Infrasound, sometimes referred to as low frequency sound, 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).[1] 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 20Hz down to 0.1Hz (and rarely to 0.001Hz). 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.[citation needed]
History and study
One of the pioneers in infrasonic research was French scientist Vladimir Gavreau.[2] 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 [these] 'nauseating vibrations'".[2]
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.[2] 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.[3][4][5] 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.
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:
Animal communication: whales, elephants,[13]hippopotamuses,[14]rhinoceroses,[15][16][17]giraffes,[18]okapis,[19]peacocks,[20] and alligators are known to use infrasound to communicate over distances—up to hundreds of miles in the case of whales. In particular, the Sumatran rhinoceros has been shown to produce sounds with frequencies as low as 3Hz which have similarities with the song of the humpback whale.[16] The roar of the tiger contains infrasound of 18Hz and lower,[21] and the purr of felines is reported to cover a range of 20 to 50Hz.[22][23][24] It has also been suggested that migrating birds use naturally generated infrasound, from sources such as turbulent airflow over mountain ranges, as a navigational aid.[25] Infrasound also may be used for long-distance communication, especially well documented in baleen whales (see whale vocalization), and African elephants.[26] The frequency of baleen whale sounds can range from 10 Hz to 31kHz,[27] and that of elephant calls from 15Hz to 35Hz. Both can be extremely loud (around 117dB), allowing communication for many kilometres, with a possible maximum range of around 10km (6mi) for elephants,[28] and potentially hundreds or thousands of kilometers for some whales.[citation needed] Elephants also produce infrasound waves that travel through solid ground and are sensed by other herds using their feet, although they may be separated by hundreds of kilometres. These calls may be used to coordinate the movement of herds and allow mating elephants to find each other.[29]
Human singers: some vocalists, including Tim Storms, can produce notes in the infrasound range.[30]
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.[34][35] 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.[36]
Research in 2013 by Jon Hagstrum of the US Geological Survey suggests that homing pigeons use low-frequency infrasound to navigate.[37]
20Hz 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 12Hz.[38] Below 10Hz it is possible to perceive the single cycles of the sound, along with a sensation of pressure at the eardrums.
From about 1000Hz, 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.[citation needed]
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.[39]
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.[40]
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.[41] Later studies, however, have linked inaudible infrasound to effects such as fullness, pressure or tinnitus, and acknowledged the possibility that it could disturb sleep.[42] 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.[43][44]
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".[45][46][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.[49]
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.[50]
Air is a very inefficient medium for transferring low frequency vibration from a transducer to the human body.[51] 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 160dB were achieved at frequencies of 2–3Hz. Test frequencies ranged from 0.5Hz to 40Hz. 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.[52][53][clarification needed]
The report "A Review of Published Research on Low Frequency Noise and its Effects"[54] 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 7.5Hz at 130dB 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 20Hz, for eight minutes at a time, at levels up to 144dB 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.[55]
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) - no more than 150 dB.[56]
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 5Hz in frequency and up to 153dB in sound pressure. They used 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."[58][59][60][61]
Infrasonic 17Hz 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 17Hz 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 17Hz tones played underneath.[62][63]
In the second concert, the pieces that were to carry a 17Hz 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 17Hz 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.[62][63]
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."[39]
Suggested relationship to ghost sightings
PsychologistRichard 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 19Hz 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 fencingfoil, 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 18.98Hz, very close to the resonant frequency of the eye given as 18Hz by NASA.[64] 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.[65]
Tandy investigated this phenomenon further and wrote a paper entitled The Ghost in the Machine.[66] 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[67][68] and Edinburgh Castle.[69][70]
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.[71] Electret-based technology offers the lowest possible background noise, because Johnson noise generated in the supporting electronics (preamplifier) is minimized.[71]
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.[72] 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.[72]
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.[72]
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).[73] IMS Infrasound stations consist of eight microbarometer sensors and space filters arranged in an array covering an area of approximately 1 to 9km2.[73][74] 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.[74] The microbarometers used are designed to monitor frequencies below approximately 20 hertz.[73] Sound waves below 20 hertz have longer wavelengths and are not easily absorbed, allowing for detection across large distances.[73]
Infrasound wavelengths can be generated artificially through detonations and other human activity, or naturally from earthquakes, severe weather, lightning, and other sources.[73] 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.[75]
Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries.
A subwoofer is a loudspeaker designed to reproduce low-pitched audio frequencies, known as bass and sub-bass, that are lower in frequency than those which can be (optimally) generated by a woofer. The typical frequency range that is covered by a subwoofer is about 20–200 Hz for consumer products, below 100 Hz for professional live sound, and below 80 Hz in THX-certified systems. Thus, one or more subwoofers are important for high-quality sound reproduction as they are responsible for the lowest two to three octaves of the ten octaves that are audible. This very low-frequency (VLF) range reproduces the natural fundamental tones of the bass drum, electric bass, double bass, grand piano, contrabassoon, tuba, in addition to thunder, gunshots, explosions, etc.
