Speed of sound


Speed of sound

Sound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how much distance such a wave travels in a certain amount of time. In SI Units with dry air at 20 °C (68 °F), the speed of sound is 343 m/s. This also equates to 1235 km/h, 767 mph, 1125 ft/s. At such speeds, sound can travel a mile in approximately five seconds. Although it is commonly used to refer specifically to air, the speed of sound can be measured in virtually any substance. Sound travels faster in liquids and non-porous solids than it does in air.

Basic concept

The transmission of sound can be illustrated by using a toy model consisting of an array of balls interconnected by springs. For real material the balls represent molecules and the springs represent the bonds between them. Sound passes through the model by compressing and expanding the springs, transmitting energy to neighboring balls, which transmit energy to "their" springs, and so on. The speed of sound through the model depends on the stiffness of the springs (stiffer springs transmit energy more quickly). Effects like dispersion and reflection can also be understood using this model.

In a real material, the stiffness of the springs is called the elastic modulus, and the mass corresponds to the density. All other things being equal, sound will travel more slowly in denser materials, and faster in stiffer ones. For instance, sound will travel faster in iron than uranium, and faster in hydrogen than nitrogen, due to the lower density of the first material of each set. At the same time, sound will travel faster in iron than hydrogen, because the internal bonds in a solid are much stronger than the gaseous bonds between hydrogen molecules. In general, solids will have a higher speed of sound than liquids, and liquids will have a higher speed of sound than gases.

Some textbooks mistakenly state that the speed of sound increases with increasing density. This is usually illustrated by presenting data for three materials, such as air, water and steel. With only these three examples it indeed appears that speed is correlated to density, yet including only a few more examples would show this assumption to be incorrect.

General formulae

In general, the speed of sound "c" is given by:c = sqrt{frac{C}{ howhere :"C" is a coefficient of stiffness : ho is the density

Thus the speed of sound increases with the stiffness of the material, and decreases with the density.For general equations of state, if classical mechanics is used, the speed of sound c is given by:c^2=frac{partial p}{partial ho}where differentiation is taken with respect to adiabatic change.

If relativistic effects are important, the speed of sound may be calculated from the relativistic Euler equations.

In a non-dispersive medium sound speed is independent of sound frequency, so the speeds of energy transport and sound propagation are the same. For audible sounds air is a non-dispersive medium. But air does contain a small amount of CO2 which "is" a dispersive medium, and it introduces dispersion to air at ultrasonic frequencies (> 28 kHz). [Dean, E. A. (August 1979). [http://handle.dtic.mil/100.2/ADA076060 Atmospheric Effects on the Speed of Sound,] Technical report of Defense Technical Information Center]

In a dispersive medium sound speed is a function of sound frequency. The spatial and temporal distribution of a propagating disturbance will continually change. Each frequency component propagates at its own phase velocity, while the energy of the disturbance propagates at the group velocity. The same phenomenon occurs with light waves -- see optical dispersion for a description.

Dependence on the properties of the medium

The speed of sound is variable and depends mainly on the temperature and the properties of the substance through of which the wave is traveling. For example, in low molecular weight gases, such as helium, sound propagates faster compared to heavier gases, such as xenon. In a given ideal gas the sound speed depends only on its temperature. At a constant temperature, the ideal gas pressure has no effect on the speed of sound, because pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly. In non-ideal gases, such as a van der Waals gas, the proportionality is not exact, and there is a slight dependence on the gas pressure, even at a constant temperature. Humidity also has a small, but measurable effect on sound speed (increase of about 0.1%-0.6%), because some oxygen and nitrogen molecules of the air are replaced by the lighter molecules of water.

Implications for atmospheric acoustics

In the Earth's atmosphere, the most important factor affecting the speed of sound is the temperature (see Details below). Since temperature and thus the speed of sound normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. [cite book
last = Everest
first = F.
title = The Master Handbook of Acoustics
publisher = McGraw-Hill
location = New York
year = 2001
isbn = 0071360972
pages = pp. 262-263
] The decrease of the sound speed with height is referred to as a negative sound speed gradient. However, in the stratosphere, the speed of sound increases with height due to heating within the ozone layer, producing a positive sound speed gradient.

