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67-457: Infrasound , sometimes referred to as low frequency sound or subsonic , describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz , as defined by the ANSI/ASA S1.1-2013 standard). 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

134-442: A specific range of frequencies . The audible frequency range for humans is typically given as being between about 20 Hz and 20,000 Hz (20 kHz), though the high frequency limit usually reduces with age. Other species have different hearing ranges. For example, some dog breeds can perceive vibrations up to 60,000 Hz. In many media, such as air, the speed of sound is approximately independent of frequency, so

201-413: A "loudness" button, known technically as loudness compensation , that boosts low and high-frequency components of the sound. These are intended to offset the apparent loudness fall-off at those frequencies, especially at lower volume levels. Boosting these frequencies produces a flatter equal-loudness contour that appears to be louder even at low volume, preventing the perceived sound from being dominated by

268-687: A 1933 paper entitled "Loudness, its definition, measurement and calculation" in the Journal of the Acoustical Society of America . Fletcher–Munson curves have been superseded and incorporated into newer standards. The definitive curves are those defined in ISO 226 from the International Organization for Standardization , which are based on a review of modern determinations made in various countries. Amplifiers often feature

335-552: A ghost—our findings support these ideas." 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

402-435: 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

469-400: A material having a low acoustic impedance and has a sufficiently thick wall to ensure structural stability. 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

536-496: 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. 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

603-635: A murder weapon. The 'ghost frequency' phenomenon is mentioned in Season 3 Episode 4 of the TV Series Evil , The Demon of the Road. Audio frequency The generally accepted standard hearing range for humans is 20 to 20,000 Hz. In air at atmospheric pressure, these represent sound waves with wavelengths of 17 metres (56 ft) to 1.7 centimetres (0.67 in). Frequencies below 20 Hz are generally felt rather than heard, assuming

670-504: 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. 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

737-519: 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

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804-402: A property utilized in the design and fabrication of the system windscreens. 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

871-593: A study coordinated by the Research Institute of Electrical Communication, Tohoku University, Japan. The study produced new curves by combining the results of several studies—by researchers in Japan, Germany, Denmark, UK, and the US. (Japan was the greatest contributor with about 40% of the data.) This has resulted in the recent acceptance of a new set of curves standardized as ISO 226:2003. The report comments on

938-538: 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. Infrasound is one hypothesized cause of death for the nine Soviet hikers who were found dead at Dyatlov Pass in 1959. US: Maximum levels for frequencies from 1 to 80 Hz are no more than 145 dB. Overall level (for all frequencies) - no more than 150 dB. The brown note

1005-404: A type 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." 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

1072-459: 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

1139-451: 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

1206-454: Is arrived at by reference to equal-loudness contours. By definition, two sine waves of differing frequencies are said to have equal-loudness level measured in phons if they are perceived as equally loud by the average young person without significant hearing impairment. The Fletcher–Munson curves are one of many sets of equal-loudness contours for the human ear, determined experimentally by Harvey Fletcher and Wilden A. Munson, and reported in

1273-466: 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 1000 Hz, the dynamic range of the auditory system decreases with decreasing frequency. This compression

1340-429: 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

1407-400: Is not good enough, given the steep rise in loudness (rising to as much as 24 dB per octave) with frequency revealed by the equal-loudness curves below about 100 Hz. A good experimenter must ensure that trial subjects really hear the fundamental and not harmonics—especially the third harmonic, which is especially strong as a speaker cone's travel becomes limited as its suspension reaches

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1474-473: 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

1541-1059: Is tasked with monitoring compliance with the Comprehensive Nuclear Test-Ban Treaty (CTBT). IMS Infrasound stations consist of eight microbarometer sensors and space filters arranged in an array covering an area of approximately 1 to 9 km. 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. The microbarometers used are designed to monitor frequencies below approximately 20 hertz. Sound waves below 20 hertz have longer wavelengths and are not easily absorbed, allowing for detection across large distances. Infrasound wavelengths can be generated artificially through detonations and other human activity, or naturally from earthquakes, severe weather, lightning, and other sources. Like forensic seismology , algorithms and other filter techniques are required to analyze gathered data and characterize events to determine if

1608-543: The amplitude of the vibration is great enough. Sound frequencies above 20 kHz are called ultrasonic . Sound propagates as mechanical vibration waves of pressure and displacement, in air or other substances. In general, frequency components of a sound determine its "color", its timbre . When speaking about the frequency (in singular) of a sound, it means the property that most determines its pitch . Higher pitches have higher frequency, and lower pitches are lower frequency. The frequencies an ear can hear are limited to