Noise pollution, or sound pollution, is the propagation of noise or sound with ranging impacts on the activity of human or animal life, most of which are harmful to a degree. The source of outdoor noise worldwide is mainly caused by machines, transport and propagation systems. Poor urban planning may give rise to noise disintegration or pollution, side-by-side industrial and residential buildings can result in noise pollution in the residential areas. Some of the main sources of noise in residential areas include loud music, transportation, lawn care maintenance, construction, electrical generators, wind turbines, explosions and people.
The low-frequency effects (LFE) channel is a band-limited audio track that is used for reproducing deep and intense low-frequency sounds in the 3–120 Hz frequency range.
Sonic and ultrasonic weapons (USW) are weapons of various types that use sound to injure or incapacitate an opponent. Some sonic weapons make a focused beam of sound or of ultrasound; others produce an area field of sound. As of 2023 military and police forces make some limited use of sonic weapons.
The brown note, also sometimes called the brown frequency or brown noise, is a hypothetical infrasonic frequency capable of causing fecal incontinence by creating acoustic resonance in the human bowel. Considered an urban myth, the name is a metonym for the common color of human faeces. Attempts to demonstrate the existence of a "brown note" using sound waves transmitted through the air have failed.
Sub-bass sounds are the deep, low-register pitches below approximately 70 Hz (C♯2 in scientific pitch notation) and extending downward to include the lowest frequency humans can hear, approximately 20 Hz (E0).
Hearing range describes the frequency range that can be heard by humans or other animals, though it can also refer to the range of levels. The human range is commonly given as 20 to 20,000 Hz, although there is considerable variation between individuals, especially at high frequencies, and a gradual loss of sensitivity to higher frequencies with age is considered normal. Sensitivity also varies with frequency, as shown by equal-loudness contours. Routine investigation for hearing loss usually involves an audiogram which shows threshold levels relative to a normal.
A tactile transducer or "bass shaker" is a device which is made on the principle that low bass frequencies can be felt as well as heard. They can be compared with a common loudspeaker, just that the diaphragm is missing. Instead, another object is used as a diaphragm. A shaker transmits low-frequency vibrations into various surfaces so that they can be felt by people. This is called tactile sound. Tactile transducers may augment or in some cases substitute for a subwoofer. One benefit of tactile transducers is they produce little or no noise, if properly installed, as compared with a subwoofer speaker enclosure.
A rotary woofer is a subwoofer-style loudspeaker which reproduces very low frequency content by using a conventional speaker voice coil's motion to change the pitch (angle) of the blades of an impeller rotating at a constant speed. The pitch of the fan blades is controlled by the audio signal presented to the voice coil, and is able to swing both positive and negative, with respect to a zero pitch spinning blade position. Since the audio amplifier only changes the pitch of the blades, it takes much less power, per dB of generated acoustic sound level, to drive a rotary woofer than to power a conventional subwoofer, which uses a moving electromagnet placed within the field of a stationary permanent magnet to drive a cone which then displaces air. Rotary woofers excel at producing sounds below 20 Hz, below the normal hearing range; when installed in the wall of a sealed room, they can produce audio frequencies below 1 Hz, a static pressure differential, by simply compressing the air in the sealed room.
Electronic pest control is the name given to any of several types of electrically powered devices designed to repel or eliminate pests, usually rodents or insects. Since these devices are not regulated under the Federal Insecticide, Fungicide, and Rodenticide Act in the United States, the EPA does not require the same kind of efficacy testing that it does for chemical pesticides.
In acoustics, microbaroms, also known as the "voice of the sea", are a class of atmospheric infrasonic waves generated in marine storms by a non-linear interaction of ocean surface waves with the atmosphere. They typically have narrow-band, nearly sinusoidal waveforms with amplitudes up to a few microbars, and wave periods near 5 seconds. Due to low atmospheric absorption at these low frequencies, microbaroms can propagate thousands of kilometers in the atmosphere, and can be readily detected by widely separated instruments on the Earth's surface.
In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Only acoustic waves that have frequencies lying between about 20 Hz and 20 kHz, the audio frequency range, elicit an auditory percept in humans. In air at atmospheric pressure, these represent sound waves with wavelengths of 17 meters (56 ft) to 1.7 centimeters (0.67 in). Sound waves above 20 kHz are known as ultrasound and are not audible to humans. Sound waves below 20 Hz are known as infrasound. Different animal species have varying hearing ranges.
Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is auditory science.
In geophysics, geology, civil engineering, and related disciplines, seismic noise is a generic name for a relatively persistent vibration of the ground, due to a multitude of causes, that is often a non-interpretable or unwanted component of signals recorded by seismometers.
Seismic or vibrational communication is a process of conveying information through mechanical (seismic) vibrations of the substrate. The substrate may be the earth, a plant stem or leaf, the surface of a body of water, a spider's web, a honeycomb, or any of the myriad types of soil substrates. Seismic cues are generally conveyed by surface Rayleigh or bending waves generated through vibrations on the substrate, or acoustical waves that couple with the substrate. Vibrational communication is an ancient sensory modality and it is widespread in the animal kingdom where it has evolved several times independently. It has been reported in mammals, birds, reptiles, amphibians, insects, arachnids, crustaceans and nematode worms. Vibrations and other communication channels are not necessarily mutually exclusive, but can be used in multi-modal communication.
