Beat Frequency
You already know about the principle of constructive and destructive interference, which applies to sound as to other waves. In essence, waves of different frequencies will occasionally match up such that their crests coincide. With two different frequencies, the rate with which the amplitude of the wave pulsates is the beat frequency and is the difference between the two frequencies, expressed with the simple equation:
One manifestation of the beat frequency is a pulsating loudness. Since this particular manifestaion only occurs when there is only slight difference in frequencies, it is useful for determining whether tuning forks are at exactly the same frequency, or for tuning instruments by adjusting one until there are no beats (and thus no difference in frequency). Below is a graphical representation of how beat frequencies work. The two slightly out of phase waves on top combine to produce the third graph by superposition.
