Infrasound

Infrasound, also known as low frequency sound or, occasionally ambiguously, subsonic (a descriptor for "less than the speed of sound"), refers to sound waves with frequencies below the typical threshold of human hearing, generally defined as 20 Hz by the ANSI/ASA S1.1-2013 standard. As frequency diminishes, human auditory sensitivity progressively decreases, necessitating a sufficiently high sound pressure level for infrasound perception. While the ear serves as the primary organ for detecting low-frequency sounds, elevated intensities of infrasound can induce perceptible vibrations throughout the body.

Infrasound, sometimes referred to as low frequency sound or (sometimes ambigously) subsonic (subsonic being a descriptor for "less than the speed of 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). 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 termed infrasonics, encompassing frequencies from below 20 Hz, typically down to 0.1 Hz, and occasionally extending to 0.001 Hz. This frequency spectrum is utilized for diverse applications, including seismic and volcanic activity monitoring, subsurface geological mapping for rock and petroleum exploration, and in medical diagnostics such as ballistocardiography and seismocardiography to analyze human cardiovascular mechanics.

Infrasound is characterized by its ability to circumvent obstacles with minimal energy loss. In musical contexts, acoustic waveguide techniques, exemplified by large pipe organs, or specialized loudspeaker designs for reproduction, such as transmission line, rotary woofer, or conventional subwoofer systems, can generate low-frequency sounds, including those approaching the infrasonic range. Subwoofers specifically engineered for infrasound generation can reproduce frequencies an octave or more lower than typical commercial subwoofers, frequently exhibiting a tenfold increase in physical dimensions.

History and Study

French scientist Vladimir Gavreau is recognized as a pioneer in infrasonic research. His initial interest in infrasonic waves emerged in 1957 within the substantial concrete facility where he and his research team were conducting their work. The team reported experiencing recurring episodes of profound and distressing nausea. Following weeks of speculation regarding the etiology of the nausea—with the team initially suspecting a pathogen or an undetected release of noxious chemical fumes within the premises—they ultimately identified a "loosely poised low speed motor" as the source, which was generating these 'nauseating vibrations'.

Upon attempting to measure the amplitude and pitch, Gavreau and his team were surprised to find that their equipment registered no audible sound. They deduced that the motor-generated sound possessed a pitch too low for human auditory perception, and that their recording apparatus lacked the capability to detect such frequencies. The existence of sound at such low frequencies had not been previously conceptualized, consequently precluding the development of detection equipment. Ultimately, the nauseating sound was identified as a 7 Hz infrasound wave, which was inducing a resonant mode within the building's ductwork and architectural structures, thereby substantially amplifying the sound. Following this serendipitous discovery, the researchers promptly commenced preparations for additional infrasonic experiments in their laboratories. One notable experiment involved an infrasonic whistle, constructed as an oversized organ pipe. Consequently, due to this and analogous occurrences, it has become standard practice in contemporary architectural construction to inspect for and mitigate infrasonic resonances within cavities, and to incorporate sound-proofing and materials possessing specialized sonic characteristics.

Sources

Infrasound can originate from both natural phenomena and anthropogenic activities:

• Natural phenomena: Infrasonic sound frequently arises from natural occurrences such as severe weather, ocean surf, lee waves, avalanches, earthquakes, volcanic eruptions, bolides, waterfalls, iceberg calving, aurorae, meteors, and various forms of lightning, including upper-atmospheric lightning. Nonlinear interactions of ocean waves during storms generate pervasive infrasound vibrations, approximately 0.2 Hz, which are termed microbaroms. The Infrasonics Program at NOAA indicates that infrasonic arrays are deployable for locating avalanches in the Rocky Mountains and for detecting tornadoes on the high plains several minutes prior to their touchdown.

