I've always been intrigued by infrasound, extreme bass frequencies that you may not be able to consciously hear but can have a profound impact on your state-of-mind (and body). Of course, the military has also been interested in exploiting this phenomenon as a non-lethal weapon. Acoustic scientists at the National Physical Laboratory in England recently conducted the first controlled infrasound experiment–on 750 people at a concert. According to this Reuters report, "The audience did not know which pieces (of music) included infrasound but 22 percent reported more unusual experiences when it was present in the music. Their unusual experiences included feeling uneasy or sorrowful, getting chills down the spine or nervous feelings of revulsion or fear."
"Some scientists have suggested that this level of sound may be present at some allegedly haunted sites and so cause people to have odd sensations that they attribute to a ghost — our findings support these ideas," said Professor Richard Wiseman, a psychologist at the University of Hertfordshire in southern England.