Chapter : 5. Sound
Minimum distance between the observer and the obstacle for echo to be heard
ECHO
The sound returning back towards the source after suffering reflection from a distance obstacle (a wall, a row of building etc.) is called an echo.
When the sound is reflected repeatedly from a number of obstacles, more than one echoes, called multiple echoes are heard. Multiple echoes may be heard one after the other when sound gets repeatedly reflected from distant high rise buildings or hills. The rolling of thunder is an example of multiple echo formation.
The two sounds–one direct and the other echo, can be heard distinctly provided the distance between the observer and the reflecting surface is large enough to allow the reflected sound to reach him without interfering with the direct sound. Since the sensation of sound persists for 1/10 second after it is produced, the echo can be heard distinctly only if it reaches at least 1/10 second after the original sound is produced.
Minimum distance between the observer and the obstacle for echo to be heard :
Let Distance between the observer and the obstacle = d
Speed of sound (in the medium) = v
Time after which echo is heard = t
Then, t = 2d/v or d = vt/2
We know, Speed of sound in air at 25ºC = 343 ms–1
For an echo to be heard distinctly,
t ≥ 0.1 s
Then d ≥
Thus, the minimum distance (in air at 25ºC) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.
The speed of sound increases with a rise in temperature. Therefore, the minimum distance in air between the observer and the obstacle for an echo to be heard clearly at temperatures higher than 25ºC is more than 17.2 m. In rooms having walls less than 17.2 m away from each other, no echo can be heard.
The sound returning back towards the source after suffering reflection from a distance obstacle (a wall, a row of building etc.) is called an echo.
When the sound is reflected repeatedly from a number of obstacles, more than one echoes, called multiple echoes are heard. Multiple echoes may be heard one after the other when sound gets repeatedly reflected from distant high rise buildings or hills. The rolling of thunder is an example of multiple echo formation.
The two sounds–one direct and the other echo, can be heard distinctly provided the distance between the observer and the reflecting surface is large enough to allow the reflected sound to reach him without interfering with the direct sound. Since the sensation of sound persists for 1/10 second after it is produced, the echo can be heard distinctly only if it reaches at least 1/10 second after the original sound is produced.
Minimum distance between the observer and the obstacle for echo to be heard :
Let Distance between the observer and the obstacle = d
Speed of sound (in the medium) = v
Time after which echo is heard = t
Then, t = 2d/v or d = vt/2
We know, Speed of sound in air at 25ºC = 343 ms–1
For an echo to be heard distinctly,
t ≥ 0.1 s
Then d ≥
Thus, the minimum distance (in air at 25ºC) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.
The speed of sound increases with a rise in temperature. Therefore, the minimum distance in air between the observer and the obstacle for an echo to be heard clearly at temperatures higher than 25ºC is more than 17.2 m. In rooms having walls less than 17.2 m away from each other, no echo can be heard.
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