Question

An echo will be heard if the minimum distance between the source and the obstacle is
1. 1 m
2. 10 m
3. 15 m
4. 17 m

Answer 1

Correct Answer - Option 4 : 17 m

The correct answer is option 4) i.e. 17 m

CONCEPT:

• Echo: The sound heard after it gets reflected from a rigid surface is called an echo.
  • Echo is received by the listener with a delay after the original sound is produced.
  • This delay occurs as the original sound persists in our ear for 0.1 s.

​​EXPLANATION:

• The original sound persists in our ear for 0.1 s. So to hear an echo, the reflected sound must be reaching our ears at least after 0.1 s.
• Assuming the speed of sound in air as 340 m/s, we can calculate the distance travelled by the sound

⇒ D = speed × time = 340 x 0.1 = 34 m.

• This 34 m is the combined to and fro distance from the source to the reflected surface.

So, one way distance = 34/2 = 17 m.

• Hence, 17 m is the minimum distance required between the reflecting surface and source to hear an echo.