When two sounds with a slightly different frequency are combined,
the listener will have the impression that there is a single sound,
but its loudness will pulsate as the contributing sounds are alternately
in phase and out of phase. For example, consider the combination of
a sound with a frequency of 441 Hz and a sound with a frequency of
Figure 1. Two sounds with slightly different frequencies
The sounds begin together and then interfere constructively. After
one-half second has elapsed, however, the first sound has vibrated
220.5 times, while the second sound has only vibrated 220 times. The
two sounds are a half-cycle apart, or 180 out of phase. At this point,
they interfere destructively, and the combined loudness is diminished.
By the end of one full second, however, both sounds have completed
full cycles—the 441st cycle for the first sound, the 440th cycle for
the second sound. The two sounds are in phase again, and their combined
loudness is greater. This alternating pattern of constructive and
destructive interference will continue as long as the two sounds are
combined. Once each second the loudness will rise, and then it will
diminish. If the difference between the two frequencies is 2 cycles
per second, then the loudness will rise and diminish twice each second.
This pulsation of loudness, produced by the combination of two sounds
of nearly the same frequency, is called BEATING.
Figure 2. Combined result of two sounds with slightly different frequencies
from Barry Truax's Handbook for Acoustic Ecology.