1675-421: The human ear is most sensitive between 2 and 5 kHz , largely due to the resonance of the ear canal and the transfer function of the ossicles of the middle ear. Fletcher and Munson first measured equal-loudness contours using headphones (1933). In their study, test subjects listened to pure tones at various frequencies and over 10 dB increments in stimulus intensity. For each frequency and intensity,

1742-517: The 1960s, in particular as part of the DIN 4550 standard for audio quality measurement , which differed from the A-weighting curve, showing more of a peak around 6 kHz. These gave a more meaningful subjective measure of noise on audio equipment, especially on the newly invented compact cassette tape recorders with Dolby noise reduction, which were characterized by a noise spectrum dominated by

1809-430: The 40-phon Fletcher–Munson curve. However, research in the 1960s demonstrated that determinations of equal-loudness made using pure tones are not directly relevant to our perception of noise. This is because the cochlea in our inner ear analyzes sounds in terms of spectral content, each "hair-cell" responding to a narrow band of frequencies known as a critical band . The high-frequency bands are wider in absolute terms than

1876-563: The Fletcher–Munson curves. The report states that it is fortunate that the 40- phon Fletcher–Munson curve on which the A-weighting standard was based turns out to have been in agreement with modern determinations. The report also comments on the large differences apparent in the low-frequency region, which remain unexplained. Possible explanations are: Real-life sounds from a reasonably distant source arrive as planar wavefronts. If

1943-484: 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. 20 Hz

2010-537: The basement of the Tourist Information Bureau next to Coventry Cathedral and Edinburgh Castle . 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. Electret-based technology offers

2077-462: The basis for a standard (ISO 226) that was considered definitive until 2003, when ISO revised the standard on the basis of recent assessments by research groups worldwide. Perceived discrepancies between early and more recent determinations led the International Organization for Standardization (ISO) to revise the standard curves in ISO ;226. They did this in response to recommendations in

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2144-429: The basis for an ISO 226 standard. The generic term equal-loudness contours is now preferred, of which the Fletcher–Munson curves are now a sub-set, and especially since a 2003 survey by ISO redefined the curves in a new standard. The human auditory system is sensitive to frequencies from about 20  Hz to a maximum of around 20,000 Hz, although the upper hearing limit decreases with age. Within this range,

2211-429: 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

2278-423: The ear, provide a flat low-frequency pressure response to the ear canal, with low distortion even at high intensities. At low frequencies, the ear is purely pressure-sensitive, and the cavity formed between headphones and ear is too small to introduce modifying resonances. Headphone testing is, therefore, a good way to derive equal-loudness contours below about 500 Hz, though reservations have been expressed about

2345-661: 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

2412-467: 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. In the second concert,

2479-557: 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

2546-433: 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. This, Tandy conjectured,

2613-536: The headphone cavity. With speakers, the opposite is true. A flat low-frequency response is hard to obtain—except in free space high above ground, or in a very large and anechoic chamber that is free from reflections down to 20 Hz. Until recently, it was not possible to achieve high levels at frequencies down to 20 Hz without high levels of harmonic distortion . Even today, the best speakers are likely to generate around 1 to 3% of total harmonic distortion, corresponding to 30 to 40 dB below fundamental. This

2680-427: The higher frequencies. BBC Research conducted listening trials in an attempt to find the best weighting curve and rectifier combination for use when measuring noise in broadcast equipment, examining the various new weighting curves in the context of noise rather than tones, confirming that they were much more valid than A-weighting when attempting to measure the subjective loudness of noise. This work also investigated

2747-509: 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

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2814-415: 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

2881-410: The introduction of sound-proofing and materials with specialized sonic properties. Infrasound can result from both natural and man-made sources: 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

2948-491: The limit of compliance. A possible way around the problem is to use acoustic filtering, such as by resonant cavity, in the speaker setup. A flat free-field high-frequency response up to 20 kHz, on the other hand, is comparatively easy to achieve with modern speakers on-axis. These effects must be considered when comparing results of various attempts to measure equal-loudness contours. The A-weighting curve—in widespread use for noise measurement —is said to have been based on

3015-401: The listener also listened to a reference tone at 1000 Hz. Fletcher and Munson adjusted the reference tone until the listener perceived that it had the same loudness as the test tone. Loudness, being a psychological quantity, is difficult to measure, so Fletcher and Munson averaged their results over many test subjects to derive reasonable averages. The lowest equal-loudness contour represents

