# Speed of sound

subsonicsound speedsound velocityMach 1sonic velocitysound velocitiessubsonic speedMachsoundvelocity of sound
The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium.wikipedia
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### Supersonic speed

supersonicsupersonic flightMach 2
Objects moving at speeds greater than are said to be traveling at supersonic speeds.
Supersonic travel is a rate of travel of an object that exceeds the speed of sound (Mach 1).

### Mach number

MachMach 2Mach 1
The ratio of the speed of an object to the speed of sound in the fluid is called the object's Mach number.
In fluid dynamics, the Mach number (M or Ma) is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound.

### Sound

audiosound wavesound waves
The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium.
As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave.

### Compressibility

compressibleisothermal compressibilitycompress
The speed of compression waves in solids is determined by the medium's compressibility, shear modulus and density.
The speed of sound is defined in classical mechanics as:

### Water

H 2 OHOliquid water
For example, (as noted above), sound travels at in air; it travels at in water (almost 4.3 times as fast as in air); and at in iron (almost 15 times as fast as in air).
The viscosity of water is about 10 −3 Pa·s or 0.01 poise at 20 C, and the speed of sound in liquid water ranges between 1400 and 1540 m/s depending on temperature.

### William Derham

Derham, William
In 1709, the Reverend William Derham, Rector of Upminster, published a more accurate measure of the speed of sound, at 1,072 Parisian feet per second.
He produced the earliest, reasonably accurate measurement of the speed of sound.

### Atmosphere of Earth

airEarth's atmosphereatmosphere
In common everyday speech, speed of sound refers to the speed of sound waves in air.
Because in an ideal gas of constant composition the speed of sound depends only on temperature and not on the gas pressure or density, the speed of sound in the atmosphere with altitude takes on the form of the complicated temperature profile (see illustration to the right), and does not mirror altitudinal changes in density or pressure.

### Church of St Laurence, Upminster

St LaurenceSt Laurence's ChurchSt Laurence's parish church
Derham used a telescope from the tower of the church of St Laurence, Upminster to observe the flash of a distant shotgun being fired, and then measured the time until he heard the gunshot with a half-second pendulum.
The tower of St Laurence's was instrumental in the first accurate measurement of the speed of sound by Rev William Derham, who was also buried in the church or churchyard and who also, by his own wish, has no memorial.

### Pierre Gassendi

GassendiGassendi, PierreGASSENDI, PETER
During the 17th century there were several attempts to measure the speed of sound accurately, including attempts by Marin Mersenne in 1630 (1,380 Parisian feet per second), Pierre Gassendi in 1635 (1,473 Parisian feet per second) and Robert Boyle (1,125 Parisian feet per second).

### Longitudinal wave

longitudinalcompressional wavecompression wave
A longitudinal wave is associated with compression and decompression in the direction of travel, and is the same process in gases and liquids, with an analogous compression-type wave in solids.
Sound's propagation speed depends on the type, temperature, and composition of the medium through which it propagates.

### Temperature

temperaturesair temperaturewarm
For a given ideal gas the molecular composition is fixed, and thus the speed of sound depends only on its temperature.

### Pierre-Simon Laplace

LaplacePierre Simon LaplacePierre-Simon de Laplace
This error was later rectified by Laplace.
Laplace in 1816 was the first to point out that the speed of sound in air depends on the heat capacity ratio.

The decrease of the speed of sound with height is referred to as a negative sound speed gradient.
In acoustics, the sound speed gradient is the rate of change of the speed of sound with distance, for example with depth in the ocean,

### Heat capacity ratio

adiabatic indexspecific heat ratioratio of specific heats
For ideal gases, the bulk modulus K is simply the gas pressure multiplied by the dimensionless adiabatic index, which is about 1.4 for air under normal conditions of pressure and temperature.
The heat capacity ratio is important for its applications in thermodynamical reversible processes, especially involving ideal gases; the speed of sound depends on that factor.

### Isaac Newton

NewtonSir Isaac NewtonNewtonian
Newton famously considered the speed of sound before most of the development of thermodynamics and so incorrectly used isothermal calculations instead of adiabatic.
He also formulated an empirical law of cooling, made the first theoretical calculation of the speed of sound, and introduced the notion of a Newtonian fluid.

### Hot chocolate effect

In heterogeneous fluids, such as a liquid filled with gas bubbles, the density of the liquid and the compressibility of the gas affect the speed of sound in an additive manner, as demonstrated in the hot chocolate effect.
The phenomenon is explained by the effect of bubble density on the speed of sound in the liquid.

### North Ockendon

North Okendon
Measurements were made of gunshots from a number of local landmarks, including North Ockendon church.
Its tower was used in the first accurate measurement of the speed of sound, by the Reverend William Derham, Rector of Upminster.

### Bell X-1

Bell X1X-1Bell X-1A
The X-1, piloted by Chuck Yeager, was the first crewed airplane to exceed the speed of sound in level flight and was the first of the X-planes, a series of American experimental rocket planes (and non-rocket planes) designed for testing new technologies.

### Helium

Hehelium IIsuperfluid helium
In low molecular weight gases such as helium, sound propagates faster as compared to heavier gases such as xenon.
Because of helium's relatively low molar (atomic) mass, its thermal conductivity, specific heat, and sound speed in the gas phase are all greater than any other gas except hydrogen.

### Kundt's tube

Kundt's tube is an example of an experiment which can be used to measure the speed of sound in a small volume.
Kundt's tube is an experimental acoustical apparatus invented in 1866 by German physicist August Kundt for the measurement of the speed of sound in a gas or a solid rod.

### Bulk modulus

bulkbulk modulicompressibility
: where K is the bulk modulus of the fluid.
In a fluid, the bulk modulus K and the density ρ determine the speed of sound c (pressure waves), according to the Newton-Laplace formula

### Relativistic Euler equations

If relativistic effects are important, the speed of sound is calculated from the relativistic Euler equations.
The relativistic Euler equations may be applied to calculate the speed of sound in a fluid with a relativistic equation of state (that is, one in which the pressure is comparable with the internal energy density e, including the rest energy; where e^C is the classical internal energy per unit mass).

### SOFAR channel

deep sound channelSOFARsound fixing and ranging (SOFAR) channel
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.
The SOFAR channel (short for Sound Fixing and Ranging channel), or deep sound channel (DSC), is a horizontal layer of water in the ocean at which depth the speed of sound is at its minimum.

### Acoustoelastic effect

The acoustoelastic effect is how the sound velocities (both longitudinal and shear wave velocities) of an elastic material change if subjected to an initial static stress field.

### Sonic boom

sonic boomssupersonic boomsonic
A sonic boom is the sound associated with the shock waves created whenever an object travelling through the air travels faster than the speed of sound.