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 - 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 ,one energized, will vibrate.
pitch - term that refers to how high or low 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 object's 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.