Sound

The speed of sound

Although sound travels quite fast, it is still possible to calculate its speed in air. To do this, the time it takes a sound to travel a measured distance must be recorded. Either the distance must be large (preferably over 50 metres) or the time recorded on an electronic timer (to eliminate errors due to reaction time).

You should be able to describe a method of measuring the speed of sound in air. You may have used a method known as the clap-echo method in the playground or you may have used a computer-timed method with two sound-activated switches.

Calculating the speed of sound

Look at the image below to see how the speed of sound is calculated.

A ringing bell emits sound waves from left to right, towards two spaced microphones. The distance between them is labelled 'Measured distance (d)' and they are connected to a laptop computer with '0.010 Sec' on the screen, next to the formula: 'speed of sound = measured distance/time on computer' Measuring speed of sound with a bell and two microphones

Speed of sound  = \frac{{measured\,distance}}{{time\,on\,computer}}

Question

Using the apparatus shown above and with the microphones placed 3.4 metres apart, the computer recorded a time of 0.01 seconds.

Calculate the speed of sound that these results give.

Speed of sound = \frac{{measured\,distance}}{{time\,on\,computer}}

= \frac{{3.4}}{{0.01}} = 340\, m\, s^{-1}

The speed of sound is different for different materials.

Sound travels fastest through solids and slowest through gases such as air.

In assessments, the speed of sound through any material will be given in the question, if required.