Question

In a room two walls are at a distance of $159 m$. Then:
A. No echo will be heard
B. An echo can be heard
С. No sound will be heard
D. None of these are true

Answer 1

Hint: The essential conditions for the formation of an echo are: the minimum distance between the source of the sound and the obstacle should be 17.2 metres, the obstacle or the reflecting surface should be rigid such as building, hill or a cliff.

Complete answer:
For the formation of echo the following conditions are necessary:
Time gap between the original and the reflected sound: In order to hear a distinct echo the reflected sound must reach our ear 0.1 second after the original sound i.e. there should be sufficient time gap between in the and reflected and reflected sound for an echo to be perceived by our ear.

Distance between the source of sound and the obstacle: There should be a minimum distance of 17.2 metres between the source producing the sound and the reflecting surface.

Nature of the reflecting surface: The obstacle from which the sound gets reflected should be rigid such as a building, hill or a cliff.

Size of the obstacle: The size of the object should be large to hear the echo.

Assuming all the other conditions are fulfilled since the given distance is $159m$ which is much greater than $17.2m$, an echo can be heard.

Hence, option B is the correct answer.

Additional information:
Echoes are used by bats, dolphins and fisherman to detect an object / obstruction.They are also used in SONAR (Sound navigation and ranging) and RADAR(Radio detection and ranging) to detect an obstacle. SONAR employs the detection of reflections of ultrasonic waves for the detection of an object whereas RADAR employs the detection of reflections of radio waves for the detection of an object. Echoes also find applications in medical science like ultrasonography and echocardiography for imaging of human organs.

Note: An echo takes at least $0.1$ second to be heard. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least $(344 m/s)\times 0.1 s = 34.4 m$. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, $17.2 m$.