16. White noise, pink noise...orange noise?
This page is mostly for fun, so don't take it
too seriously!
You've certainly heard of 'white noise', which
is calqued
into Chinese as ¥Õ¦â¾¸µ bai2se4 zao4yin1. It is not hard to understand why
this term was chosen. White noise contains an equal distribution of energy spread
over all frequencies, just as white light is composed of all of the colors of
the spectrum of visible light; here
is a more detailed explanation. Click here
for an audio sample.
You hear white noise when no signal is being
broadcast over a television channel or radio station. It's not just an annoying,
unpleasant sound – some people use it to sleep better, e.g. by running
a fan during the night. This works because steady sound at all frequencies can
mask all kinds of other noises that might disturb a light sleeper. There are
even commercial
products that emit white noise to block out the snoring noises of
one's partner, to help an infant sleep more soundly, or to drown out a conversation
you want to keep private! White noise can also be used to determine the frequency
response of mechanical equipment.
The analogy with colors of light does not stop
with white noise. Another very commonly used kind of noise is called 'pink
noise'. Instead of having equal energy at all frequencies, with pink
noise the energy within each successive octave is the same, but it is
reduced by 3 dB per octave, i.e. each octave has half the
power of the preceding one. This is analogous to light that tends toward the
red or lower end of the visible light spectrum. Pink noise gives more
weight to the lower frequencies to compensate for the increased number of frequencies
of each higher octave. Also, human hearing is most sensitive to sounds in the
1-4kHz range, so pink noise makes the higher frequencies less prominent and
less hard on the ear. Some
claim that pink noise is the most common kind of noise found in nature,
e.g. the churning of ocean waves. Pink noise is also often used for testing
the frequency response of audio equipment. You can produce white noise with
a program to generate random numbers, and you can turn white noise into pink
noise by filtering it – though this is more difficult to do than it sounds!
A less commonly referred to 'color' of noise is
'brown
noise'. This is supposed to simulate Brownian motion, a kind of random
motion that shifts in steady increments. 'Brown noise' decreases in power by
6 dB per octave. Listen to a sample here.
The reverse of 'brown noise' is 'violet noise' (or 'purple noise'), in which
the power of each octave increases by 6 dB.
There
are still other 'colors' of noise, but they are mostly curiosities, good for
a little synaesthetic
fun, rather than being very practical concepts. Here is a collection of a whole
rainbow
of noise types:
http://www.ptpart.co.uk/show.php?contentid=71
More
on the connection between the visual and auditory senses: The "Forry,
Wrong Number" unit of "A
Moment of Science" mentioned that, in addition to frequency
cut-offs, a further reason why it is difficult to hear some words clearly over
the phone is the lack of visual cues we get from watching the face of
the person talking. How much of a difference do these really make in our understanding
of speech? Go on to the next page, and you may be surprised to see just how
much difference they can make!
Next: The
McGurk Effect (with
videos)