. In the midst of a glacier canyon, he makes a loud holler. He hears an echo 1.22 seconds later. The air temperature is 20 degrees C. How far away are the canyon walls?

Answer = 209 m

The speed of the sound wave at this temperature is 343 m/s (using the equation described in the Tutorial). The distance can be found using d = v • t resulting in an answer of 343 m. Use 0.61 second for the time since 1.22 seconds refers to the round-trip distance.

9. Two sound waves are traveling through a container of unknown gas. Wave A has a wavelength of 1.2 m. Wave B has a wavelength of 3.6 m. The velocity of wave B must be __________ the velocity of wave A.

a. one-ninth b. one-third c. the same as d. three times larger than

The speed of a wave does not depend upon its wavelength, but rather upon the properties of the medium. The medium has not changed, so neither has the speed.

10. Two sound waves are traveling through a container of unknown gas. Wave A has a wavelength of 1.2 m. Wave B has a wavelength of 3.6 m. The frequency of wave B must be __________ the frequency of wave A.

Since Wave B has three times the wavelength of Wave A, it must have one-third the frequency. Frequency and wavelength are inversely related.

• Interference and Beats

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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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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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The Nature of Sound

Introduction.

Sound is a longitudinal, mechanical wave.

Sound can travel through any medium, but it cannot travel through a vacuum. There is no sound in outer space.

Sound is a variation in pressure. A region of increased pressure on a sound wave is called a compression (or condensation). A region of decreased pressure on a sound wave is called a rarefaction (or dilation).

The sources of sound

• vibrating solids
• rapid expansion or compression (explosions and implosions)
• Smooth (laminar) air flow around blunt obstacles may result in the formation of vortices (the plural of vortex) that snap off or shed with a characteristic frequency. This process is called vortex shedding and is another means by which sound waves are formed. This is how a whistle or flute produces sound. Also the aeolian harp effect of singing power lines and fluttering venetian blinds.

What are the different characteristics of a wave? What are the things that can be measured about waves? Amplitude, frequency (and period), wavelength, speed, and maybe phase. Deal with each one in that order.

amplitude, intensity, loudness, volume

Amplitude goes with intensity, loudness, or volume. That's the basic idea. The details go in a separate section .

[ISO 226:2003]

• Unlike our ears and hydrophones, fish ears don't detect sound pressure, which is the compression of molecules. Instead, they perceive something called particle motion, the tiny back-and-forth movements of particles in response to sound waves.

speed of sound

The speed of sound depends upon the type of medium and its state. Sound generally travels faster in solids than in liquids than in gases.

The speed of sound in a medium is generally affected by two things: elasticity and inertia. This is the Newton-Laplace equation. Laplace added the γ (gamma) correction factor for ideal gases.

For solids…

For fluids (which incudes liquids and gases)…

For ideal gases…

Hi there. The speed of sound is faster in solids that have some stiffness like steel and slower in those that are softer like rubber.

Basically liquids. Actually, basically water.

The speed of sound in water is somewhat difficult to determine since it is affected by temperature, pressure, density, and salinity (or the amount of any other other dissolved substances). The speed of sound in water can be found using this emprically derived equation …

v  = 1449.2 + 4.6  T  − 0.055  T 2  + 0.00029  T 3  + (1.34 − 0.010  T )( s  − 35) + 0.16  h

Generally, an increase in temperature and salinity will increase the speed of sound in water. Usually, ocean salinity is estimated at around 35 ppt, so the equation above is essentialy a function of temperature and depth.

Acoustic Thermometry of Ocean Climates (ATOC)

• in water, sounds below 1 kHz travel much farther than higher frequencies
• shipping noise is loudest in the 30 to 200 Hz range [lowest piano note to middle of cello]
• blue and fin wales are the loudest sound in the 17 to 30 Hz range
• "In pre-industrial times, the low frequency range of 15 to 300 Hz in which most of the baleen whales sing was the quietest part of the sound spectrum, nestled between the subsonic ramblings of earthquakes and the higher pitched rattle of wind, waves and rain." Bob Holmes. "Noises Off." New Scientist. 1 March 1997: 30–33.

ideal gases

Let's be honest. All we really care about is the speed of sound in air.

• The speed of sound in air is approximately 345 m/s (about 1250 kph, 770 mph, 1100 ft/s).
• The speed of sound in air is nearly the same for all frequencies and amplitudes.
• It increases with temperature.

The speed of sound in air as a function of temperature can be found using these approximate equations…

• the linear one
• the one with the square root in it

Where should this go?

frequency, pitch, tone

The frequency of a sound wave is called it pitch . High frequency sounds are said to be "high pitched" or just "high"; low frequency sounds are said to be "low pitched" or just "low".

human hearing and speech

Humans are generally capable of hearing sounds between 20 Hz and 20 kHz (although I can't hear sounds above 13 kHz). Sounds with frequencies above the range of human hearing are called ultrasound . Sounds with frequencies below the range of human hearing are called infrasound .

