This is some scary stuff right here. Proceed with caution.
Infrasound is sound waves with a frequency so low (lower than 20 Hz) that humans can’t hear it. (On the opposite side of the spectrum you’ll find ultrasound, think of dog whistles.) You can still feel it though. Know that feeling when the music’s real loud and the bass kicks in? I usually feel it in my stomach, the vibrations in the air are affecting me physically. It’s nothing harmful though, aside from possible damage to your hearing. But if you do the same with infrasound then you may be in for a painful ride.
The resonant frequency of something is the rate at which it naturally vibrates. For example, the resonant frequency of the eyes is roughly 17 Hz. If you play that sound loudly enough, it’ll resonate with your eyes and interesting stuff will start to happen. At lower volumes, you may start seeing grey ghost-like shadows in your vision. Turn the amp up to 11 and you’ll be doing some serious damage.
Most of your body has low resonant frequency, so infrasound is where it’s at if you want to do some damage. Scary thing is, because the frequency of that sound is so low, you wouldn’t even hear it! You’d never know what’s going on. You’ll have trouble understanding speech and sound at 130 dB of infrasound. Move that up to 150, you’ll get nauseous. If you’re going with infrasound from 0.5 to 8 Hz then at 166 dB it’ll affect your lungs so you’ll have trouble breathing. A tad louder, 177 dB, and the sound will cause your lungs to inhale and exhale, essentially forcing you to breathe.
Oh and at 240 dB you can blow up someone’s head. I wish I was kidding. (Thanks to Game Theory for source!)