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How loud does something have to be to kill someone? I heard once on BattleBots that it took 150 decibels to stop the human heart. Is this true? If so, I'm afraid, since I work in a VERY loud job environment (I assemble 747 engines). So is there a fatal decibel level? And if so, is it really 150 decibels? Please tell me. I'm scared.

--Nick, via the Straight Dope Message Board

Nick, just a hint: next time you broach this topic with a scientist over lunch, don't let it slip that you got your information from BattleBots. A 150-decibel noise isn't loud enough to kill you, and even if it were, the immediate cause of death in most cases wouldn't be direct trauma of the heart. But the TV show was right in the big-picture sense: if a noise were loud enough, you'd die.

What we're talking about here is high-energy-impulse noise, also known as blast overpressure or air blast, which is "the sharp instantaneous rise in ambient atmospheric pressure resulting from explosive detonation or firing of weapons" (N.M. Elsayed, "Toxicology of Blast Overpressure," Toxicology, 1997). What, you say this isn't noise in the usual sense? Nonsense. It's no different from a thunderclap or a sonic boom. It's true you might not hear a truly titanic air blast, but that's because your eardrums would shatter, and worst case you'd be dead.

The lethal effects of air blast first became evident during World War I, when soldiers were found dead in the vicinity of an explosion despite having no external injuries. There was plenty of internal damage, though, particularly to hollow organs such as the ear, lung, and gastrointestinal tract. Partly for that reason the exact cause of death due to air blast has been debated for many years. The prevailing theory at the moment is air embolism originating in the lungs. The blast's immense pressure on the chest ruptures delicate lung tissue, admitting air bubbles into the arteries that travel to the heart, brain, and other organs and cause sudden death.

How loud does a noise have to be to kill? Let's see. The threshold of ear pain is around 130 to 140 decibels, roughly equivalent to a jet engine at a range in the tens of meters. Eardrum rupture occurs around 160 decibels (conventional sound) to 185 decibels (nonperiodic blast pressure). Lung rupture and presumably embolism due to air blast occur at 200 decibels. German physicist Juergen Altmann, who has written about the physiological effects of high-intensity sound (see PeaceProgram/acousticweapons.pdf) tells me the threshold for suffocation or embolism following lung rupture is 2.6 to 11 times atmospheric pressure, depending on pulse duration. The point is, instant death requires a noise that's pretty damn loud.

That's not to say lesser noises are harmless. The medical literature is full of reports of people with medical conditions who heard a loud noise and then, say, suffered a seizure and died. Some say (relatively) low-level noise can be bad for you even if you're (relatively) healthy. The subject remains controversial. One infamous study claimed that jet noise boosted the death rate among people living near the Los Angeles International Airport. But the effect disappeared when other scientists reviewed the data, and chances are it's a crock.

Another possible use of nonfatal doses of noise is "acoustic weapons," now under secret development by the military. Supposedly, acoustic weapons will use infrasonic sound or some such to incapacitate rather than kill an enemy, turning him into a bleeding, puking mess. Or maybe they'll kill him, using baseball-sized "acoustic bullets." Despite persistent reports in the press and much hand-wringing about the dire import of this new technology by human rights groups, no one has actually seen a practical demonstration of such a weapon, and very little has appeared in the scientific journals. Then again, I suppose very little had appeared in the scientific journals about the Manhattan Project before they crisped Hiroshima. All I'm saying is, while there's no question the military is putting some serious money into acoustic weapons research, a lot of people are skeptical. But if something does come of it, we're ready: the U.S. military already has experience serenading the likes of Manuel Noriega with ear-shattering rock 'n' roll. If that fails, there's always gangsta rap.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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