• Typical sounds produced by human speech have frequencies on the order of 100 to 1,000 Hz.
• The peak sensitivity of human hearing is around 4,000 Hz.
• Interaural Time Difference (ITD)
• Interaural Phase Difference (IPD) Phase differences are one way we localize sounds. Only effective for wavelengths greater than 2 head diameters (ear-to-ear distances).
• Interaural Level Difference (ILD) Sound waves diffract easily at wavelengths larger than the diameter of the human head (around 500 Hz wavelength equals 69 cm). At higher frequencies the head casts a "shadow". Sounds in one ear will be louder than the other.
• 1,400 different pitches
• (whistle register?)
• modal — the usual speaking register
• vocal fry — the lowest of the three vocal registers

More in the next section.

• avalanches: location, depth, duration
• meteors: altitude, direction, type, size, location
• ocean waves: storms at sea, magnitude, spectra
• severe weather: location, intensity
• tornadoes: detection, location, warning, core radius, funnel shape, precursors
• turbulence: aircraft avoidance, altitude, strength, extent
• earthquakes: precursors, seismic-acoustic coupling
• volcanoes: location, intensity
• Elephants, whales, hippos, rhinoceros, giraffe, okapi, and alligator are just a few examples of animals that create infrasound.
• Some migratory birds are able to hear the infrasonic sounds produced when ocean waves break. This allows them to orient themselves with coastlines.
• An elephant is capable of hearing sound waves well below our the human hearing limitation (approximately 30 Hertz). Typically, an elephant's numerous different rumbles will span between 14 and 35 Hertz. The far reaching use of high pressure infrasound opens the elephant's spatial experience far beyond our limited capabilities.
• Silent Thunder, Katy Payne
• microchiropterans a.k.a. microbats: carnivorous bats ( not fruit bats or flying foxes)
• cetaceans : dolphins, porpoises, orcas, whales
• two bird species: swiftlets and oilbirds
• some visually impaired humans have learned this technique
• echo sounding
• fish finders
• medical ultrasonography (the images generated are called sonograms ).

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Speed of Sound in Physics

In physics, the speed of sound is the distance traveled per unit of time by a sound wave through a medium. It is highest for stiff solids and lowest for gases. There is no sound or speed of sound in a vacuum because sound (unlike light ) requires a medium in order to propogate.

What Is the Speed of Sound?

Usually, conversations about the speed of sound refer to the speed of sound of dry air (humidity changes the value). The value depends on temperature.

• at 20 ° C or 68 ° F: 343 m/s or 1234.8 kph or 1125ft/s or 767 mph
• at 0 ° C or 32 ° F: 331 m/s or 1191.6 kph or 1086 ft/s or 740 mph

Mach Numher

The Mach number is the ratio of air speed to the speed of sound. So, an object at Mach 1 is traveling at the speed of sound. Exceeding Mach 1 is breaking the sound barrier or is supersonic . At Mach 2, the object travels twice the speed of sound. Mach 3 is three times the speed of sound, and so on.

Remember that the speed of sound depends on temperature, so you break sound barrier at a lower speed when the temperature is colder. To put it another way, it gets colder as you get higher in the atmosphere, so an aircraft might break the sound barrier at a higher altitude even if it does not increase its speed.

Solids, Liquids, and Gases

The speed of sound is greatest for solids, intermediate for liquids, and lowest for gases:

v solid > v liquid >v gas

Particles in a gas undergo elastic collisions and the particles are widely separated. In contrast, particles in a solid are locked into place (rigid or stiff), so a vibration readily transmits through chemical bonds.

Here are examples of the difference between the speed of sound in different materials:

• Diamond (solid): 12000 m/s
• Copper (solid): 6420 m/s
• Iron (solid): 5120 m/s
• Water (liquid) 1481 m/s
• Helium (gas): 965 m/s
• Dry air (gas): 343 m/s

Sounds waves transfer energy to matter via a compression wave (in all phases) and also shear wave (in solids). The pressure disturbs a particle, which then impacts its neighbor, and continues traveling through the medium. The speed is how quickly the wave moves, while the frequency is the number of vibrations the particle makes per unit of time.

The Hot Chocolate Effect

The hot chocolate effect describes the phenomenon where the pitch you hear from tapping a cup of hot liquid rises after adding a soluble powder (like cocoa powder into hot water). Stirring in the powder introduces gas bubbles that reduce the speed of sound of the liquid and lower the frequency (pitch) of the waves. Once the bubbles clear, the speed of sound and the frequency increase again.

