delanceyplace.com 3/22/11 - how loud is loud

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: The Science and Psychology of Beautiful Sounds, from Beethoven to the Beatles and Beyond

publisher:

Little, Brown

date:

Copyright 2010 by John Powell

pages:

85-87
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