Reflection of sound waves

Echoes

Sound waves can reflect off surfaces.

We hear reflected sound waves as echoes.

Hard, smooth surfaces are particularly good at reflecting sound.

This is why empty rooms produce lots of echoes.

Soft, rough surfaces are good at absorbing sound.

This is why rooms with carpets and curtains do not usually produce lots of echoes.

Sound travels at a constant speed in a medium.

If we know the speed of sound and the time it takes for the echo to be detected, we can use the equation:

Speed = \frac{\text{distance}}{\text{time}} to work out distances.

Or

Distance = speed x time.

curriculum-key-fact
Remember in the time taken to hear an echo the sound has travelled to the object and back again. To calculate the distance to the object use half of the time (or calculate half the total distance there and back)

Example

A fishing boat sounds its foghorn.

The echo from a nearby cliff is heard after 5 s.

If the speed of sound is 340 m/s calculate the distance between the fishing boat and the cliff.

Answer

Distance = speed x time

Speed = 340 m/s

Time for sound to travel to the cliff and back again = 5 s

time for sound to travel to the cliff = 5\div 2 = 2.5 s

Distance to the cliff = 340 m/s x 2.5 s

Distance to the cliff = 850 m

The distance between the fishing boat and the cliff is 850 m.