Infrasound
Infrasound, sometimes referred to as low frequency sound or incorrectly subsonic, describes sound waves with a frequency below the lower limit of human audibility. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. Although the ear is the primary organ for sensing low sound, at higher intensities it is possible to feel infrasound vibrations in various parts of the body.
The study of such sound waves is sometimes referred to as infrasonics, covering sounds beneath 20 Hz down to 0.1 Hz. People use this frequency range for monitoring earthquakes and volcanoes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography to study the mechanics of the human cardiovascular system.
Infrasound is characterized by an ability to get around obstacles with little dissipation. In music, acoustic waveguide methods, such as a large pipe organ or, for reproduction, exotic loudspeaker designs such as transmission line, rotary woofer, or traditional subwoofer designs can produce low-frequency sounds, including near-infrasound. Subwoofers designed to produce infrasound are capable of sound reproduction an octave or more below that of most commercially available subwoofers, and are often about 10 times the size.
History and study
One of the pioneers in infrasonic research was French scientist Vladimir Gavreau. His interest in infrasonic waves first came about in 1957 in the large concrete building that he and his research team were working in. The group was experiencing bouts of periodic and deeply unpleasant nausea. After weeks of speculation on the source of the nausea — the team was convinced that it was a pathogen or an untraced leak of noxious chemical fumes in the facility — they discovered that a "loosely poised low speed motor... was developing 'nauseating vibrations'".When Gavreau and the team attempted to measure an amplitude and pitch, they were shocked when their equipment detected no audible sound. They concluded the sound being generated by the motor was so low in pitch that it was below their biological ability to hear, and that their recording equipment was not capable of detecting these frequencies. Nobody had conceived that sound might exist at such low frequencies, and so no equipment had been developed to detect it. Eventually, it was determined that the sound inducing the nausea was a 7 cycle per second infrasound wave that was inducing a resonant mode in the ductwork and architecture of the building, significantly amplifying the sound. In the wake of this serendipitous discovery, the researchers soon got to work preparing further infrasonic tests in the laboratories. One of his experiments was an infrasonic whistle, an oversized organ pipe. As a result of this and similar incidents, it has become routine in new architecture construction to inspect for and eliminate any infrasonic resonances in cavities and the introduction of sound-proofing and materials with specialized sonic properties.
Animal reaction
Some animals have been thought to perceive the infrasonic waves going through the earth, caused by natural disasters, and to use these as an early warning. An example of this is the 2004 Indian Ocean earthquake and tsunami. Animals were reported to have fled the area hours before the actual tsunami hit the shores of Asia. It is not known for sure that this is the cause; some have suggested that it may have been the influence of electromagnetic waves, and not of infrasonic waves, that prompted these animals to flee.Research in 2013 by Jon Hagstrum of the US Geological Survey suggests that homing pigeons use low-frequency infrasound to navigate.
Human reactions
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 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 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 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 of respondents reporting feeling uneasy or sorrowful, getting chills down the spine or nervous feelings of revulsion or fear.
In presenting the evidence to the 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."