How Music Works

[Sara S]

from delancyplace.com:

In today's excerpt - we do not hear two instruments being played at the same time
as being twice as loud as one. In fact, when we hear 100 instruments, we perceive
them as being only four times as loud as a single instrument. That is due to two
things - first, unless they are absolutely identical, the pressure ripples (sound
waves) from multiple instruments partially cancel each other out, and second, our
brains operate in a manner so as to protect itself by dampening the effect of increasingly
loud noise:


"If we have two instruments (such as glockenspiels), we only get double the effect
if the up-down-up-down pressure ripples (sound waves, which have the effect of alternating
increased and decreased pressure) from them are perfectly in step with each other.
If so, they can act together to give a [perfectly synchronized] UP-DOWN-UP-DOWN
pressure ripple.
"But, when we hit both instruments, you can bet your life that we don't hit them
exactly at the same time, so the pressure ripples from the two instruments won't
be in step when they reach the microphone. This means that sometimes the 'pressure
up' part of one ripple will be trying to raise the air pressure as the 'pressure
down' part of the other is trying to lower it. if the wave patterns were perfectly
out of step, the up-down-up-down of one of them would be canceled out by the down-up-down-up
of the other - and we wouldn't hear a note at all.
"This is weird but true - it's how some farmers protect their hearing when they
are driving noisy tractors all day. They buy 'active ear defenders' which look like
headphones. Inside each of the earpieces is a microphone and a speaker connected
to some electronics. The microphone listens to the sound which is about to reach
your eardrum and makes the speaker produce the same pressure wave - but out of step
with the original one. The idea is that when the two pressure waves meet, one of
them tries to raise the pressure at the same time as the other tries to lower it
- so nothing much happens and the eardrum is left in peace. In practice the sound
waves are too complicated for this to work exactly, but it does reduce most of the
noise.
"Going back to our glockenspiels, the canceling out is nowhere near perfect because
it would be too difficult to organize - the sound waves are coming from different
places in the room and also bouncing off the walls, and it's incredibly unlikely
that you would hit the instruments at precisely the right times to get the ripple
patterns exactly out of step just at the point where they meet the microphone. What
actually happens is that we do get more sound pressure from two instruments than
we would from one - but there is some interference from the low-pressure bits of
one wave pattern with the high-pressure bits of the other, so there is some canceling
out.
"If more instruments are involved, the amount of canceling out gets more serious.
The pressure of the air next to the microphone can only be higher than normal (pushing
the microphone inward) or lower than normal (pulling it outward): it can't be both
at once. If we play forty glockenspiels, each of our forty glockenspiels has an
'up pressure' or 'down pressure' vote at any point in time -but a lot of these votes
cancel each other out. If a forty-first glockenspielist joins our little party,
then his note will be mostly canceled - though a little bit will get
through to contribute to the overall loudness.
"This effect is not the only one involved in our appreciation of loudness. If it
was, 100 instruments would sound ten times as loud as one. But we perceive 100
instruments as being only four times as loud as one. This extra diminution in perceived
loudness is the result of the way we humans are designed - so let's
have a look at that.
"Why don't our brains add up sounds normally? The surprising answer is that our
brains and ears add up sounds in an unusual way in order to help us stay alive.
From the times of the earliest cavemen to the present day, we have used our ears
to help us avoid danger. This is one of the main reasons we have ears in the first
place (although they are also useful for supporting your sunglasses). To
be effective, your ears have to be able to hear very quiet noises (like the sound
of someone creeping up on you), but also they must not get damaged by loud noises
(such as thunder). It wouldn't be any good if you had excellent hearing for quiet
noises but your ears stopped working after the first loud noise you heard.
"Our ears are organized in such a way that quiet noises can be heard clearly but
any increase in the volume of the noise has progressively less and less impact.
This effect is also true of our other four senses: smell, taste, sight and touch.
Six smelly socks aren't six times as smelly as one on its own (even though each
of the socks is releasing the same amount of smell) and ten salted peanuts in your
mouth aren't five times as salty as two of them (even though you now have five times
as much salt on your tongue). If you light 100 candles one at a time in a dark room
you get the same effect as you got with the [glockenspiels] - the first one makes
the biggest difference and the eighty-seventh makes hardly any difference. If you
are daft enough to stick a pin in your fingertip then it will hurt, but if you stick
a second one in (next to the first one) the pain will not be
doubled."
Author: John Powell
Title: How Music Works
Publisher: Little, Brown
Date: Copyright 2010 by John Powell
Pages: 85-87
How Music Works: The Science and Psychology of Beautiful Sounds, from Beethoven
to the Beatles and Beyond
by John Powell by Little, Brown and Company
Hardcover