• Animal communication: Various animal species, including whales, elephants, hippopotamuses, rhinoceroses, giraffes, okapis, peacocks, and alligators, are recognized for their use of infrasound in long-distance communication, with whales communicating over hundreds of miles. Specifically, the Sumatran rhinoceros produces sounds as low as 3 Hz, exhibiting similarities to the humpback whale's song. Tigers' roars incorporate infrasound at 18 Hz and below, while feline purrs reportedly span 20 to 50 Hz. Furthermore, migrating birds are hypothesized to utilize naturally occurring infrasound, such as that generated by turbulent airflow over mountain ranges, for navigation. Infrasound facilitates long-distance communication, particularly well-documented in baleen whales and African elephants. Baleen whale vocalizations range from 10 Hz to 31 kHz, and elephant calls from 15 Hz to 35 Hz. Both types of vocalizations can be exceptionally loud (approximately 117 dB), enabling communication across many kilometers; elephants may communicate up to 10 km (6 mi), while some whales potentially communicate over hundreds or thousands of kilometers. Elephants also generate infrasound waves that propagate through the ground, detected by other herds via their feet, even when separated by hundreds of kilometers. These calls likely serve to coordinate herd movements and facilitate mate location.

• Human singers: Certain vocalists, such as Tim Storms, possess the ability to generate notes within the infrasound spectrum.

• Anthropogenic sources: Infrasound can originate from human activities, including sonic booms and both chemical and nuclear explosions, or from mechanical apparatuses like diesel engines, wind turbines, and purpose-built mechanical transducers (e.g., industrial vibration tables). Additionally, specific loudspeaker configurations are capable of reproducing exceptionally low frequencies, encompassing large-scale rotary woofer subwoofers, along with substantial horn-loaded, bass-reflex, sealed, and transmission line loudspeakers.

Animal responses

Certain animal species are hypothesized to detect infrasonic waves propagating through the Earth, which are generated by natural disasters, and to utilize these as an early warning mechanism. A notable instance is the 2004 Indian Ocean earthquake and tsunami, where animals reportedly evacuated affected regions hours prior to the tsunami's impact on Asian coastlines. However, the definitive cause remains unconfirmed; alternative theories propose that electromagnetic waves, rather than infrasonic waves, may have instigated these animals' flight.

Research conducted in 2013 by Jon Hagstrum of the U.S. Geological Survey indicates that homing pigeons employ low-frequency infrasound for navigational purposes.

Human physiological responses

The conventional lower limit of human auditory perception is established at 20 Hz. Nevertheless, under optimal conditions and at elevated volumes, individuals can discern pure sine wave tones as low as 12 Hz. Frequencies below 10 Hz may be perceived as individual sound cycles, accompanied by a sensation of pressure within the eardrums.

Commencing from approximately 1,000 Hz, the auditory system's dynamic range diminishes as frequency decreases. This phenomenon of compression is evident in equal-loudness-level contours, suggesting that even minor increases in sound level can transform perceived loudness from barely perceptible to pronounced. When considered alongside the inherent variability in auditory thresholds across a population, this effect implies that a very low-frequency sound, inaudible to some individuals, might be perceived as loud by others.

A particular study has posited that infrasound could induce sensations of awe or fear in humans. Furthermore, it has been hypothesized that because infrasound is not consciously detected, it might contribute to a vague perception among individuals that unusual or supernatural occurrences are unfolding.

A researcher affiliated with Sydney University's Auditory Neuroscience Laboratory indicates accumulating evidence suggesting that infrasound might impact the human nervous system through vestibular system stimulation, an effect demonstrated in animal models to be analogous to motion sickness.

A 2006 study investigating the effects of wind turbine sound emissions on proximate communities linked perceived infrasound to symptoms like annoyance and fatigue, varying with intensity, with minimal evidence for physiological impacts of infrasound below the threshold of human perception. Subsequent research, however, has connected inaudible infrasound to sensations such as aural fullness, pressure, or tinnitus, and recognized its potential to disrupt sleep patterns. Furthermore, additional investigations have indicated correlations between turbine noise levels and self-reported sleep disturbances among nearby residents, though the precise contribution of infrasound to this phenomenon remains incompletely elucidated.

In a study conducted at Ibaraki University in Japan, researchers reported that electroencephalogram (EEG) tests demonstrated that infrasound generated by wind turbines constituted a source of annoyance for technicians operating in close proximity to contemporary large-scale units.

