Or, more specifically, how to kill people with it!*
Sound, as we all know, is a physical thing. It comes from vibrations moving through matter (including air, of course). A sound begins when something causes a vibration. This vibration creates a longitudinal wave traveling through matter. This wave is actually a pressure wave. If said pressure wave hits our eardrums, then we hear a sound.
The volume or loudness of a sound is based on the amplitude of these vibration-created pressure waves. A loud sound will have a larger oscillation between the high and low pressures of the waves when compared to a soft sound, meaning that loud sounds have higher high pressure sections and lower low pressure sections than soft sounds.
This is kind of convenient because it allows there to be a threshold for what we can consider to be the “loudest” sound—if the low pressure gets too low, it hits vacuum-level and cannot go lower. So the loudest sound is something that creates low pressure sections between waves that are nearly vacuums.
In fact, what we consider to be the “threshold of sound” on the loud side of the scale (at least on earth) occurs at 194 decibels.** And where do we hit the point where sound can be deadly? Somewhere around 185-200 decibels.
Here are a few good resources/articles that talk about sound in general as well as what happens when you start approaching (or passing!) that “here is where sound gets deadly” threshold.
It’s actually kind of disturbing to think about. But also really cool.
*Sound killing people was actually something I focused on in my 2012 NaNoWriMo, “Whistler’s Father.” A scientist and an artist were working together to create the “perfect” sound—something that would boost mood, health, and would overall make people “better” if they were to hear it. But it turned out that the perfect sound was actually deadly if it was listened to for too long; the scientist in my story ends up killing himself via this perfect sound because he becomes addicted to it and is unable to stop listening to it in time to prevent his own death. Yeah, that was a cheery NaNo.
**You can get sounds louder than this, but the vibrations that create them don’t create waves (again, because of that low pressure threshold) but they still create something. Things like the atomic bombs dropped on Japan and the eruption of Krakatoa, for example, were louder than 194 decibels, and were very destructive if we even just consider the sounds (or rather, the spikes in atmospheric pressure) they produced.