10 Horror Movies That Put Insane Effort Into Details Nobody Noticed

This level of hidden detail here is truly insane. So insane in fact, that it requires a quick explanation before identifying its use in Paranormal Activity and elsewhere. So, infrasound is an undetectable level of noise; human ears are designed to pick up sound between the frequency of 20 Hz to 22,000 Hz, sounds below 20Hz can be detected but only at extremely high volumes. Infrasound is measured at anything below 20Hz, got it?

The overall effect of infrasound is rumoured to produce intense feelings of "awe or fear in humans. It has also been suggested that since it is not consciously perceived, it may make people feel vaguely that odd or supernatural events are taking place."

With that in mind, its use in horror movies is rather useful to say the least. Within Paranormal Activity, it is almost impossible to identify a specific sequence as its use occurs throughout much of the film. Gaspar Noé used this technique in the first thirty minutes of his intensely disturbing thriller Irreversible, to great effect, albeit almost subconscious.

The effect of infrasound is designed to be imperceptible yet within Paranormal Activity some distinct rumblings are audible and therefore above the 20Hz range - it is the effect of the unheard sonic range that really begins to play tricks with your mind. If you get freaked out in a movie for no apparent reason, this could be why.