### Re: How can you measure how fast sound travels?

Date: Tue Feb 27 10:09:11 2001
Posted By: Bruno Putzeys, Staff, Electroacoustics and Analog Electronics, Philips ITCL
Area of science: Physics
ID: 982959679.Ph
Message:
```
Hi!

It seems I'm getting all the "speed of sound" questions. Below you find
measuring speed of sound in air, the "Kundt tube":

For science classes this experiment is the easiest to perform and the most
instructive. You will find that the Kundt tube experiment has two
disadvantages namely that it is only practical in air and that it measures
the speed of sound inside a tube, which is not necessarily the same as the
speed of sound in free air (though the difference is admittedly small).

The latter is actually a basic problem in determining the speed of sound
in general. It is not only dependent upon the medium (and the state it is
in) but also on the shape of the medium. This is particularly tricky when
the wavelength is of the same order as the smallest dimension of the
medium. Conversely, the speed of sound is wavelength dependent.

Something like the kundt tube experiment can be used to determine the
speed of transversal waves in solids. Such waves are found on metal plates
which are made to resonate. For example, ever tried playing a cymbal using
a violin bow? If you put some dry sand on the cymbal the standing waves
generating the shrieking sounds will be visualised by the sand settling
down on the nodes.

For longitudinal waves in liquids and solids it is not easy to visualise
standing wave patterns. Some indirect way of detection must be used
instead. A bar of iron can be struck with a hammer on one end and the
ringing requency measured. For liquids I can't really think of an easy way.

Probably the most generally applicable method of measuring the speed of
sound is the direct method: apply a pulse (a very short burst of energy)
and measure at some distance of the source how much time it took the
impulse to make its way there. To eliminate measurement errors caused by
the unknown delay in the source you might use two identical sound
transducers (for gases: microphones, for liquids: hydrophones, for solids:
accelerometers) spaced a known distance apart and measure the time
difference between those two. It be said that such a setup is quite
specialised and expensive.

Conclusion: Practical experiments not involving precision measurement
equipment rely on creating and detecting standing waves in the medium to
be measured. From there, the speed of sound can be calculated.

Regards,

Bruno

```

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