Very low frequency sound, or infrasound, apparently has strong negative emotional effects. While we already know music can powerfully manipulate the emotions, infrasound is special because people aren't consciously aware of it. Conventional audio equipment, even subwoofers, don't go this low, but creating these sounds isn't that expensive, so why don't we see groups using sub-sub woofers to create these feelings of unease?

Lots of people have suggested using them for games and movies, but I don't think that makes much sense: games and movies can already create the emotions they want through ordinary music. The strength of infrasound is that people don't know you're doing it. Imagine having a generator producing the sound in an otherwise silent "haunted house". Or a duplicitous leader having infrasound play before they arrived but stop immediately when they strode onto stage. Or a religion that worked it into some ritual, perhaps timing its cessation to coincide with baptism.

There's something about an ability to manipulate people's emotions without them knowing that you're doing so that just lends itself so well to unethical behavior. I'm surprised people aren't taking advantage of it yet.

(Or maybe they are and I should carry an infrasonic detector [1].)

[1] I hear various kinds of animals work. But the last time I tried to use an animal as a portable detector it didn't work well.