Speed of Sound Formulas

There are several formulas for calculating the speed of sound. Here are a few of the most common ones:

For gases these approximations work in most situations:

For this formula, use the Celsius temperature of the gas.

v = 331 m/s + (0.6 m/s/C)•T

Here is another common formula:

v = (γRT) 1/2

• γ is the ratio of specific heat values or adiabatic index (1.4 for air at STP )
• R is a gas constant (282 m 2 /s 2 /K for air)
• T is the absolute temperature (Kelvin)

The Newton-Laplace formula works for both gases and liquids (fluids):

v = (K s /ρ) 1/2

• K s is the coefficient of stiffness or bulk modulus of elasticity for gases
• ρ is the density of the material

So solids, the situation is more complicated because shear waves play into the formula. There can be sound waves with different velocities, depending on the mode of deformation. The simplest formula is for one-dimensional solids, like a long rod of a material:

v = (E/ρ) 1/2

• E is Young’s modulus

Note that the speed of sound decreases with density! It increases according to the stiffness of a medium. This is not intuitively obvious, since often a dense material is also stiff. But, consider that the speed of sound in a diamond is much faster than the speed in iron. Diamond is less dense than iron and also stiffer.

Factors That Affect the Speed of Sound

The primary factors affecting the speed of sound of a fluid (gas or liquid) are its temperature and its chemical composition. There is a weak dependence on frequency and atmospheric pressure that is omitted from the simplest equations.

While sound travels only as compression waves in a fluid, it also travels as shear waves in a solid. So, a solid’s stiffness, density, and compressibility also factor into the speed of sound.

Speed of Sound on Mars

Thanks to the Perseverance rover, scientists know the speed of sound on Mars. The Martian atmosphere is much colder than Earth’s, its thin atmosphere has a much lower pressure, and it consists mainly of carbon dioxide rather than nitrogen. As expected, the speed of sound on Mars is slower than on Earth. It travels at around 240 m/s or about 30% slower than on Earth.

What scientists did not expect is that the speed of sound varies for different frequencies. A high pitched sound, like from the rover’s laser, travels faster at around 250 m/s. So, for example, if you listened to a symphony recording from a distance on Mars you’d hear the various instruments at different times. The explanation has to do with the vibrational modes of carbon dioxide, the primary component of the Martian atmosphere. Also, it’s worth noting that the atmospheric pressure is so low that there really isn’t any much sound at all from a source more than a few meters away.

Speed of Sound Example Problems

Find the speed of sound on a cold day when the temperature is 2 ° C.

The simplest formula for finding the answer is the approximation:

v = 331 m/s + (0.6 m/s/C) • T

Since the given temperature is already in Celsius, just plug in the value:

v = 331 m/s + (0.6 m/s/C) • 2 C = 331 m/s + 1.2 m/s = 332.2 m/s

You’re hiking in a canyon, yell “hello”, and hear an echo after 1.22 seconds. The air temperature is 20 ° C. How far away is the canyon wall?

The first step is finding the speed of sound at the temperature:

v = 331 m/s + (0.6 m/s/C) • T v = 331 m/s + (0.6 m/s/C) • 20 C = 343 m/s (which you might have memorized as the usual speed of sound)

Next, find the distance using the formula:

d = v• T d = 343 m/s • 1.22 s = 418.46 m

But, this is the round-trip distance! The distance to the canyon wall is half of this or 209 meters.

If you double the frequency of sound, it double the speed of its waves. True or false?

This is (mostly) false. Doubling the frequency halves the wavelength, but the speed depends on the properties of the medium and not its frequency or wavelength. Frequency only affects the speed of sound for certain media (like the carbon dioxide atmosphere of Mars).

• Everest, F. (2001). The Master Handbook of Acoustics . New York: McGraw-Hill. ISBN 978-0-07-136097-5.
• Kinsler, L.E.; Frey, A.R.; Coppens, A.B.; Sanders, J.V. (2000). Fundamentals of Acoustics (4th ed.). New York: John Wiley & Sons. ISBN 0-471-84789-5.
• Maurice, S.; et al. (2022). “In situ recording of Mars soundscape:. Nature. 605: 653-658. doi: 10.1038/s41586-022-04679-0
• Wong, George S. K.; Zhu, Shi-ming (1995). “Speed of sound in seawater as a function of salinity, temperature, and pressure”. The Journal of the Acoustical Society of America . 97 (3): 1732. doi: 10.1121/1.413048

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How Sound Travels

Most recent answer: 10/22/2007

(published on 10/22/2007)

Follow-up on this answer

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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.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.

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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.

Search Our Facts

More information.

• 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