Let’s say we can hear a beat frequency of 2Hz from two tuning forks. The first tuning fork has a frequency of 100Hz. If wax is applied on the 2nd tuning fork, the beat frequency becomes 4Hz, what is the frequency of the 2nd tuning fork?

A wave not only travels in space, but it also propagates through time, so if the two waves can produce interference by overlapping in space, they should also produce an interference pattern when they overlap in time; this phenomenon is called beats.  A beat pattern is characterized by a wave whose amplitude is changing at a regular rate.