Editing Speed of sound (section)

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

The earliest reasonably accurate estimate of the speed of sound in air was made by [[William Derham]] and acknowledged by [[Isaac Newton]]. Derham had a telescope at the top of the tower of the [[Church of St Laurence, Upminster|Church of St Laurence]] in [[Upminster]], England. On a calm day, a synchronized pocket watch would be given to an assistant who would fire a shotgun at a pre-determined time from a conspicuous point some miles away, across the countryside. This could be confirmed by telescope. He then measured the interval between seeing gunsmoke and arrival of the sound using a half-second pendulum. The distance from where the gun was fired was found by triangulation, and simple division (distance/time) provided velocity. Lastly, by making many observations, using a range of different distances, the inaccuracy of the half-second pendulum could be averaged out, giving his final estimate of the speed of sound. Modern stopwatches enable this method to be used today over distances as short as 200–400 metres, and not needing something as loud as a shotgun.

===Single-shot timing methods===
The simplest concept is the measurement made using two [[microphone]]s and a fast recording device such as a [[Digital data|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:
# The distance between the microphones ({{mvar|x}}), called microphone basis.
# The time of arrival between the signals (delay) reaching the different microphones ({{mvar|t}}).

Then {{math|1=''v'' = ''x''/''t''}}.

===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In these methods, the [[time]] measurement has been replaced by a measurement of the inverse of time ([[frequency]]).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 [[Node (physics)|nodes]] and [[antinode]]s 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 (material)|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 {{math|(1 + 2''n'')''λ''/4}} where ''n'' is an integer. As the [[antinode|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''.

===High-precision measurements in air===
The effect of impurities can be significant when making high-precision measurements. Chemical [[desiccant]]s can be used to dry the air, but will, in turn, contaminate the sample. The air can be dried cryogenically, but this has the effect of removing the carbon dioxide as well; therefore many high-precision measurements are performed with air free of carbon dioxide rather than with natural air. A 2002 review<ref>Zuckerwar, Handbook of the speed of sound in real gases, p. 52</ref> found that a 1963 measurement by Smith and Harlow using a cylindrical resonator gave "the most probable value of the standard speed of sound to date." The experiment was done with air from which the carbon dioxide had been removed, but the result was then corrected for this effect so as to be applicable to real air. The experiments were done at {{val|30|u=degC}} but corrected for temperature in order to report them at {{val|0|u=degC}}. The result was {{nobreak|331.45 ± 0.01 m/s}} for dry air at STP, for frequencies from {{val|93|u=Hz}} to {{nobreak|1,500 Hz}}.