Physics > Speed of Sound

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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