Infrasound is sound at frequencies lower than the low frequency end of human hearing threshold at 20 Hz. It is known, however, that humans can perceive sounds below this frequency at very high pressure levels. Infrasound can come from many natural as well as man-made sources, including weather patterns, topographic features, ocean wave activity, thunderstorms, geomagnetic storms, earthquakes, jet streams, mountain ranges, and rocket launchings. Infrasounds are also present in the vocalizations of some animals. Low frequency sounds can travel for long distances with very little attenuation and can be detected hundreds of miles away from their sources.
A nuclear detonation detection system (NDDS) is a device or a series of devices that are able to indicate, and pinpoint a nuclear explosion has occurred as well as the direction of the explosion. The main purpose of these devices or systems was to verify compliance of countries that signed nuclear treaties such as the Partial Test Ban treaty of 1963 (PTBT) and the Treaty of Tlatelolco.
The infrasonic sensing array, designed by the U.S. Army Research Laboratory (ARL), was a military device that detected and located explosive events with inaudible frequencies at long ranges, such as artillery, missiles, and helicopters through the use of the array process. It was also used for direction-finding and positioning for navigational purposes and for detecting atmospheric events for battlefield weather prediction.
Elephants communicate via touching, visual displays, vocalisations, seismic vibrations, and semiochemicals.
↑ Garces, M.; Willis, M. (2006). Modeling and Characterization of Microbarom Signals in the Pacific (Report). 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.
↑ "Microbaroms". Infrasonic Signals. University of Alaska Fairbanks, Geophysical Institute, Infrasound Research Group. Archived from the original on 15 February 2008. Retrieved 22 November 2007. The ubiquitous five-second-period infrasonic signals called "microbaroms", which are generated by standing sea waves in marine storms, are the cause of the low-level natural-infrasound background in the passband from 0.02 to 10Hz.
↑ Payne, Katharine B.; Langbauer, William R.; Thomas, Elizabeth M. (1986). "Infrasonic calls of the Asian elephant (Elephas maximus)". Behavioral Ecology and Sociobiology. 18 (4): 297–301. doi:10.1007/BF00300007. S2CID1480496.
↑ von Muggenthaler, E.K.; Stoughton, J.W.; Daniel, J.C., Jr. (1993). "Infrasound from the rhinocerotidae". In Ryder, O.A. (ed.). Rhinoceros biology and conservation: Proceedings of an international conference, San Diego, U.S.A. Zoological Society of San Diego. pp.136–140.
↑ Marrin, West (3 November 2004). Infrasonic signals in the environment(PDF). Acoustics 2004: Transportation Noise & Vibration — The New Millennium. Gold Coast, Australia: Australian Acoustical Society. Archived from the original(PDF) on 26 March 2012.
↑ E. Von Muggenthaler: Infrasound from the okapi, invited presentation, student competition award, proceedings from the 1992 American Association for the Advancement of Science (A.A.A.S) 158th conference, 1992
↑ Work by Muggenthaler et al, also referred to in: The Secret Of A Tiger's Roar, ScienceDaily, 1 December 2000, American Institute of Physics, Inside Science News Service (1 December 2000), Retrieved 25 December 2011
↑ Von Muggenthaler, E., Perera, D. (2002), The cat's purr: a healing mechanism?, In review, presented 142nd Acoustical Society of America International Conference, 2001.
↑ von Muggenthaler, (2006) The Felid Purr: A Biomechanical Healing Mechanism, Proceedings from the 12th International Low Frequency Noise and Vibration Conference, pp. 189–208
↑ W. John Richardson; Charles R. Greene, Jr.; Charles I. Malme; Denis H. Thomson (1995). Marine Mammals and Noise. Academic Press. ISBN978-0-12-588440-2.
↑ 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.
↑ Salt, Alec N.; Kaltenbach, James A. (19 July 2011). "Infrasound From Wind Turbines Could Affect Humans". Bulletin of Science, Technology & Society. 31 (4): 296–302. doi:10.1177/0270467611412555. S2CID110190618.
↑ 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.
↑ Abbasi, Milad; Monnazzam, Mohammad Reza; Zakerian, Sayedabbolfazl; Yousefzadeh, Arsalan (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.
↑ 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.
↑ Arnot, Chris (11 July 2000). "Ghost buster". The Guardian. London.
↑ Lowe, Justin (3 October 2017). "'The Sound': Film Review". The Hollywood Reporter. Retrieved 29 March 2021. Continuing her search, Kelly soon encounters ghostly apparitions and overwhelming low-frequency sound waves below the threshold of human hearing that emanate from a source she's unable to identify with her audio gear.
Sub-surface windscreen for the measurement of outdoor infrasound Qamar A. Shams, Cecil G. Burkett and Toby Comeaux NASA Langley Research Center, Allan J. Zuckerwar Analytical Services and Material, and George R. Weistroffer Virginia Commonwealth University
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
