Infrasound is Scary Stuff
Pressure waves are delightfully versatile. JS Bach used them to dazzle musicians and mathematicians alike, medical technicians use them to image internal structures, and engineers investigate petroleum deposits and rock formations. What’s less known, however, is that properly tuned pressure waves can scare the living daylights out of human beings.
A Mk I human ear in good repair can detect longitudinal pressure waves – “sound,” to humanities majors – between 20 and 16,000 Hz. This is more than adequate for daily use; our vocal range averages between 80 Hz and 1100 Hz, on the low side of the mean for mammalian hearing. In absolutely perfect conditions – an enclosed, pressurized chamber, high intensity (volume), attentive subject with good hearing – subjects can detect sounds as low as 2 Hz.
In nature, sounds below the 20 Hz threshold are almost exclusively associated with terrifying events. Elephants and whales exploit them for long distance communication, but infrasonic noise is more commonly produced by Scary Things. Large cat species roar with extreme low frequency tones, stunning prey. Whales do the same thing. The alarm and challenge bellows of alligators are partially infrasonic. Earthquakes, floods and storms, which animals detect and flee from well ahead of humans, make tell-tale low frequency sounds.
For the most part, we don’t hear any of this. Once you drop below the 20 Hz range, however, interesting resonances occur within the human body. The physiological and psychological effects of those resonances present the ideal cocktail of half-seen specters and formless dread for a perfectly engineered haunted house.
Better Haunting Through Infrasound
In the early days of the US space program, NASA generated a lot of research on the subject of direct physical vibration from low frequency sources, where the subject was in contact with the source. Very high intensity (around 160 dB) vibrations in the .5 Hz to 40 Hz range were generated; sounds at 20 Hz and below caused a number of physiological effects, ranging from nausea and inhibited communication to visual hallucinations and motor ataxia.
Those numbers only speak to direct, mechanical vibration; exposure through sound alone wasn’t part of the study. Two interesting field tests come to mind which demonstrate the use of indirect infrasound – sound transmitted through the air – to create exactly the sensations we associate with haunted houses.
In the first case, a dodgy extractor fan in a laboratory associated with Coventry University was outputting vibrations of 18.98 Hz. The laboratory was purported to be haunted; ghosts were sighted out of the corner of the eye, people constantly felt watched, and the whole space was namelessly, formlessly wrong. The fan was “exorcised” by a lecturer named Vic Tandy, who had borrowed the lab to work on his fencing foil. Oscillations in the foil, when clamped, gave him the first vital clue to the root of the haunting. With the extractor repaired, no further haunting sensations, ghost sightings, or disturbing events were reported. (Tandy wrote up his experience in the Journal of the Society for Psychical Research, available in PDF.)
In a second case, a team of scientists lead by Dr. Richard Lord of the National Physical Laboratory, Richard Wisemen from the University of Hertfordshire, and Sarah Angliss, composer and engineer, played 17 Hz sounds under selections of contemporary music in a concert hall. Audience members, unaware of which pieces in the program were accompanied by infrasound, were then polled for emotional and physiological reactions. (Over the course of multiple performances, the experimental and control pieces were randomized to eliminate collusion between showings.)
Twenty-two percent of audience members reported predictable reactions to infrasound exposure, characterized as chills down the spine, unexplained sorrow, nausea, and fear.
In each case, the scientists involved viewed their discoveries as a triumph of reason over the supernatural. Our takeaway was a little different: With an 18.98 Hz generator and the right resonating body, you seriously scare the crap out of people.
Engineering Your Haunted Infrasound Generator
There are a number of ways to engineer an infrasound generator for your haunted house. For our purposes, a sine wave generator connected to a self-amplified subwoofer would probably do just fine. If you’re willing to spend a fair bit of money, as well as rent a U-Haul, you can try to reproduce Park and Robertson’s field-portable infrasound generator (pdf), which can produce detectable sound over five kilometers. Why not haunt your entire neighborhood?
The infrasound generator used by Angliss, Lord and Wisemen in their concert series is a fine middle ground between barely sufficient and legally actionable. It consisted of a six-meter length of corrugated drain conduit, a long-stroke subwoofer, a sine wave generator, and a power amp with the ability to faithfully pass 10 Hz sound. Detailed build guides aren’t available, but the right engineer could certainly reconstruct it from available information.
Here’s what we know:
- According to Angliss, the pipe functioned as quarter-wavelength, open cylinder resonator, with a frequency of 17.4 Hz. She gives us a length of approximately six meters, but you could dial that in with the available information and a little math.
- Most subwoofers seem to cut off frequency response at around 19 – 20 Hz. THX-approved subwoofers go from 80 Hz and down; your typical home unit tops out at 200 Hz. Getting a 19 Hz subwoofer and playing with the resonator dimensions should be adequate.
- You can practically run a sine wave generator on your cell phone these days. If you lack specialized equipment, this can be modeled in software.
- The only special requirement mentioned for the power amp is that it feature a flat frequency response at 10 Hz. (“Flat frequency” means just that frequency and nothing else.)
Remember, this unit was powerful enough to generator measurable effects in 165 people out of a crowd of over 700, in a concert hall. Powering one up in your haunted house will put you on the map.
Go forth, mad engineers, and scare the candy out of those little monsters!