Practical formula for dry air

The approximate speed of sound in dry (0% humidity) air, in metres per second (m·s-1), at temperatures near 0 °C, can be calculated from: :c_{mathrm{air = 331{.}3 + (0{.}606 cdot vartheta) mathrm{m cdot s^{-1,where vartheta is the temperature in degrees Celsius (°C).

This equation is derived from the first two terms of the Taylor expansion of the following much more accurate equation:

:c_{mathrm{air = 331.3 sqrt{1+frac{vartheta}{273.15 mathrm{m cdot s^{-1

The value of 331.3 m/s, which represents the 0 °C speed, is based on theoretical (and some measured) values of the heat capacity ratio, gamma, as well as on the fact that at 1 atm real air is very well described by the ideal gas approximation. Commonly found values for the speed of sound at 0 °C may vary from 331.2 to 331.6 due to the assumptions made when it is calculated. If ideal gas gamma is assumed to be 7/5 = 1.4 exactly, the 0 °C speed is calculated (see section below) to be 331.3 m/s, the coefficient used above.

This equation is correct to a much wider temperature range, but still depends on the approximation of heat capacity ratio being independent of temperature, and will fail, particularly at higher temperatures. It gives good predictions in relatively dry, cold, low pressure conditions, such as the Earth's stratosphere. A derivation of these equations will be given in a later section.

Details

peed in ideal gases and in air

For a gas, "K" (the bulk modulus in equations above, equivalent to C, the coefficient of stiffness in solids) is approximately given by:K = gamma cdot p thus c = sqrt{gamma cdot {p over ho

Where::gamma is the adiabatic index also known as the "isentropic expansion factor". It is the ratio of specific heats of a gas at a constant-pressure to a gas at a constant-volume(C_p/C_v), and arises because a classical sound wave induces an adiabatic compression, in which the heat of the compression does not have enough time to escape the pressure pulse, and thus contributes to the pressure induced by the compression.:"p" is the pressure.:" ho" is the density

Using the ideal gas law to replace p with "nRT"/"V", and replacing "ρ" with "nM"/"V", the equation for an ideal gas becomes:

:c_{mathrm{ideal = sqrt{gamma cdot {p over ho = sqrt{gamma cdot R cdot T over M}= sqrt{gamma cdot k cdot T over m}

where
*c_{mathrm{ideal is the speed of sound in an ideal gas.
*R (approximately 8.3145 J·mol-1·K-1) is the molar gas constant. [http://physics.nist.gov/cgi-bin/cuu/Value?r]
*k is the Boltzmann constant
*gamma (gamma) is the adiabatic index (sometimes assumed 7/5 = 1.400 for diatomic molecules from kinetic theory, assuming from quantum theory a temperature range at which thermal energy is fully partitioned into rotation (rotations are fully excited), but none into vibrational modes. Gamma is actually experimentally measured over a range from 1.3991 to 1.403 at 0 degrees Celsius, for air. Gamma is assumed from kinetic theory to be exactly 5/3 = 1.6667 for monoatomic molecules such as noble gases).
*T is the absolute temperature in kelvins.
*M is the molar mass in kilograms per mole. The mean molar mass for dry air is about 0.0289645 kg/mol.
*m is the mass of a single molecule in kilograms.

This equation applies only when the sound wave is a small perturbation on the ambient condition, and the certain other noted conditions are fulfilled, as noted below. Calculated values for c_{mathrm{air have been found to vary slightly from experimentally determined values.U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976.]

Newton famously considered the speed of sound before most of the development of thermodynamics and so incorrectly used isothermal calculations instead of adiabatic. His result was missing the factor of gamma but was otherwise correct.

Numerical substitution of the above values gives the ideal gas approximation of sound velocity for gases, which is accurate at relatively low gas pressures and densities (for air, this includes standard Earth sea-level conditions). Also, for diatomic gases the use of gamma, = 1.4000 requires that the gas exist in a temperature range high enough that rotational heat capacity is fully excited (i.e., molecular rotation is fully used as a heat energy "partition" or reservoir); but at the same time the temperature must be low enough that molecular vibrational modes contribute no heat capacity (i.e., insigificant heat goes into vibration, as all vibrational quantum modes above the minimum-energy-mode, have energies too high to be populated by a significant number of molecules at this temperature). For air, these conditions are fulfilled at room temperature, and also temperatures considerably below room temperature (see tables below). See the section on gases in heat capacity for a more complete discussion of this phenomenon.

