The Physics of Sound

A wave is described by several distinct properties. Frequency is the number of cycles per second, measured in hertz, and is perceived as pitch; human hearing covers roughly 20 Hz to 20 kHz. Wavelength equals the speed of sound divided by the frequency: a 20 Hz tone is about 17 m long, 1 kHz about 34 cm and 20 kHz about 17 mm. These dimensions explain why low frequencies bend around obstacles and are hard to direct, while high frequencies are strongly directional and are absorbed by air and soft surfaces.

Amplitude is the size of the pressure variation and is perceived as loudness. Phase is the timing relationship between two waves. Two copies of the same sound that arrive together reinforce each other, and when one is delayed by half a period they cancel. A reflection that arrives shortly after the direct sound therefore produces comb filtering, a series of notches in the frequency response. The first notch falls at the frequency whose half period equals the delay, that is one divided by twice the delay. A 1 ms reflection creates a notch near 500 Hz and further notches at every odd multiple of that frequency. The notches move with the listening position, so equalization cannot remove them.