3082-480: The low-frequency bands, and therefore "collect" proportionately more power from a noise source. However, when more than one critical band is stimulated, the signals to the brain add the various bands to produce the impressions of loudness. For these reasons equal-loudness curves derived using noise bands show an upwards tilt above 1 kHz and a downward tilt below 1 kHz when compared to the curves derived using pure tones. Various weighting curves were derived in

3149-407: The lowest possible background noise, because Johnson noise generated in the supporting electronics (preamplifier) is minimized. 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

3216-403: The mid-frequencies where the ear is most sensitive. The first research on the topic of how the ear hears different frequencies at different levels was conducted by Fletcher and Munson in 1933. Until recently, it was common to see the term Fletcher–Munson used to refer to equal-loudness contours generally, even though a re-determination was carried out by Robinson and Dadson in 1956, which became

3283-644: The monitoring system was generated by the 2013 Chelyabinsk meteor . The 2017 film The Sound uses infrasound as a major plot element. In "Fermata", a 2020 episode of the Franco-Belgian TV series Astrid et Raphaëlle , infrasound from a generator hidden in the pipe organ of the Grand Auditorium in the Maison de la Radio et de la Musique , the Paris headquarters of Radio France , is used as

3350-495: The other ear. This combined effect of head-masking and pinna reflection is quantified in a set of curves in three-dimensional space referred to as head-related transfer functions (HRTFs). Frontal presentation is now regarded as preferable when deriving equal-loudness contours, and the latest ISO standard is specifically based on frontal and central presentation. Because no HRTF is involved in normal headphone listening, equal-loudness curves derived using headphones are valid only for

3417-497: 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 –

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3484-439: 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. In presenting

3551-410: 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

3618-417: 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 (and rarely to 0.001 Hz). People use this frequency range for monitoring earthquakes and volcanoes, charting rock and petroleum formations below

3685-415: The quietest audible tone—the absolute threshold of hearing . The highest contour is the threshold of pain . Churcher and King carried out a second determination in 1937, but their results and Fletcher and Munson's showed considerable discrepancies over parts of the auditory diagram. In 1956 Robinson and Dadson produced a new experimental determination that they believed was more accurate. It became

3752-700: The response of human hearing to tone-bursts, clicks, pink noise and a variety of other sounds that, because of their brief impulsive nature, do not give the ear and brain sufficient time to respond. The results were reported in BBC Research Report EL-17 1968/8 entitled The Assessment of Noise in Audio Frequency Circuits . The ITU-R 468 noise weighting curve, originally proposed in CCIR recommendation 468, but later adopted by numerous standards bodies ( IEC , BSI , JIS , ITU )

3819-399: The size. 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

3886-411: The source of sound is directly in front of the listener, then both ears receive equal intensity, but at frequencies above about 1 kHz the sound that enters the ear canal is partially reduced by the head shadow , and also highly dependent on reflection off the pinna (outer ear). Off-centre sounds result in increased head masking at one ear, and subtle changes in the effect of the pinna, especially at

3953-524: The special case of what is called side-presentation , which is not how we normally hear. The Robinson–Dadson determination used loudspeakers , and for a long time the difference from the Fletcher–Munson curves was explained partly on the basis that the latter used headphones. However, the ISO report actually lists the latter as using compensated headphones, though it doesn't make clear how Robinson–Dadson achieved compensation . Good headphones, well sealed to

4020-407: 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

4087-470: The surprisingly large differences, and the fact that the original Fletcher–Munson contours are in better agreement with recent results than the Robinson–Dadson, which appear to differ by as much as 10–15 dB, especially in the low-frequency region, for reasons not explained. According to the ISO report, the Robinson–Dadson results were the odd one out, differing more from the current standard than did

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4154-449: The validity of headphone measurements when determining the actual threshold of hearing, based on the observation that closing off the ear canal produces increased sensitivity to the sound of blood flow within the ear, which the brain appears to mask in normal listening conditions. At high frequencies, headphone measurement becomes unreliable, and the various resonances of pinnae (outer ears) and ear canals are severely affected by proximity to

4221-409: The wavelength of the sound waves (distance between repetitions) is approximately inversely proportional to frequency. Equal-loudness contour An equal-loudness contour is a measure of sound pressure level , over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon and

4288-493: 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

4355-411: 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 [these] '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

4422-424: 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

4489-458: 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

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