If temperatures in degrees Celsius(°C) are to be used to calculate air speed in the region near 273 kelvins, then Celsius temperature vartheta = T - 273.15 may be used.

:c_{mathrm{ideal = sqrt{gamma cdot R cdot T} = sqrt{gamma cdot R cdot (vartheta + 273.15)}

:c_{mathrm{ideal = sqrt{gamma cdot R cdot 273.15} cdot sqrt{1+frac{vartheta}{273.15

For dry air, where vartheta, (theta) is the temperature in degrees Celsius(°C).

Making the following numerical substitutions: R = R_*/M_{mathrm{air, where R_* = 8.315410 cdot mathrm{J cdot mol^{-1 cdot K^{-1} is the molar gas constant, M_{mathrm{air = 0.0289645 cdot mathrm{kg cdot mol^{-1, and using the ideal diatomic gas value of gamma, = 1.4000

Then:

:c_{mathrm{air = 331.3 mathrm{m cdot s^{-1 sqrt{1+frac{vartheta}{273.15

Using the first two terms of the Taylor expansion:

:c_{mathrm{air = 331.3 mathrm{m cdot s^{-1 (1 + frac{vartheta}{2 cdot 273.15}) ,

:c_{mathrm{air = 331{.}3 + (0{.}606 cdot vartheta) mathrm{m cdot s^{-1,

The derivation includes the two approximate equations which were given in the introduction. For Celsius temperatures which are negative, the second term of the equation right hand side, is negative.

Effects due to wind shear

The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. [cite book | last = Everest | first = F. | title = The Master Handbook of Acoustics | publisher = McGraw-Hill | location = New York | year = 2001 | isbn = 0071360972 | pages = pp. 262-263 ] Wind shear of 4 m/s/km can produce refraction equal to a typical temperature lapse rate of 7.5 °C/km. [cite book | last = Uman | first = Martin | title = Lightning | publisher = Dover Publications | location = New York | year = 1984 | isbn = 0486645754 ] Higher values of wind gradient will refract sound downward toward the surface in the downwind direction, [cite book | last = Volland | first = Hans | title = Handbook of Atmospheric Electrodynamics | publisher = CRC Press | location = Boca Raton | year = 1995 | isbn = 0849386470 | pages = p. 22] eliminating the acoustic shadow on the downwind side. This will increase the audibility of sounds downwind. This downwind refraction effect occurs because there is a wind gradient; the sound is not being carried along by the wind. [cite book | last = Singal | first = S. | title = Noise Pollution and Control Strategy | publisher = Alpha Science International, Ltd | location = | year = 2005 | isbn = 1842652370 | pages = p. 7 | quote = It may be seen that refraction effects occur only because there is a wind gradient and it is not due to the result of sound being convected along by the wind.]

For sound propagation, the exponential variation of wind speed with height can be defined as follows:cite book | last = Bies | first = David | title = Engineering Noise Control; Theory and Practice | publisher = Spon Press | location = London | year = 2003 | isbn = 0415267137 | pages = p. 235 | quote = As wind speed generally increases with altitude, wind blowing towards the listener from the source will refract sound waves downwards, resulting in increased noise levels.]

: U(h) = U(0) h ^ zeta

: frac {dU} {dH} = zeta frac {U(h)} {h}

where:

: U(h) = speed of the wind at height h, and U(0) is a constant : zeta = exponential coefficient based on ground surface roughness, typically between 0.08 and 0.52: frac {dU} {dH} = expected wind gradient at height h

In the 1862 American Civil War Battle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle, [cite book | last = Cornwall | first = Sir | title = Grant as Military Commander | publisher = Barnes & Noble Inc | location = | year = 1996 | isbn = 1566199131 pages = p. 92] because they could not hear the sounds of battle only six miles downwind. [cite book | last = Cozzens | first = Peter | title = The Darkest Days of the War: the Battles of Iuka and Corinth | publisher = The University of North Carolina Press | location = Chapel Hill | year = 2006 | isbn = 0807857831 ]