Jürgen Altmann of the Technical University of Dortmund, an expert in sonic weaponry, has stated that robust evidence is lacking to substantiate infrasound as a cause of nausea and vomiting.

Elevated sound pressure levels from subwoofer arrays at musical performances have been implicated in instances of lung collapse, particularly among individuals in close proximity to the subwoofers, with a heightened susceptibility noted in tall, slender smokers.

In September 2009, a London student, Tom Reid, succumbed to sudden arrhythmic death syndrome (SADS) in a nightclub, having previously reported that intense bass frequencies from the venue's speakers were affecting his heart. The subsequent inquest concluded with a verdict of natural causes, though certain specialists posited that the bass frequencies might have served as a precipitating factor.

Atmospheric transmission proves highly inefficient for conveying low-frequency vibrations from a transducer to the human organism. Conversely, direct mechanical coupling of the vibration source to the human body presents a potentially hazardous scenario. Concerned about the detrimental impacts of rocket flight on astronauts, the U.S. space program commissioned vibration experiments. These involved mounting cockpit seats on vibration tables to directly transmit "brown note" and other frequencies to human participants. Sound pressure levels reaching 160 dB were attained at frequencies between 2 and 3 Hz. The tested frequency spectrum spanned from 0.5 Hz to 40 Hz. Participants experienced symptoms including motor ataxia, nausea, visual disturbances, diminished task performance, and communication difficulties. Researchers hypothesize that these experiments form the foundational basis for the contemporary urban legend concerning the "brown note" and its purported physiological consequences.

The publication titled "A Review of Published Research on Low Frequency Noise and its Effects" compiles extensive studies on high-level infrasound exposure in both human and animal populations. For example, in 1972, Borredon subjected 42 young men to 7.5 Hz tones at 130 dB for a duration of 50 minutes. This exposure elicited no adverse effects beyond self-reported drowsiness and a minor elevation in blood pressure. Subsequently, in 1975, Slarve and Johnson exposed four male participants to infrasound ranging from 1 to 20 Hz, for eight-minute intervals, at sound pressure levels up to 144 dB. No detrimental effects were observed, apart from slight ear discomfort. Experiments involving high-intensity infrasound on animals yielded quantifiable alterations, including cellular modifications and ruptured blood vessel walls.

Infrasound has been proposed as a potential cause of death for the nine Soviet hikers discovered deceased at Dyatlov Pass in 1959.

Occupational Health Standards

United States: The maximum permissible sound pressure levels for frequencies ranging from 1 to 80 Hz are capped at 145 dB. The cumulative sound pressure level across all frequencies must not exceed 150 dB.

The "Brown Note" Phenomenon

The "brown note" refers to a hypothetical infrasonic frequency purportedly capable of inducing fecal incontinence through the generation of acoustic resonance within the human bowel. Experimental efforts to substantiate the existence of a "brown note" via airborne sound waves have proven unsuccessful.

In February 2005, the television program MythBusters undertook an investigation to ascertain the veracity of the "brown note" phenomenon. Their experiments involved testing frequencies as low as 5 Hz and sound pressure levels reaching 153 dB. The equipment utilized included a subwoofer typically employed for large-scale rock concerts, specifically modified to achieve enhanced bass extension. The purported physiological effects failed to manifest. Consequently, the program concluded that the "brown note" myth was unsubstantiated.

Infrasonic 17 Hz Tone Experimentation

On May 31, 2003, British researchers conducted a large-scale experiment involving approximately 700 participants. These individuals were exposed to music subtly infused with 17 Hz sine waves, generated at a barely perceptible volume by an extended-stroke subwoofer positioned within a seven-meter plastic sewer pipe. The experimental concert, named Infrasonic, was hosted at the Purcell Room⁠‍—‍‌, a performance venue within Central London's Southbank Centre⁠‍—‍‌. The event comprised two performances, each featuring four distinct musical compositions. Notably, two pieces in each concert incorporated the underlying 17 Hz tones.

