It’s a now-legendary movie scene: in Back to the Future, Marty McFly (Michael J. Fox) plays a single guitar chord through an amplifier that’s cranked up so loud, it slams him against the wall.
But can a loud sound really knock somebody off their feet?
Not only can it bowl people over, says Chris Waltham, a University of British Columbia astronomy and physics professor who specializes in music acoustics — it can actually kill people.
According to Waltham, the sound waves that we hear are actually tiny fractions of fluctuations in air pressure. Just how tiny? Air pressure is 100,000 pascals, and the human limit of hearing is roughly one-100,000th of a pascal—or one-10 billionth of air pressure.
In fact, Waltham says an undamaged human ear can hear sound waves where the air is barely moving the width of an atom.
Once you get up over the 100,000th of air pressure, you’re going to cause damage to the human ears. In principle, if you get hit with a sound wave where the highs of the pressure are double the air pressure, and the lows are a vacuum, not only will it blow out your ears, but it can kill you.
"If air pressure doubles and then it goes into a vacuum, you can do serious physical damage. It’s the same as if you’re in a spaceship and you suddenly open the door," says Waltham, who adds that, technically, sound is already used to cause harm — but not with a guitar and giant amps like Marty McFly's.
"Essentially that’s what a stun grenade is," he says. "Rather than fragments of metal hitting you it’s just the air pressure oscillating violently."