Physics > Speed of Sound

The speed of sound is a measurement of sound waves travelling through a specific medium. Under normal atmospheric conditions, the speed of sound travelling through air is 343 m/s (metres per second) or 769.5 miles per hour.

The speed of the sound waves varies depending upon the medium through which the waves travel. For example, waves travel faster through water than they do through air. Temperature of air will also change the speed of sound. When air heats up it becomes less dense which allows sound to travel faster through it.

Sound waves are essentially compression waves, otherwise known as longitudinal waves. Sound wave models can be replicated using a slinky spring

Pupils are sometimes required to measure the speed of sound using a clapper or otherwise noisy device, a stopwatch and a long tape measure of trundle wheel. The clapper is positioned at one end of the sports field (or school hall although the longer the distance between, the more accurate the measurements) and a listener with a stopclock is positioned at the other end. As the clapper is set off, the listener starts the stopclock and stops it when they can actually hear the sound reach them. Because of the dependence upon human reactions, this measurement is not particularly accurate although they should end up with a speed near what it should be.

Because we know that

Speed = distance/time

we can put into that equation the distance between the clapper and the hearer (in metres) and the time it took the sound to travel that distance (in seconds). Dividing the distance by the time taken should give us a figure not too far away from 344m/s.

More accurate measurements could be taken with a datalogger and microphone. Graphical analysis could show a peak when the noise is heard as long as the logging of time starts exactly when the clapper is set off.

Synchronised stopclocks could be another way to achieve more accuracy. For example the clapper could be set off at an exactly set time which eliminates a certain amount of human response time. The time taken for the sound to reach the hearer would then be the difference between that set time and the subsequently elapsed time.

Speed of sound in normal air is 343 m/s. In water the speed is 1,433 m/s. The table below shows the speed of sound in other familiar materials.

Medium	Velocity (m/s)	Velocity (ft/s)
Aluminum	4877	16000
Brass	3475	11400
Brick	4176	13700
Concrete	3200-3600	10500-11800
Copper	3901	12800
Cork	366-518	1200-1700
Diamond	12000	39400
Glass	3962	13000
Glass, Pyrex	5640	18500
Gold	3240	10630
Hardwood	3962	13000
Iron	5130	16830
Lead	1158	3800
Lucite	2680	8790
Rubber	40-150	130-492
Steel	6100	20000
Water	1433	4700
Wood (hard)	3960	13000
Wood	3300-3600	10820-11810