Tables

In the standard atmosphere:

"T"0 is 273.15 K (= 0 °C = 32 °F), giving a theoretical value of 331.3 m·s-1 (= 1086.9 ft/s = 1193 km·h-1 = 741.1 mph = 644.0 knots). Values ranging from 331.3-331.6 may be found in reference literature, however.
"T"20 is 293.15 K (= 20 °C = 68 °F), giving a value of 343.2 m·s-1 (= 1126.0 ft/s = 1236 km·h-1 = 767.8 mph = 667.2 knots).
"T"25 is 298.15 K (= 25 °C = 77 °F), giving a value of 346.1 m·s-1 (= 1135.6 ft/s = 1246 km·h-1 = 774.3 mph = 672.8 knots).

In fact, assuming an ideal gas, the speed of sound "c" depends on temperature only, not on the pressure or density (since these change in lockstep for a given temperature and cancel out). Air is almost an ideal gas. The temperature of the air varies with altitude, giving the following variations in the speed of sound using the standard atmosphere - "actual conditions may vary".

:"vartheta" is the temperature in °C:"c" is the speed of sound in m·s-1:"ρ" is the density in kg·m-3:"Z" is the characteristic acoustic impedance in N·s·m-3 ("Z"="ρ"·"c")

Given normal atmospheric conditions, the temperature, and thus speed of sound, varies with altitude:

Effect of frequency and gas composition

The medium in which a sound wave is travelling does not always respond adiabatically, and as a result the speed of sound can vary with frequency. [A B Wood, A Textbook of Sound (Bell, London, 1946)]

The limitations of the concept of speed of sound due to extreme attenuation are also of concern. The attenuation which exists at sea level for high frequencies applies to successively lower frequencies as atmospheric pressure decreases, or as the mean free path increases. For this reason, the concept of speed of sound (except for frequencies approaching zero) progressively loses its range of applicability at high altitudes.:U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976. ]

The molecular composition of the gas contributes both as the mass (M) of the molecules, and their heat capacities, and so both have an influence on speed of sound. In general, at the same molecular mass, monatomic gases have slightly higher sound speeds (over 9% higher) because they have a higher gamma (5/3 = 1.66...) than diatomics do (7/5 = 1.4). Thus, at the same molecular mass, the sound speed of a monatomic gas goes up by a factor of

{ c_{mathrm{gas: monatomic over c_{mathrm{gas: diatomic } = sqrt5 / 3} over {7 / 5 = sqrt{25 over 21} = 1.091...

This gives the 9 % difference, and would be a typical ratio for sound speeds at room temperature in helium vs. deuterium, each with a molecular weight of 4. Sound travels faster in helium than deuterium because adiabatic compression heats helium more, since the helium molecules can store heat energy from compression only in translation, but not rotation. Thus helium molecules (monatomic molecules) travel faster in a sound wave and transmit sound faster. (Sound generally travels at about 70% of the mean molecular velocity in gases).

Note that in this example we have assumed that temperature is low enough that heat capacities are not influenced by molecular vibration (see heat capacity). However, vibrational modes simply cause gammas which decrease toward 1, since vibration modes in a polyatomic gas gives the gas additional ways to store heat which do not affect temperature, and thus do not affect molecular velocity and sound velocity. Thus, the effect of higher temperatures and vibrational heat capacity acts to increase the difference between sound speed in monatomic vs. polyatomic molecules, with the speed remaining greater in monatomics.

Mach number

Mach number, a useful quantity in aerodynamics, is the ratio of an object's speed to the speed of sound in the medium through which it is passing (again, usually air). At altitude, for reasons explained, Mach number is a function of temperature.

Aircraft flight instruments, however, operate using pressure differential to compute Mach number; not temperature. The assumption is that a particular pressure represents a particular altitude and, therefore, a standard temperature. Aircraft flight instruments need to operate this way because the impact pressure sensed by a Pitot tube is dependent on altitude as well as speed.

