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Infrasound

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Infrasound arrays at infrasound monitoring station in Qaanaaq, Greenland

Infrasound, sometimes referred to as low-frequency sound, describes sound waves with a frequency below the lower limit of audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. The ear is the primary organ for sensing infrasound, but 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. and rarely to 0.001 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 heart.

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.[citation needed]

Definition

Infrasound is defined by the American National Standards Institute as "sound at frequencies less than 20 Hz."

History and study

The Allies of World War I first used infrasound to locate artillery.[1] One of the pioneers in infrasonic research was French scientist Vladimir Gavreau.[2] His interest in infrasonic waves first came about in his laboratory during the 1960s, when he and his laboratory assistants experienced shaking laboratory equipment and pain in the eardrums, but his microphones did not detect audible sound. He concluded it was infrasound caused by a large fan and duct system, and soon got to work preparing tests in the laboratories. One of his experiments was an infrasonic whistle, an oversized organ pipe.[3][4][5]

Sources

Patent for a double bass reflex loudspeaker enclosure design intended to produce infrasonic frequencies ranging from 5 to 25 hertz, of which traditional subwoofer designs are not readily capable.

Infrasound can result from both natural and man-made sources:

  • Human singers: some vocalists, including Tim Storms, can produce notes in the infrasound range.[29]

Animal reaction

Further information: Tsunami § Possible animal reaction, and Rayleigh wave § Possible animal reaction
See also: P-wave

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.[33][34] 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.[35]

Research in 2013 by Jon Hagstrum of the US Geological Survey suggests that homing pigeons use low-frequency infrasound to navigate.[36]

Human reactions

See also: Brown note

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.[37] 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 1000 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.[38]

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.[39]

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.[40] Later studies, however, have linked inaudible infrasound to effects such as fullness, pressure or tinnitus, and acknowledged the possibility that it could disturb sleep.[41] 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.[42][43]

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.”[44][45][46]

Jürgen Altmann of the Dortmund University of Technology, an expert on sonic weapons, has said that there is no reliable evidence for nausea and vomiting caused by infrasound.[47]

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.[48]

In September 2009, London student Tom Reid died of sudden arrhythmic death syndrome (SADS) after complaining that "loud bass notes" 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.[49]

Air is a very inefficient medium for transferring low frequency vibration from a transducer to the human body.[50] 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.[51][52]

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 middle ear discomfort. Tests of high-intensity infrasound on animals resulted in measurable changes, such as cell changes and ruptured blood vessel walls.

In February 2005 the television show MythBusters used twelve Meyer Sound 700-HP subwoofers—a model and quantity that has been employed for major rock concerts.[53][54] Normal operating frequency range of the selected subwoofer model was 28 Hz to 150 Hz[55] but the 12 enclosures at MythBusters had been specially modified for deeper bass extension.[56] Roger Schwenke and John Meyer directed the Meyer Sound team in devising a special test rig that would produce very high sound levels at infrasonic frequencies. The subwoofers' tuning ports were blocked and their input cards were altered. The modified cabinets were positioned in an open ring configuration: four stacks of three subwoofers each. Test signals were generated by a SIM 3 audio analyzer, with its software modified to produce infrasonic tones. A Brüel & Kjær sound level analyzer, fed with an attenuated signal from a model 4189 measurement microphone, displayed and recorded sound pressure levels.[56] The hosts on the show tried a series of frequencies as low as 5 Hz, attaining a level of 120 decibels of sound pressure at 9 Hz and up to 153 dB at frequencies above 20 Hz, but the rumored physiological effects did not materialize.[56] The test subjects all reported some physical anxiety and shortness of breath, even a small amount of nausea, but this was dismissed by the hosts, noting that sound at that frequency and intensity moves air rapidly in and out of one's lungs. 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 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.[57][58]

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.[57][58]

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."[38]

Suggested relationship to ghost sightings

Psychologist Richard Wiseman of the University of Hertfordshire suggests that the odd sensations that people attribute to ghosts may be caused by infrasonic vibrations. Vic Tandy, experimental officer and part-time lecturer in the school of international studies and law at Coventry University, along with Dr. Tony Lawrence of the University's psychology department, wrote in 1998 a paper called "Ghosts in the Machine" for the Journal of the Society for Psychical Research. Their research 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 vice. Although there was nothing touching it, the blade started to vibrate wildly. Further investigation led Tandy to discover that the extractor fan in the lab was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye given as 18 Hz by NASA.[59] 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.[60]

