Question

Explain how the speed of sound can be determined by the method of echo.

Answer 1

The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off. As the speed of sound is 340 m/s, the distance travelled by sound in 0.1 second is 34 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly. Hence, by measuring the time taken for the observer to hear the echo and the distance between the observer and the obstacle can be used to determine the speed of sound. The relationship between speed, time and distance is given as, $Speed=\frac{Distance}{Time}$