Answer to Question #162244 in Optics for Ulrich

Question

An airplane mechanic notices that the sound from a twin-engine aircraft rapidly varies in loudness when both engines are running. What could be causing this vibration from loud to soft?

Answer 1

Since the two airplane engines are working at almost similar speeds, their frequencies are also almost same. However small difference in the rotational speed of their fans or the frequencies that they are shaking their surrounding air would cause the beat phenomenon.

When two waves of frequency $f_1 and f_2$ interfere, we here beat with frequency

$$f = | f_1 - f_2|$$

This phenomenon is called Acoustic Beating.