20 Hz is considered the normal low-frequency limit of human hearing. When pure sine waves are reproduced under ideal conditions and at very high volume, a human listener will be able to identify tones as low as 12 Hz. Below 10 Hz it is possible to perceive the single cycles of the sound, along with a sensation of pressure at the eardrums. From about 1,000 Hz, the dynamic range of the auditory system decreases with decreasing frequency. This compression is observable in the
equal-loudness-level contours, and it implies that even a slight increase in level can change the perceived loudness from barely audible to loud. Combined with the natural spread in
thresholds within a population, its effect may be that a very low-frequency sound which is inaudible to some people may be loud to others. One study has suggested that infrasound may cause 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. A scientist working at Sydney University's Auditory Neuroscience Laboratory reports growing evidence that infrasound may affect some people's nervous system by stimulating the
vestibular system, and this has shown in animal models an effect similar to
sea sickness. In research conducted in 2006 focusing on the impact of sound emissions from wind turbines on the nearby population, perceived infrasound has been associated to effects such as annoyance or fatigue, depending on its intensity, with little evidence supporting physiological effects of infrasound below the human perception threshold. Later studies, however, have linked inaudible infrasound to effects such as fullness, pressure or tinnitus, and acknowledged the possibility that it could disturb sleep. Other studies have also suggested associations between noise levels in turbines and self-reported sleep disturbances in the nearby population, while adding that the contribution of infrasound to this effect is still not fully understood. In a study at
Ibaraki University in Japan, researchers said EEG tests showed that the infrasound produced by
wind turbines was "considered to be an annoyance to the technicians who work close to a modern large-scale wind turbine". Jürgen Altmann of the
Technical University of Dortmund, an expert on
sonic weapons, has said that there is no reliable evidence for nausea and vomiting caused by infrasound. High volume levels at concerts from subwoofer arrays have been cited as causing
lung collapse in individuals who are very close to the subwoofers, especially for smokers who are particularly tall and thin. In September 2009, London student Tom Reid died in a club of
sudden arrhythmic death syndrome (SADS) after complaining that "loud bass notes" from the club's speakers were "getting to his heart". The inquest recorded a verdict of natural causes, although some experts commented that the bass could have acted as a trigger. Air is a very inefficient medium for transferring low frequency vibration from a transducer to the human body. Mechanical connection of the vibration source to the human body, however, provides a potentially dangerous combination. The U.S. space program, worried about the harmful effects of rocket flight on astronauts, ordered vibration tests that used cockpit seats mounted on vibration tables to transfer "brown note" and other frequencies directly to the human subjects. Very high power levels of 160 dB were achieved at frequencies of 2–3 Hz. Test frequencies ranged from 0.5 Hz to 40 Hz. Test subjects suffered motor ataxia, nausea, visual disturbance, degraded task performance and difficulties in communication. These tests are assumed by researchers to be the nucleus of the current
urban myth surrounding the "brown note" and its effects. The report "A Review of Published Research on Low Frequency Noise and its Effects" contains a long list of research about exposure to high-level infrasound among humans and animals. For instance, in 1972, Borredon exposed 42 young men to tones at 7.5 Hz at 130 dB for 50 minutes. This exposure caused no adverse effects other than reported drowsiness and a slight blood pressure increase. In 1975, Slarve and Johnson exposed four male subjects to infrasound at frequencies from 1 to 20 Hz, for eight minutes at a time, at levels up to 144 dB
SPL. There was no evidence of any detrimental effect other than mild ear discomfort. Tests of high-intensity infrasound on animals resulted in measurable changes, such as cell changes and ruptured blood vessel walls. Infrasound is one hypothesized cause of death for the nine Soviet hikers who were
found dead at Dyatlov Pass in 1959.
Hygienic standards in the workplace US: Maximum levels for frequencies from 1 to 80 Hz are no more than 145 dB. Overall level (for all frequencies) is no more than 150 dB.
Brown note The brown note is a hypothetical infrasonic frequency capable of causing fecal incontinence by creating acoustic resonance in the human bowel. Attempts to demonstrate the existence of a "brown note" using sound waves transmitted through the air have failed. In February 2005 the television show
MythBusters attempted to verify whether the "brown note" was a reality. They tested notes down to 5 Hz in frequency and up to 153 dB in
sound pressure. They used a type of subwoofer used for major rock concerts, and which had been specially modified for deeper bass extension. The rumored physiological effects did not materialize. The show declared the brown note myth "busted."
Infrasonic 17 Hz tone experiment On 31 May 2003, a group of UK researchers held a mass experiment, where they exposed some 700 people to music laced with soft 17 Hz
sine waves played at a level described as "near the edge of hearing", produced by an extra-long-stroke subwoofer mounted two-thirds of the way from the end of a seven-meter-long plastic sewer pipe. The experimental concert (entitled
Infrasonic) took place in the
Purcell Rooma concert and performance venue which is part of
Central London's
Southbank Centreover 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 feeling uneasy or sorrowful, getting chills down the spine or nervous feelings of revulsion or fear. This, Tandy conjectured, was why he had seen a ghostly figure—it was, he believed, an optical illusion caused by his eyeballs resonating. The room was exactly half a wavelength in length, and the desk was in the centre, thus causing a
standing wave which caused the vibration of the foil. Tandy investigated this phenomenon further and wrote a paper entitled
The Ghost in the Machine. He carried out a number of investigations at various sites believed to be haunted, including the basement of the Tourist Information Bureau next to
Coventry Cathedral and
Edinburgh Castle. == Detection and measurement ==