Assuming air to be an ideal gas, the formula to compute Mach number in a subsonic compressible flow is derived from Bernoulli's equation for "M"<1:Olson, Wayne M. (2002). "AFFTC-TIH-99-02, "Aircraft Performance Flight Testing"." (PDF). Air Force Flight Test Center, Edwards AFB, CA, United States Air Force.]

:{M}=sqrt{5left [left(frac{q_c}{P}+1 ight)^frac{2}{7}-1 ight] }

where:M is Mach number:q_c is impact pressure and :P is static pressure.

The formula to compute Mach number in a supersonic compressible flow is derived from the Rayleigh Supersonic Pitot equation:

:{M}=0.88128485sqrt{left [left(frac{q_c}{P}+1 ight)left(1-frac{1}{ [7M^2] } ight)^{2.5} ight] }

where:M is Mach number:q_c is impact pressure measured behind a normal shock :P is static pressure.

As can be seen, "M" appears on both sides of the equation. The easiest method to solve the supersonic "M" calculation is to enter both the subsonic and supersonic equations into a computer spread sheet such as Microsoft Excel, OpenOffice.org Calc, or some equivalent program. First determine if "M" is indeed greater than 1.0 by calculating "M" from the subsonic equation. If "M" is greater than 1.0 at that point, then use the value of "M" from the subsonic equation as the initial condition in the supersonic equation. Then perform a simple iteration of the supersonic equation, each time using the last computed value of "M", until "M" converges to a value--usually in just a few iterations.

Experimental methods

A range of different methods exist for the measurement of sound in air.

ingle-shot timing methods

The simplest concept is the measurement made using two microphones and a fast recording device such as a digital storage scope. This method uses the following idea.

If a sound source and two microphones are arranged in a straight line, with the sound source at one end, then the following can be measured:

1. The distance between the microphones ("x"), called microphone basis.2. The time of arrival between the signals (delay) reaching the different microphones ("t")

Then "v" = "x" / "t"

An older method is to create a sound at one end of a field with an object that can be seen to move when it creates the sound. When the observer sees the sound-creating device act they start a stopwatch and when the observer hears the sound they stop their stopwatch. Again using "v = x" / "t" you can calculate the speed of sound. A separation of at least 200 m between the two experimental parties is required for good results with this method.

Other methods

In these methods the time measurement has been replaced by a measurement of the inverse of time (frequency).

Kundt's tube is an example of an experiment which can be used to measure the speed of sound in a small volume. It has the advantage of being able to measure the speed of sound in any gas. This method uses a powder to make the nodes and antinodes visible to the human eye. This is an example of a compact experimental setup.

A tuning fork can be held near the mouth of a long pipe which is dipping into a barrel of water. In this system it is the case that the pipe can be brought to resonance if the length of the air column in the pipe is equal to "({1+2n}λ/4)" where "n" is an integer. As the antinodal point for the pipe at the open end is slightly outside the mouth of the pipe it is best to find two or more points of resonance and then measure half a wavelength between these.

Here it is the case that "v" = "fλ"

Non-gaseous media

peed in solids

In a solid, there is a non-zero stiffness both for volumetric and shear deformations. Hence, in a solid it is possible to generate sound waves with different velocities dependent on the deformation mode. A sound wave generating volumetric deformations is called longitudinal and a transversal wave generates shear deformations. The velocities of these two different sound waves can be calculated in isotropic solids by:

:c_{mathrm{long = sqrt{frac{E}{ ho:c_{mathrm{trans = sqrt{frac{G}{ ho

where

:"E" is Young's modulus:"G" is Shear modulus:"ρ" (rho) is density

Thus in steel the speed of longitudinal waves is approximately 5,100 m·s-1. In beryllium, a substance with relatively high stiffness and low density the speed of longitudinal waves is 12,870 m·s-1. [http://www.sizes.com/natural/sound.htm Accessed Jan 5, 2007]

In a solid with lateral dimensions much larger than the wavelength, the sound velocity is higher. It is found by replacing Young's modulus "E" in the above formula by the plane wave modulus "M", which can be expressed in terms of the Young's modulus and Poisson's ratio as::M = E frac{1- u}{1- u-2 u^2}

peed in liquids

In a fluid the only non-zero stiffness is to volumetric deformation (a fluid does not sustain shear forces).

