Today, I’ll “go around the bend” here and bring up something unusual that I had touched on earlier; infrasound and elephants. This time I will look at how infrasound affects humans from experiments conducted on human subjects.
20Hz is considered the bottom limit of the frequency range of human hearing. When pure sine waves are reproduced under the right conditions and at very high volume, a person will be able to hear tones as low as 12 Hertz. Below 10 Hertz, it is possible to perceive the single cycles of the sound, along with a sensation of pressure on the eardrums.
“The dynamic range of the auditory system decreases with decreasing frequency. This compression can be seen in the equal-loudness-level contours, and it implies that a slight increase in level can change the perceived loudness from barely audible, to loud. Combined with the natural spread within a population, it may have the effect that a very low-frequency sound which is inaudible to some people may be loud to others.”
This is the funny part; a study has suggested that infrasound may cause feelings of awe or fear in humans. It also was suggested that since it is not consciously perceived, it may make people feel vaguely that odd or supernatural events are taking place. This was derived from an experiment done in 2003 in the UK where a mass experiment was conducted with some 700 people listening to music laced with soft 17 Hertz sign waves, played at barely audible levels. “The experimental concert (entitled Infrasonic) took place in the Purcell Room over the course of two performances, each consisting of four musical pieces. Two of the pieces in each concert had 17 Hz tones played underneath. In the second concert, the pieces that were to carry a 17 Hz undertone were swapped so that test results would not focus on any specific musical piece. The participants were not told which pieces included the low-level 17 Hz near-infrasonic tone. The presence of the tone resulted in a significant number (22%) of respondents reporting anxiety, uneasiness, extreme sorrow, nervous feelings of revulsion or fear, chills down the spine, and feelings of pressure on the chest. In presenting the evidence to British Association for the Advancement of Science, Professor Richard Wiseman said, ‘These results suggest that low frequency sound can cause people to have unusual experiences even though they cannot consciously detect infrasound. 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.’ ”
Here’s an interesting story by Vic Tandy, a lecturer at Coventry University, who suggested that an infrasonic signal of 19 Hz might be responsible for some ghost sightings. Tandy was working late one night alone in a supposedly haunted laboratory at Warwick, when he felt very anxious and could detect a grey blob out of the corner of his eye. When Tandy turned to face the grey blob, there was nothing.
The following day, Tandy was working on his fencing foil, with the handle held in a vise. Although there was nothing touching it, the blade started to vibrate wildly. It turns out the extractor fan in the lab was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye (18 Hertz). This was why Tandy had seen the optical illusion caused by his eyeballs resonating. The room was exactly half a wavelength in length, and the desk was in the center, thus causing a standing wave which caused the vibration of the foil. That is a VERY BIG RELIEF! I thought someone was after me…
The part that is not funny, however, is the fact that this sound is heard in and around many homes much of the time. This can contribute to annoyance, sleep disturbance, headaches, and nausea. Other things can be affected including blood pressure, respiratory rate, and balance. Just because infrasound is generally below the human hearing range, it does not mean the ear will not suffer damage if the sound level reaches 140 dB. Remember, all sound is sound waves perceived as alternating compression/decompression of the air at varying speeds. Even if you just feel it and don’t hear it, the fact remains that your ear drums are like drums, sensitive not just to frequencies, but air pressure changes. If the air pressure changes are severe enough, you can lose your hearing just like some divers have suffered when reaching a depth too quickly, not giving the eustachian tubes time to equalize the pressure. It may only take one or a few wave cycles to suck or blow holes into your ear drums.
If you do hear/feel such sounds around the home or at work, consider bringing this up and get help as I mentioned in the Noise Pollution entry a few days ago. Because this is infrasonic, ear plugs may not help, so you may end up having to move or get the sound source issue resolved.