Question

Repetitions of sound due to reflection of original sound heard by a large and hard obstacle is known as
A) Compression
B) Rarefaction
C) Persistence of hearing
D) Echo

Answer 1

Hint: Sound bounces back when it falls on a hard surface and reflected sounds obey the same laws of reflection as it is followed by light. Hard solid surfaces are the best reflecting surfaces for sound. Hard surfaces include walls, metal sheet, cliff etc.

Complete step-by-step solution:
The repetition of original sound which is heard after the sound is reflected from a distant, dense and rigid object like high building, cliff etc. is called an echo.
To hear an echo, there must be a time gap of 0.1 second in original sound and the reflected sound. If this time gap is not present then echo should not be heard.
An echo is a single reflection of a sound wave from a surface. Reverberation is the sound created by superposition of echoes.
Necessary Conditions for the formation of an Echo
1. Time gap between the original sound and reflected sound must be 0.1 second. This time is calculated at \[{{20}^{0}}C\] temperature when speed of sound is\[344\dfrac{m}{s}\].
2. Minimum distance between the source of sound and reflector sound is at least 17.2m.
3. The reflecting surface should be hard enough.
Compression and rarefaction is obtained when sound waves travel in a medium, they are produced in an alternate manner, it does not have any concern with the repetitions of sound after reflection from hard surfaces. So these options are invalid.
So it is concluded that Repetitions of sound due to reflection of original sound heard by a large and hard obstacle is known as echo.
So, the correct option is D.

Note: We should note that the distance for distant echo to be heard at \[{{20}^{0}}C\] is 17.2 m. On a hot summer day the temperature increases and so speed of sound also increases, so the minimum distance required for the echo to be heard will also increase. Hence no echo is heard on a hot day because the condition of echo is not satisfied.