Tandy investigated this phenomenon further and wrote a paper entitled The Ghost in the Machine.[61] 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[62][63] and Edinburgh Castle.[64][65]

Infrasound for nuclear detonation detection

Infrasound is one of several techniques used to identify if a nuclear detonation has occurred. A network of 60 infrasound stations, in addition to seismic and hydroacoustic stations, comprise the International Monitoring System (IMS) that is tasked with monitoring compliance with the Comprehensive Nuclear Test-Ban Treaty (CTBT).[66] IMS Infrasound stations consist of eight microbarometer sensors and space filters arranged in an array covering an area of approximately 1 to 9 km2.[66][67] The space filters used are radiating pipes with inlet ports along their length, designed to average out pressure variations like wind turbulence for more precise measurements.[67] The microbarometers used are designed to monitor frequencies below approximately 20 hertz.[66] Sound waves below 20 hertz have longer wavelengths and are not easily absorbed, allowing for detection across large distances.[66]

Infrasound wavelengths can be generated artificially through detonations and other human activity, or naturally from earthquakes, severe weather, lightning, and other sources.[66] Like forensic seismology, algorithms and other filter techniques are required to analyze gathered data and characterize events to determine if a nuclear detonation has actually occurred. Data is transmitted from each station via secure communication links for further analysis. A digital signature is also embedded in the data sent from each station to verify if the data is authentic.[68]

Detection and measurement

NASA Langley has designed and developed an infrasonic detection system that can be used to make useful infrasound measurements at a location where it was not possible previously. The system comprises an electret condenser microphone PCB Model 377M06, having a 3-inch membrane diameter, and a small, compact windscreen.[69] Electret-based technology offers the lowest possible background noise, because Johnson noise generated in the supporting electronics (preamplifier) is minimized.[69]

The microphone features a high membrane compliance with a large backchamber volume, a prepolarized backplane and a high impedance preamplifier located inside the backchamber. The windscreen, based on the high transmission coefficient of infrasound through matter, is made of a material having a low acoustic impedance and has a sufficiently thick wall to ensure structural stability.[70] Close-cell polyurethane foam has been found to serve the purpose well. In the proposed test, test parameters will be sensitivity, background noise, signal fidelity (harmonic distortion), and temporal stability.

The microphone design differs from that of a conventional audio system in that the peculiar features of infrasound are taken into account. First, infrasound propagates over vast distances through the Earth's atmosphere as a result of very low atmospheric absorption and of refractive ducting that enables propagation by way of multiple bounces between the Earth's surface and the stratosphere. A second property that has received little attention is the great penetration capability of infrasound through solid matter – a property utilized in the design and fabrication of the system windscreens.[70]

Thus the system fulfills several instrumentation requirements advantageous to the application of acoustics: (1) a low-frequency microphone with especially low background noise, which enables detection of low-level signals within a low-frequency passband; (2) a small, compact windscreen that permits (3) rapid deployment of a microphone array in the field. The system also features a data acquisition system that permits real time detection, bearing, and signature of a low-frequency source.[70]

The Comprehensive Nuclear-Test-Ban Treaty Organization Preparatory Commission uses infrasound as one of its monitoring technologies, along with seismic, hydroacoustic, and atmospheric radionuclide monitoring. The loudest infrasound recorded to date by the monitoring system was generated by the 2013 Chelyabinsk meteor.[71]

