Beats: A throbbing variation in the loudest of sounds 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, 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 the frequency. Also called resonant frequency.
Pitch: Term that refers to how high or low sound frequencies appears to be.
Rarefaction: A disturbance in air 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