[Dixielandjazz] How sound works-- interesting clip

Tue Mar 22 06:03:56 PDT 2011

To:  DJMLand Musicians & Jazzfans lists\

From:  Norman Vickers, Jazz Society of Pensacola

Interesting article about our perception of sound forwarded to me by
Pensacola Jazzfan Howard Martin.  I hope some of you will find it of
interest.

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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

 http://ecx.images-amazon.com/images/I/51zRfs-FB0L._SL160_.jpgHow 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

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