Non-periodic vibrations that are largely random and do not convey
a sense of definite pitch are called NOISE. Also, random frequency
WHITE NOISE has an equal distribution of random energy per
band within the range of frequencies audible to the human ear. For
example, there is the same amount of energy between 100 and 200 Hz
as there is between 6100 and 6200 Hz or between 15,700 and 15,800
Hz. This is the sound of tape hiss, or the sound heard on a radio
or TV channel after the station assigned to that channel has left
the air (the random "snow" seen on the television screen can be thought
of as the visual analog of the white noise being heard.
PINK NOISE has an equal distribution of random energy per
octave within the range of audible frequencies. For example, there
is the same amount of energy between 100 and 200 Hz as there is between
200 and 400 Hz, 400 and 800 Hz, 800 and 1600 Hz, 1600 and 3200 Hz,
3200 and 6400 Hz, and 6400 and 12,800 Hz.
Relative to WHITE NOISE, the energy distribution of PINK
NOISE is more sparse in the higher frequency ranges and more concentrated
in the lower frequencies ( just as white light includes an even distribution
of frequencies of visible light, whereas red light consists primarily
of the lower frequencies of visible light). Examples of PINK NOISE
include the sounds of ocean waves crashing ashore, of distant thunder,
and of bass drums.
from Barry Truax's Handbook for Acoustic Ecology.