Echo

Echo is formed when a sound is reflected by large, hard and flat surfaces like buildings, walls and cliffs. It is a distinct, reflected sound wave from a surface. Sometimes, we will hear many echoes when there are many reflecting surfaces.

Note that reflection of sound occurs just like the reflection of light.

Application of Echo
  • Find the depth of a sea 
  • Locate the shoals of fish

Echo Calculation

(1) If the total time taken includes time taken for returning echo, 
  • total time taken needs to divide by two or 
  • total distance traveled by sound is twice the distance between the sound source and reflecting surface due to its echo. 
Formula:
(2) However, if the total time taken does not include time taken for returning echo, then use normal formula to calculate speed.

Example:

A fishing boat uses a sonar to detect a shoal of fish.  A short pulse of sound waves is emitted from the boat, and the echo from the shoal is detected 0.1 s later.  The waves travel through sea water at 1500 m/s. How far below the boat is the shoal of fish?

Answer:

Speed of sound = distance⁄(time taken/2)
                  1500 = distance/(0.1 / 2)
            distance = 75.0 m

Excel@Physics