Beats: a throbbing variation in the loudness of sound caused by interference when two tones of slightly different frequencies are sounded together.
Compression: in mechanics, the act of squeezing material and reducing its volume. In sound, a pulse of compressed air ( or other matter): opposite of rarefaction.
Forced Vibration: the vibration of an object that is made to vibrate by another vibrating object that is nearby. The sounding board in a musical instrument amplifies the sound through forced vibration.
Infrasonic: term applied to sound pitch too low to be heard by the human ear, that is below 20 hertz.
Natural Frequency: a frequency at which an elastic object, once energized, will vibrate. Minimum energy is required to continue vibration at that frequency. Also called resonant frequency.