[in the Arena]

That would be a "certain ultra LOW frequency", below 20 Hrz. It's the amplitude (volume), at a frequency below the threshold of hearing, that causes severe distress.

How does that work ?

[ScreamingFist]

"How does that work ?"

Remember your last rock concert? Remember when the keyboardist hits that REALLY deep note that vibrates the entire arena and holds it, the crowd goes crazy, the lasers come on etc.? Now picture that happening, but the note is lower than you can hear (typical human hearing is 20 Hertz to 20,000 Hertz), your body still "feels" the note. Imagine "feeling" that note for a couple days and you start to get the idea.