Hence the speed of sound in a fluid is given by:c_{mathrm{fluid = sqrt {frac{K}{ ho

where:"K" is the bulk modulus of the fluid

Water

The speed of sound in water is of interest to anyone using underwater sound as a tool, whether in a laboratory, a lake or the ocean. Examples are sonar, acoustic communication and acoustical oceanography. See [http://www.dosits.org/ Discovery of Sound in the Sea] for other examples of the uses of sound in the ocean(by both man and other animals). In fresh water, sound travels at about 1497 m/s at 25 °C. See [http://www.npl.co.uk/acoustics/techguides/soundpurewater/ Technical Guides - Speed of Sound in Pure Water] for an online calculator.

eawater

In salt water that is free of air bubbles or suspended sediment, sound travels at about 1500 m/s. The speed of sound in seawater depends on pressure (hence depth), temperature (a change of 1 °C ~ 4 m/s), and salinity (achange of 1‰ ~ 1 m/s), and empirical equations have been derived to accurately calculate sound speed from these variables. [ [http://handle.dtic.mil/100.2/ADB199453 APL-UW TR 9407 High-Frequency Ocean Environmental Acoustic Models Handbook] , pp. I1-I2.] Other factors affecting sound speed are minor. For more information see Dushaw et al.

A simple empirical equation for the speed of sound in sea water with reasonable accuracy for the world's oceans is due to Mackenzie: [cite journal
author = Mackenzie, Kenneth V.
title = Discussion of sea-water sound-speed determinations
year = 1981
journal = Journal of the Acoustical Society of America
volume = 70
issue = 3
pages = 801-806
doi = 10.1121/1.386919
] :"c"("T", "S", "z") = "a"1 + "a"2"T" + "a"3"T"2 + "a"4"T"3 + "a"5("S" - 35) + "a"6"z" + "a"7"z"2 + "a"8"T"("S" - 35) + "a"9"Tz"3where "T", "S", and "z" are temperature in degrees Celsius, salinity in parts per thousand and depth in metres, respectively. The constants "a"1, "a"2, ..., "a"9 are::"a"1 = 1448.96, "a"2 = 4.591, "a"3 = -5.304×10-2, "a"4 = 2.374×10-4, "a"5 = 1.340, "a"6 = 1.630×10-2, "a"7 = 1.675×10-7, "a"8 = -1.025×10-2, "a"9 = -7.139×10-13with check value 1550.744 m/s for "T"=25 °C, "S"=35‰, "z"=1000 m. This equation has a standard error of 0.070 m/s for salinities between 25 and 40 ppt. See [http://www.npl.co.uk/acoustics/techguides/soundseawater/ Technical Guides - Speed of Sound in Sea-Water] for an online calculator.

Other equations for sound speed in sea water are accurate over a wide range of conditions, but are far more complicated, e.g., that by V. A. Del Grosso [cite journal
author = Del Grosso, V. A.
title = New equation for speed of sound in natural waters (with comparisons to other equations)
year = 1974
journal = Journal of the Acoustical Society of America
volume = 56
issue = 4
pages = 1084-1091
doi = 10.1121/1.1903388
] and the Chen-Millero-Li Equation. [cite journal
last = Meinen | first = Christopher S.
coauthors = Watts, D. Randolph
title = Further evidence that the sound-speed algorithm of Del Grosso is more accurate than that of Chen and Millero
year = 1997
journal = Journal of the Acoustical Society of America
volume = 102
issue = 4
pages = 2058-2062
doi = 10.1121/1.419655
] cite journal
last = Dushaw |first = Brian D.
coauthors = Worcester, P.F.; Cornuelle, B.D.; and Howe, B.M.
title = On equations for the speed of sound in seawater
year = 1993
journal = Journal of the Acoustical Society of America
volume = 93
issue = 1
pages = 255-275
doi = 10.1121/1.405660
]

peed in plasma

The speed of sound in a plasma for the common case that the electrons are hotter than the ions (but not too much hotter) is given by the formula (see here):c_s = (gamma ZkT_e/m_i)^{1/2} = 9.79 imes10^5,(gamma ZT_e/mu)^{1/2},mbox{cm/s}In contrast to a gas, the pressure and the density are provided by separate species, the pressure by the electrons and the density by the ions. The two are coupled through a fluctuating electric field.

