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Decibels of DEATH!

Ear Protection

When I see the word “decibel,” I think two things. First, I think “Noise.” Then, I think “Oh god, decibels confuse the hell out of me…”

Well, I think I finally understand the decibel. It’s kind of a weird unit, but it’s also nifty, and it showcases one of the coolest things in mathematics: the logarithm.

Here’s how you compute the decibel-level of a sound. First, you figure out the acoustic power of that sound, probably using a microphone (or using a sound engineer who has a microphone and understands better than I do the difference between “acoustic power” and “sound amplitude.”) The acoustic power tells you the maximum pressure the sound wave exerts on things (say, your eardrums). Most of the time, you measure that sound pressure in pascals. Take that sound pressure and divide it by 20 micro-pascals. 20 micro-pascals is a semi-arbitrary reference point. It’s about the sound pressure where a 1000-Hertz sine wave first becomes audible to a human ear. It’s not a lot of pressure. The pressure 10 meters underwater is about twice what it is at sea level, which means the overpressure is about 1 atmosphere (1000 hectopascals. I’d like to note that Hectopascal would be a good name for a movie villain.) Well, the depth of water it would take to get an overpressure of 20 micropascals is 2 nanometers, which is about the diameter of a strand of DNA. Did you know human ears were that sensitive? I didn’t.

Anyway, you can use decibels to express a wide range of noise levels without using too many digits (Because, let’s face it, we all start zoning out after you get beyond about six digits, give or take.) To get the decibel number, you divide your sound pressure by 20 micropascals, take the base-10 logarithm of that, and multiply the result by 20. For example: a sound pressure of 20 micropascals gives you 20 * log10(20/20) = 20 * log10(1) = 20 * 0 = 0 dB.

With a title like Decibels of Death, you knew this article was going to be all about extremes. The quietest officially-measured place in the world is the anechoic chamber at Orfield Laboratories. It’s a room encased in a foot-thick concrete vault. The room itself sits on I-beams which are on springs to isolate external vibration. The inside of the room is full of wedge-shaped foam blocks which prevent echoes and dampen the sound from outside even further. The Guinness Book of World Records measured the sound level in the Orfield anechoic chamber at -9.4 decibels. That works out to a sound pressure level of 6.8 micropascals. To produce an overpressure that small, you’d only need a layer of water 0.612 nanometers thick. At that point, it’s less a puddle and more a molecular stack. That’s pretty damn quiet.

It’s actually intolerably quiet, apparently. The longest anybody’s ever spent in the chamber is 45 minutes, according to that Daily Mail article I linked above. I’ve heard stories about people who freaked out in the chamber because, all of a sudden, they can hear their heartbeats. And some people have auditory hallucinations when deprived of sound long enough, which probably makes the Orfield chamber even scarier.

So -9.4 dB is quiet enough to make you crazy. 0 dB is the threshold of hearing. 10 dB is about the quietest environment you or I will ever experience, and that’s only if we don’t breathe too loud. 25 dB is a very quiet room. According to a funky app I’ve got on my smartphone, the noise level at this desk is 51 dB. The EPA (the US environmental agency) recommends your everyday environment not exceed 70 dB. 85 dB can cause hearing damage over the long-term. 130 dB is painful. 150 dB can rupture your eardrums. This is what I was talking about earlier: logarithmic scales allow you to convert numbers orders of magnitude apart into nice numbers with low digit counts, which makes it easier to compare them side-by-side. When we started out, back at -9.4 decibels, the pressures were so low they were almost impossible to measure. Now, they’re so high they’re doing organ damage.

And speaking of organ damage… The strength of a blast wave is measured by its overpressure, just like the strength of a sound wave. In this fascinating and unnerving paper, some doctors report the effects of 62,000-pascal blast waves on rats. They speak of “minimal to mild alveolar hemorrhages,” as though there were such a thing as a mild case of bleeding fucking lungs. The upshot of all this is that, although 150 dB may burst your eardrums, 189.9 decibels (which is the decibel equivalent of 62,000 pascals overpressure) can actually damage your guts.

But if you’re catching a 190-decibel blast, you’ve got more serious things to worry about than bleeding lungs. Yes, really. There are a lot of reports of soldiers who have been hit by blasts from roadside bombs and car bombs and other such nasty things. Some of these soldiers, although they didn’t hit their heads on anything and nothing hit them in the head, developed serious cognitive problems: difficulty concentrating and short-term memory loss, enough to pretty much spoil their day-to-day lives. In another experiment, rats exposed to a blast overpressure of 20 kilopascals (180 decibels) experienced similar symptoms, and when they were dissected, had lots of dying brain cells. Which is all really pretty damn sad.

As it turns out, there’s actually technically a maximum sound pressure, at least if you want an undistorted sound wave. A pure tone has the shape of a sine wave: the pressure rises a certain amount above atmospheric, drops in a graceful sinusoidal curve, falls that same amount below atmospheric pressure, returns to atmospheric pressure, rinse and repeat. The thing about these kinds of sine waves is that, after their maximum overpressure, they have to drop that far below atmospheric pressure. And if the sound pressure of your sine wave happens to be greater than atmospheric pressure, that can’t happen: pressure is a number that doesn’t go any lower than zero, which is a vacuum. So a sine wave with a sound pressure larger than 1013 hectopascals (1 atmosphere) will sound all right when the pressure goes up, but will get cut off (“clipped,” the sound-engineer people call it) when it goes down. (And when I say “Will sound all right” I mean “Will rupture your aorta, destroy your lungs, tear your limbs off, and knock your house down,” as we learned from nuclear tests.) The maximum for unclipped sound, therefore, is 194.1 decibels.

But since we’re already blowing everything up, why worry about a little distortion? You know the Barrett M82? The big-ass .50-caliber sniper rifle? That one from that movie The Hurt Locker? The big scary one? Well, when that thing fires, its cartridge sees a blast wave of 265.5 decibels, which is just one more reason not to live in a rifle barrel.

You would experience 270 decibels if you were standing about 100 meters (350 feet) from a 1-megaton nuclear bomb when it went off. I use “experience” loosely here, since you wouldn’t have long to enjoy the racket before you were spread over an alarmingly large area.

Now, let’s say you were standing on the surface of a star just as it went supernova. Well, you’d be exposed to a blast pressure of something in the (very rough) neighborhood of 476 decibels, which I’m pretty sure the EPA would classify as “potentially hazardous.”

As it turns out, there’s a maximum pressure that is still physically meaningful, at least according to our current understanding of physics. It’s called the Planck Pressure, and it’s very large. It’s the kind of pressure you get inside black holes. It’s the kind of pressure the universe experienced (we think) right after the Big Bang. The Big Bang had a noise rating of 2,367.3 decibels. The explosion that set the current universe in motion had a pressure which can be quantified in five significant digits.

That’s what I mean about logarithmic scales being awesome. They turn unimaginable cosmic numbers into nice, manageable, comprehensible numbers. You’d better believe I’m going to be playing with logarithmic scales again soon. Which sounds way dirtier than I intended.

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One thought on “Decibels of DEATH!

  1. Pingback: A race between a snail and a beam of light. (Fun with logarithms part 2) | Sublime Curiosity

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