See also

References

Notes
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  4. ^ Gavreau V., infrasound, in: Science journal 4(1) 1968, p. 33
  5. ^ Gavreau V., "Sons graves intenses et infrasons" in: Scientific Progress – la Nature (Sept. 1968) pp. 336–344
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  7. ^ Fee, David; Matoza, Robin S. (1 January 2013). "An overview of volcano infrasound: From hawaiian to plinian, local to global". Journal of Volcanology and Geothermal Research. 249: 123–139. doi:10.1016/j.jvolgeores.2012.09.002. ISSN 0377-0273.
  8. ^ Johnson, Jeffrey Bruce; Ripepe, Maurizio (15 September 2011). "Volcano infrasound: A review". Journal of Volcanology and Geothermal Research. 206 (3): 61–69. doi:10.1016/j.jvolgeores.2011.06.006. ISSN 0377-0273.
  9. ^ Garces, M.; Willis, M. (2006). "Modeling and Characterization of Microbarom Signals in the Pacific". Archived from the original on 11 February 2009. Retrieved 24 November 2007. Naturally occurring sources of infrasound include (but are not limited to) severe weather, volcanoes, bolides, earthquakes, mountain waves, surf, and, the focus of this research, nonlinear ocean wave interactions. {{cite journal}}: Cite journal requires |journal= (help)
  10. ^ Haak, Hein (1 September 2006). "Probing the Atmosphere with Infrasound : Infrasound as a tool" (PDF). CTBT: Synergies with Science, 1996–2006 and Beyond. Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization. Archived from the original (PDF) on 2 July 2007. Retrieved 24 November 2007.
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  29. ^ Hsu, Christine (24 August 2012). "Man With World's Deepest Voice Hits Notes That Only Elephants Can Hear". Medical Daily. Retrieved 2 August 2016. American singer Tim Storms who also has the world's widest vocal range can reach notes as low as G-7 (0.189Hz) [...] so low that even Storms himself cannot hear it.
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  35. ^ Nature. Can Animals Predict Disaster? – PBS: posted November 2005.
  36. ^ Knight, Kathryn (2013). Disappearing homing Pigeon mystery solved. The Company of Biologists. Retrieved 2013-01-31
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  39. ^ King, Simon (12 June 2015). "Wind farm effect on balance 'akin to seasickness': scientist". News Corp Australia.
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  42. ^ Abbasi, Milad; Monnazzam, Mohammad Reza; Zakerian, SayedAbbolfazl; Yousefzadeh, Arsalan (June 2015). "Effect of Wind Turbine Noise on Workers' Sleep Disorder: A Case Study of Manjil Wind Farm in Northern Iran". Fluctuation and Noise Letters. 14 (2): 1550020. Bibcode:2015FNL....1450020A. doi:10.1142/S0219477515500200.
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  47. ^ The Pentagon considers ear-blasting anti-hijack gun — New Scientist
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  49. ^ Loud bass music ‘killed student’ Tom Reid, Metro, retrieved 18 June 2010
  50. ^ Tempest, W. Infrasound and low frequency vibration (1977). Academic Press Inc. (London) Ltd
  51. ^ ProSoundWeb: some effects of low end (bulletin board entry by Tom Danley)
  52. ^ The Matterhorn
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  57. ^ a b Infrasonic concert, Purcell Room, London, 31 May 2003, sponsored by the sciart Consortium with additional support by the National Physical Laboratory (NPL)
  58. ^ a b Sounds like terror in the air Sydney Morning Herald, 9 September 2003.
  59. ^ NASA Technical Report 19770013810
  60. ^ infrasound
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  62. ^ Tandy, V. (July 2000). "Something in the cellar" (PDF). Journal of the Society for Psychical Research. 64.3 (860). Archived from the original (PDF) on 29 September 2011.
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  66. ^ a b c d e Monitoring, Government of Canada, Natural Resources Canada, Nuclear Explosion. "IMS Infrasound Network". can-ndc.nrcan.gc.ca. Retrieved 25 April 2017.{{cite web}}: CS1 maint: multiple names: authors list (link)
  67. ^ a b Australia, c\=AU\;o\=Australia Government\;ou\=Geoscience (15 May 2014). "Infrasound Monitoring". www.ga.gov.au. Retrieved 25 April 2017.{{cite web}}: CS1 maint: multiple names: authors list (link)
  68. ^ "Infrasound monitoring: CTBTO Preparatory Commission". www.ctbto.org. Retrieved 25 April 2017.
  69. ^ a b Development and installation of an infrasonic wake vortex detection system By Qamar A. Shams and Allan J. Zuckerwar, NASA Langley Research Center, Hampton VA USA, WakeNet-Europe 2014, Bretigny, France.
  70. ^ a b c NASA Langley Researchers Nab Invention of the Year for Infrasound Detection System By Joe Atkinson, 2014, NASA Langley Research Center
  71. ^ Paul Harper (20 February 2013). "Meteor explosion largest infrasound recorded". The New Zealand Herald. APN Holdings NZ. Retrieved 31 March 2013.
Bibliography
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