Sound is a longitudinal wave that is transmitted through the air and detected by our ears. Good human ears can detect sound from about 50 Hz to about 20,000 Hz (20 kHz); this can be called the audible range of frequencies. As people grow older, we loose our ability to detect high frequencies so the highest frequencies that a middle-aged person is able to detect may be about 15,000Hz or even 12,000Hz.
Sound with frequencies below about 50 Hz may be felt by our bodies rather than heard by our ears. Such very low frequencies are called infrasonic.
Sound with frequencies above our hearing sensitivity are known as ultrasonic.
The lowest frequency (and the longest wavelength) is known as the fundamental frequency. Remember,
so restrictions on wavelengths are also restrictions on frequencies. The pitch of a sound is determined by this fundamental frequency, the lowest frequency that is present. Which additional harmonics are present and their amplitudes will determine the quality or timbre of the sound--what the instrument sounds like.
"Beats" refers to the increasing and decreasing volume or amplitude that is heard when two sounds of slightly different frequencies are heard together. The beat frequency is the difference in the two frequencies that are being combined.
Sound from a moving source sounds different than from a stationary source. If the source is moving toward you, the sound will be higher in pitch. If the source is moving away from you, the sound will be lower in pitch. This is known as the Doppler effect.
A "sonic boom" or shock wave of increased air pressure is produced when an object travels faster than the speed of sound. Such speeds are know as supersonic.
(C) 2003, Doug Davis; all rights reserved