During the subsequent concert, the musical selections containing the 17 Hz undertone were interchanged to prevent any bias in the test results toward a particular composition. Participants remained unaware of which pieces contained the subtle 17 Hz near-infrasonic tone. The introduction of this tone led to a notable proportion (22%) of respondents reporting sensations of unease, sorrow, chills, or nervous feelings of revulsion or fear.

Presenting these findings to the British Association for the Advancement of Science, Professor Richard Wiseman stated, "These results indicate that low-frequency sound can induce unusual experiences in individuals, even when infrasound is not consciously perceived. Certain scientists have theorized that such sound levels might exist in purportedly haunted locations, thereby eliciting peculiar sensations that people attribute to spectral entities—our research corroborates these hypotheses."

Proposed Correlation with Apparitional Sightings

Richard Wiseman, a psychologist at the University of Hertfordshire, posits that infrasonic vibrations could be responsible for the unusual sensations individuals often attribute to ghosts. In 1998, Vic Tandy, an experimental officer and part-time lecturer in the School of International Studies and Law at Coventry University, collaborated with Dr.Tony Lawrence from the University's psychology department to author a paper titled "Ghosts in the Machine," published in the Journal of the Society for Psychical Research. Their investigation proposed that a 19 Hz infrasonic signal might account for certain ghost sightings. Tandy recounted an incident where, while working alone late one night in a purportedly haunted laboratory at Warwick, he experienced intense anxiety and perceived a grey amorphous shape in his peripheral vision. Upon directly looking at the perceived shape, it vanished.

The subsequent day, Tandy was engaged in work on his fencing foil, which was secured by its handle in a vice. Despite no external contact, the blade began to vibrate erratically. Subsequent inquiry revealed that the laboratory's extractor fan was emitting a frequency of 18.98 Hz, a value remarkably close to the eye's resonant frequency, which NASA has identified as 18 Hz. Tandy hypothesized that this phenomenon explained his earlier perception of a ghostly figure, attributing it to an optical illusion resulting from the resonance of his eyeballs. Furthermore, the room's dimensions, being precisely half a wavelength in length with the desk centrally positioned, created a standing wave that induced the foil's vibration.

Tandy subsequently conducted further research into this phenomenon, culminating in a paper titled The Ghost in the Machine. His investigations extended to several locations reputed to be haunted, such as the basement of the Tourist Information Bureau adjacent to Coventry Cathedral and Edinburgh Castle.

Infrasound Detection and Measurement Methodologies

NASA Langley has engineered and implemented an infrasonic detection system capable of performing valuable infrasound measurements in environments previously inaccessible for such data collection. This system incorporates an electret condenser microphone, specifically the PCB Model 377M06, featuring a 3-inch membrane diameter, alongside a compact windscreen. The utilization of electret-based technology ensures minimal background noise, primarily by reducing Johnson noise produced within the associated electronics, such as the preamplifier.

The microphone incorporates high membrane compliance, a substantial backchamber volume, a prepolarized backplane, and a high-impedance preamplifier internally situated within the backchamber. The windscreen, leveraging infrasound's high transmission coefficient through various media, is fabricated from a low acoustic impedance material and possesses a wall of adequate thickness to ensure structural integrity. Closed-cell polyurethane foam has demonstrated efficacy for this application. The proposed testing protocol will evaluate key parameters including sensitivity, background noise, signal fidelity (specifically harmonic distortion), and temporal stability.

The microphone design diverges from conventional audio system designs by specifically integrating the unique characteristics of infrasound. Firstly, infrasound traverses extensive distances within the Earth's atmosphere, facilitated by minimal atmospheric absorption and refractive ducting, which enables propagation via multiple reflections between the Earth's surface and the stratosphere. A less frequently acknowledged characteristic is infrasound's substantial penetration capability through solid materials, a property leveraged in the design and construction of the system's windscreens.

