Speed of sound
The speed of sound c (from Latin celeritas, "velocity") varies depending on the medium through which the sound waves pass. It is usually quoted in describing properties of substances (e.g. see the article on sodium). In conventional use and in scientific literature sound velocity v is the same as sound speed c. Sound velocity c or velocity of sound should not be confused with sound particle velocity v, which is the velocity of the individual particles.
More commonly the term refers to the speed of sound in air. The speed varies depending on atmospheric conditions; the most important factor is the temperature. The humidity has very little effect on the speed of sound, while the static sound pressure (air pressure) has none. Sound travels slower with an increased altitude (elevation if you are on solid earth), primarily as a result of temperature and humidity changes. An approximate speed (in metres per second) can be calculated from:
Basic concept
One can understand the transmission of sound using a simple toy model of materials, one that consists of a number of atoms or molecules represented by balls, and the bonds between them as springs. Sound compresses and expands the springs, transmitting that energy to the balls around it. Effects like dispersion and reflection can be understood simply under this model.
The speed of sound in this model is effected primarily by two factors, the number of balls that need to be moved, and strength of the springs. If there are more balls to move the sound will travel more slowly. Stronger springs, on the other hand, will speed the transmission up.
In a real material, the former measure is referred to as density, and the later a modulus. All other things being equal, sound will travel slower in denser materials, and faster in "springier" ones. For instance, sound will travel faster in aluminium 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 aluminium than hydrogen, as the internal bonds in aluminium are much stronger. Generally solids will have a higher speed of sound than liquids or gasses.
Note that many textbooks claim that the speed of sound increases with increasing density. They typically present only three data points to refer to, steel, water and air. Given only these three examples it indeed appears that speed is related to density, yet including only a few more demonstrates this to be incorrect. This is an extremely misleading claim, yet is commonly taught in grade school texts.
Experimental 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) 2. The time delay between the signal 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 obsever 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 ) 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
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