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Sound: Echo Calculations

An 'echo' is a sound reflection.

Sound waves are reflected according to the law of reflection - like light:

angle of incidence = angle of reflection

But whereas a light beam 'spreads out' very little, sound 'spreads out' quite a lot - and we therefore do not think of it as being 'directed' in a path from a speaker, we consider it to spread out everywhere!

The vidclip below shows us that sound is reflected just like light.

Two curved reflectors are used. One person stads at the focus of the one dish and speaks into it. The spreading sound waves are reflected from the dish. A parallel beam of sound waves projects across to the opposite dish. That second dish reflects the sound waves and focuses them at the focal point of the second reflector, and the sound can then be heard clearly.

 

Sonar

Gets its name from "sound navigation ranging"), is a technique for detecting and determining the distance and direction of underwater objects by acoustic means. Sound waves emitted by the object, or reflected from it (as an echo), are detected by sonar apparatus and calculations can be used to determine the position of the object.

Animals also use echos to locate objects by their own inbuilt sonar systems.

In exams you might be asked to perform echo calculations:



Question: If the time between the horn sounding and the people on the ship hearing the echo from the ship is 4 seconds, how far away is the ship from the cliff ?
 

speed = distance travelled by the sound
time taken

distance travelled by the sound = speed x time taken = 330 x 4 = 1320 m

this is 'distance from ship to cliff' + 'distance from cliff to ship'

\ distance between ship and cliff is 1320/2 = 660 m