An airplane traveling at one third the speed of sound (i. e., 114 m/s) emits a sound of frequency 3.72 khz. at what frequency does a stationary listener hear the sound as the plane approaches? answer in units of khz.

f'=5.58kHz

Explanation:

This is an example of the Doppler effect, the formula is:

Where f is the actual frequency, is the observed frequency, is the velocity of the sound waves, the velocity of the observer (which is negative if the observer is moving away from the source)  and the velocity of the source  (which is negative if is moving towards the observer). For this problem: