[tpoacher]

Apparently this is a thing that was used in old horror movies.

Subsonic music would play just before a scary encounter, creating a feeling of uneasiness to the audience without any consciously perceptible stimulous, and thus priming the audience for the horror to come.

[tenthirtyam]

The interwebz once told me that the human eye resonates at 19Hz or so, which apparently when triggered is very useful for inducing a sense of fear.