An echo is heard when minimum distance of the reflecting surface is
An echo is heard when minimum distance of the reflecting surface is Correct Answer 17 m
CONCEPT:
- Persistence of Hearing: The human ear can identify two different sounds if the time interval between them is less than 0.1 second.
- Reflection of sound: The sound reflects back when it strikes any surface and can be heard to us again at certain conditions.
- Echo: When we hear the reflection of our sound after a certain interval of time, it is called echo.
- For echo to happen, the time interval between we speak, and the reflected sound reaches us should be less than 0.1 seconds. Only then our ear can distinguish between two sounds.
- Speed of sound in air is approximately equal to 340 m /s at standard room temperature.
CALCULATION:
Speed of Sound v = 340 m/s
The time interval between a human ear and the reflecting surface should be at least equal to the time interval of the persistence of hearing.
t = 0.1 s
Let the minimum distance of reflecting surface be d
distance travelled by sound x = distance travelled in reaching the surface + distance travelled by reflected sound
x = d + d = 2d
Now, we know that distance travelled = speed × time
⇒ 2d = v × t
⇒ 2d = 340 m/s × 0.1s = 34 m
⇒ d = 34 m /2 = 17 m
So, the minimum distance should be at least 17 m.
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