Directly below, on the floor, is a source of sound that is emitting at f=440 Hz.

A microphone is suspended from a spring attached to the ceiling. Directly below, on the floor, is a source of sound that is emitting at f=440 Hz. The microphone vibrates vertically in simple harmonic motion (SHM) with a period of 2.0 seconds. The difference between the maximum and minimum sound frequencies detected by the microphone is 2.1 Hz.