Consequently, this system addresses several instrumentation prerequisites that are beneficial for acoustic applications. Firstly, it incorporates a low-frequency microphone exhibiting exceptionally low background noise, thereby facilitating the detection of subtle signals within a low-frequency passband. Secondly, a compact windscreen design allows for the rapid deployment of a microphone array in diverse field environments. Furthermore, the system integrates a data acquisition component capable of real-time detection, bearing determination, and signature analysis of low-frequency sources.

Infrasound in Nuclear Detonation Detection

Infrasound constitutes one of several methodologies employed to ascertain the occurrence of nuclear detonations. The International Monitoring System (IMS), responsible for verifying adherence to the Comprehensive Nuclear Test-Ban Treaty (CTBT), integrates a network of 53 infrasound stations alongside seismic and hydroacoustic stations. Each IMS infrasound station comprises an array of eight microbarometer sensors and space filters, distributed over an area ranging from approximately 1 to 9 km². These space filters, characterized by radiating pipes with strategically placed inlet ports, are engineered to mitigate pressure fluctuations, such as wind turbulence, thereby enabling more accurate measurements. The microbarometers employed are specifically calibrated to detect frequencies below approximately 20 hertz. Infrasonic waves, with wavelengths exceeding those of audible sound, exhibit reduced atmospheric absorption, facilitating their detection over considerable distances.

Infrasonic waves can originate from anthropogenic sources, such as detonations and other human activities, or from natural phenomena, including earthquakes, severe meteorological events, and lightning. Analogous to forensic seismology, sophisticated algorithms and filtering methodologies are indispensable for analyzing collected data and characterizing events to definitively ascertain the occurrence of a nuclear detonation. Data from each station is securely transmitted through dedicated communication links for subsequent analysis. To ensure data authenticity, a digital signature is embedded within the data transmitted from each station.

The Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization employs infrasound as a monitoring technology, complementing seismic, hydroacoustic, and atmospheric radionuclide surveillance. The most intense infrasound event registered by the monitoring system until recently was attributed to the 2013 Chelyabinsk meteor, which was detected by 20 of the system's stations. Subsequently, the commission reported that the magnitude of infrasound generated by the 2022 Hunga Tonga–Hunga Haʻapai eruption, detected by all 53 stations, significantly surpassed that of the Chelyabinsk event.

Infrasound in Popular Culture

The 2017 cinematic production, The Sound, incorporates infrasound as a pivotal narrative component.

The 2020 episode "Fermata" from the Franco-Belgian television series Astrid et Raphaëlle depicts infrasound, emanating from a generator concealed within the pipe organ of the Grand Auditorium at the Maison de la Radio et de la Musique (Radio France's Paris headquarters), as a lethal instrument.

The 'ghost frequency' phenomenon is referenced in Season 3, Episode 4 of the television series *Evil*, specifically in the episode The Demon of the Road, and in Season 23, Episode 3 of the television series *NCIS*, in the episode titled The Sound and the Fury.

References

Bibliography

• Gundersen, P. Erik. The Handy Physics Answer Book. Visible Ink Press, 2003.
• Chedd, Graham. Sound: From Communications to Noise Pollution. Doubleday & Company, 1970.
• O'Keefe, Ciaran, and Sarah Angliss. "The Subjective Effects of Infrasound in a Live Concert Setting." In CIM04: Conference on Interdisciplinary Musicology. Graz, Austria: Graz UP, 2004, 132–133.
• The program Discovery's Biggest Shows was broadcast on The Discovery Channel, India, at 8:00 PM Indian Standard Time on October 7, 2007.

• NOAA Infrasonics Program (archived)
• Los Alamos Infrasound Monitoring Laboratory (archived)
• Tahira, Makoto, Masahiro Nomura, Yosihiro Sawada, and Kosuke Kamo. "Infrasonic and Acoustic-Gravity Waves Generated by the Mount Pinatubo Eruption of June 15, 1991."
• Shams, Qamar A., Cecil G. Burkett, Toby Comeaux (NASA Langley Research Center), Allan J. Zuckerwar (Analytical Services and Material), and George R. Weistroffer (Virginia Commonwealth University). "Sub-surface Windscreen for the Measurement of Outdoor Infrasound."