Gradients

When sound spreads out evenly in all directions, the intensity drops in proportion to the inverse square of the distance. However, in the ocean there is a layer called the 'deep sound channel' or SOFAR channel which can confine sound waves at a particular depth, allowing them to travel much further. In the SOFAR channel, the speed of sound is lower than that in the layers above and below. Just as light waves will refract towards a region of higher index, sound waves will refract towards a region where their speed is reduced. The result is that sound gets confined in the layer, much the way light can be confined in a sheet of glass or optical fiber.

A similar effect occurs in the atmosphere. Project Mogul successfully used this effect to detect a nuclear explosion at a considerable distance.

References

* Applied Physics Laboratory - University of Washington, 1994

ee also

*Second sound
*Sound barrier
*SOFAR channel
*Underwater acoustics

External links

* [http://www.sengpielaudio.com/calculator-speedsound.htm Calculation: Speed of sound in air and the temperature]
* [http://www.sengpielaudio.com/SpeedOfSoundPressure.pdf Speed of sound - temperature matters, not air pressure]
* [http://www.pdas.com/atmos.htm Properties Of The U.S. Standard Atmosphere 1976]
* [http://www.mathpages.com/home/kmath109/kmath109.htm The Speed of Sound] at MathPages
* [http://www.bustertests.co.uk/answer/how-to-measure-the-speed-of-sound-in-a-laboratory/ How to measure the speed of sound in a laboratory]
* [http://twt.mpei.ac.ru/MAS/Worksheets/wspWPT.mcd Speed of sound in water and water steam as function of pressure & temperature]
* [http://www.acoustics.salford.ac.uk/schools/index1.htm Teaching resource for 14-16yrs on sound including speed of sound]
* [http://www.npl.co.uk/acoustics/techguides/soundpurewater/ Technical Guides - Speed of Sound in Pure Water]
* [http://www.npl.co.uk/acoustics/techguides/soundseawater/ Technical Guides - Speed of Sound in Sea-Water]
* [http://www.newbyte.co.il/calc.html NewByte standard atmosphere calculator and speed converter]
* [http://space.newscientist.com/article/mg19826504.200-did-sound-once-travel-at-light-speed.html?feedId=online-news_rss20 Did sound once travel at light speed?] - If the speed of sound was greater just after the big bang, it could solve a longstanding mystery over the universe's background temperature ("New Scientist", 9 April 2008)


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  • speed of sound — ▪ physics       speed at which sound waves propagate through different materials. In particular, for dry air at a temperature of 0 °C (32 °F), the modern value for the speed of sound is 331.29 metres (1,086.9 feet) per second. The speed of sound… …   Universalium

  • speed of sound — Synonyms and related words: Mach, Mach number, Mach one, Mach two, air speed, escape velocity, ground speed, knots, shock wave, sonic barrier, sonic boom, sonic speed, sonics, sound barrier, subsonic speed, subsonics, supersonic speed,… …   Moby Thesaurus

  • speed of sound — The speed at which sound travels in a given medium under specified conditions. The speed of sound at sea level in the International Standard Atmosphere is 1108 ft/second, 658 knots, 1215 km/hour. See also hypersonic; sonic; subsonic; supersonic;… …   Military dictionary

  • speed of sound — garso greitis statusas T sritis Standartizacija ir metrologija apibrėžtis Fazinis garso bangų sklidimo tampriojoje terpėje greitis: c = ω/k = λf; čia ω – kampinis dažnis, k – kampinis bangos skaičius, λ – bangos ilgis, f – virpesių dažnis.… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

  • Speed Of Sound —    In dry air, at 0° C (32° F), sound travels at 331.6 meters per second (1,088 feet per second). As the temperature increases, so does the speed of sound. See Mach, Hypersonic, Transonic and Subsonic …   The writer's dictionary of science fiction, fantasy, horror and mythology


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