Question

A horn sounds a 351 Hz tone. How fast would you have to run away from it for it to sound like 279 Hz? (v=343 m/s)

Answer 1

The problem can be solved using the Doppler effect in sound.

Doppler effect defined as the apparent frequency heard by an observer either because of the motion of the source, or the observer, or both.

Here the horn is the source and you are the observer. Also, the horn is stationary (so velocity will be zero) and you are moving away from it Then according to the Doppler effect, the apparent frequency you hear in this condition is given by,

where,

Given,

From equation (1), the velocity of the observer is given by,

Therefore, the velocity of the observer is given by,

(Ans.)