Beats - A throbbing variation in the loudness of sound caused by interference when two tones of slightly different frequencies are sounded together.
Compression - (a) In mechanics, the act of squeezing material and reducing its volume. (b) 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.
Pitch - Term that refers to how high or low sound frequencies appear to be.
Rarefaction - A disturbance in air (or matter) in which the pressure is lowered. Opposite of compression.
Resonance - A phenomenon that occurs when the frequency of forced vibrations on an object matches the objects natural frequency, and a dramatic increase in amplitude results.
Ultrasonic - Term applied to sound frequencies above 20,000 hertz, the normal upper limit of human hearing.