In 2003, scientists subjected 750 volunteers to a strange experiment, in the form of a fear and anxiety-inducing concert. 

For a while, people had suspected that low-frequency sounds, beyond our hearing, can have strange effects on humans. But in 2003, there had been little in the way of research. And so, scientists from the National Physical Laboratory in England decided to subject people to infrasound at a concert where infrasound would be played to them underneath regular music.

"For our experiment we wanted to establish if infrasound in music generated unusual psychological effects, even in listeners unaware of its presence," Sarah Angliss, a composer and sound designer who worked on the project, explains on her website.

"Of particular interest were its reputed emotional effects. Infrasound is used in sacred music, for instance during cathedral organ recitals, and there is debate about why it’s used. Some people say it adds a sense of awe to the music – it puts a shiver down your spine. Others say that giant infrasonic organ pipes are nothing more than ‘an expensive way to make a draught’," Angliss explained in a separate blog. 

"Stranger still, infrasound has also been detected at some ostensibly haunted sites...where it may also be making people feel very uneasy."

In one example she referred to, scientists working at a laboratory in Coventry, England, reported feelings of depression and shivers, and various other strange goings on, including "figures" being seen in the corner of one scientist's vision. 

When one scientist noticed that a foil blade in a vice was vibrating wildly, he investigated and found the apparent cause of all their trouble; a new extractor fan in the lab was creating infrasound, a low-frequency sound wave below human hearing. Once the fan was switched off, the problems disappeared.

The team from the National Physical Laboratory decided to play sound at 17.5 Hertz (Hz), just beyond human hearing, generated through the use of a 7-meter (23-foot) pipe at audience members at a London concert hall. 

The infrasound was only played during certain pieces, and audience members did not know which. Despite this, 22 percent of audience members reported strange effects during pieces laced with infrasound. These included feelings of sorrow, chills down the spine, general unease, and feelings of revulsion and fear.

“These results suggest that low frequency sound can cause people to have unusual experiences even though they cannot consciously detect infrasound,” Professor Richard Wiseman, psychologist at the University of Hertfordshire said at the time, per NBC News.

“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,” he added.

Subsequent studies, including one that deliberately set about creating a "haunted room", have shown evidence that infrasound can produce effects similar to hauntings in humans. Of course, not all hauntings can be explained in this way, with other explanations ranging from hoaxes to carbon monoxide poisoning, and the genuinely unknown.