Infrasonics

Infrasound, also known as low-frequency sound, refers to sound waves that have a frequency lower than the lower range of audibility (about 20 Hz). Because hearing becomes less sensitive as frequency decreases, the sound pressure should be reasonably large for humans to experience infrasound. Although the ear is the main organ for sensing low sound, infrasound vibrations can be felt in different areas of the body at excessive levels.

The research of these sound waves is known as infrasonics (infrasonic wave), and it includes sounds below 20 Hz and up to 0.1 Hz (and rarely to 0.001 Hz). This frequency range is used to monitor earthquakes and volcanoes, to map rock and petroleum deposits beneath the earth's surface, and to examine the mechanisms of the heart in ballistocardiography and seismocardiography.

Infrasound is distinguished by its ability to circumvent barriers with minimal dissipation. Low-frequency sounds, such as near-infrasound, can be produced in music using acoustic waveguide techniques as in a large pipe organ or, for reproduction, exotic loudspeaker designs including a rotary woofer, transmission line, or conventional subwoofer (infrasound meter) designs. Subwoofers designed to produce infrasound can reproduce sound an octave or even less than that of several commonly produced infrasound meters and are typically 10 times larger.

Gavreau Infrasound

Vladimir Gavreau, also known as Vladimir Gavronsky, was a French scientist who studied the biological effects of infrared sound.

Gavreau took birth in Russia. When he and his lab assistants encountered pain in their eardrums and moving testing laboratory equipment in the 1960s, when there was no audible sound picked up on his microphones, he became interested in infrasonic waves. An infrasonic whistle was one of Gavreau's experiments (gavreau infrasound), which some claim has resulted in a field of inquiry with military applications.

Infrasound Sources

Both natural and man-made sources can produce infrasonic bass.

Natural Sources

Lee waves, Severe weather, surf, avalanches, lightning, volcanoes, earthquakes, bolides, waterfalls, aurorae, iceberg calving, meteors, and upper-atmospheric lightning are all examples of natural events that produce infrasonic sound. Microbaroms are pervasive infrasound vibrations approximately 0.2 Hz produced by nonlinear ocean wave interactions in ocean storms. Infrasonic arrays could be used to identify avalanches in the Rocky Mountains and also to identify tornadoes on the great plains some minutes until they touch down, as per NOAA's Infrasonics Program.

Animal Communication: Elephants, whales, rhinoceroses, hippopotamuses, giraffes, peacocks, okapis, and alligators have all been observed using infrasound to communicate over long distances, with whales communicating over hundreds of miles. The Sumatran rhinoceros, in specific, was shown to make sounds with frequencies very low as 3 Hz, which are similar to the humpback whale's song.

Human Singers: Some vocalists, such as Tim Storms, are capable of producing infrasound notes.

Man-Made Sources

Human processes including sonic booms and explosions (both chemical and nuclear) can produce infrasound, as could machineries like wind turbines, diesel engines, and uniquely engineered mechanical transducers (industrial vibration tables).

Human Reactions

The usual low-frequency limit of hearing range for humans is set at 20 Hz. A person listener would be capable of detecting tones as small as 12 Hz when pure sine waves are manufactured under optimal conditions even at very high volume. Below 10 Hz, single sound cycles can be heard, together with a pressure feeling in the ear holes.

The dynamic range of the auditory system decreases significantly with lowering frequency starting around 1000 Hz. The equal-loudness-level contours show this distortion, which means that even a small rise in level will alter the perceived loudness from hardly audible to loud. When this effect is coupled with the natural spread of thresholds within a population, a much more low-frequency sound that is inaudible to a certain person might become detectable to someone else.

Humans may experience feelings of awe or anxiety as a result of infrasound, as per research. It's also been recommended that because it's not actively perceived, it could give people the impression that strange or supernatural events are occurring. Infrasound might impact certain people's nervous systems by activating the vestibular system, as per a scientist from Sydney University's Auditory Neuroscience Laboratory, and it has been displayed in animal models to have an equivalent effect as that of seasickness.

Detection and Measurement

NASA Langley successfully developed and produced an infrared detection system that could be used to take useful infrared measurements in places where they were previously impossible. The system includes a 3-inch membrane diameter electret condenser microphone PCB Model 377M06 and a small, portable windscreen. Because Johnson noise produced in the promoting electronics (preamplifier) is minimised, electret-based technology has the lowest possible background noise.

With a significant back chamber volume, a pre-polarized backplane, and a high impedance preamplifier located within the back chamber, the microphone has high membrane compliance. The windscreen is composed of a material with a small acoustic impedance and a relatively thick wall to guarantee structural stability, considering the high transmission coefficient of infrasound via matter. Closed-cell polyurethane foam has proven to be effective in this application. Sensitivity, signal fidelity (harmonic distortion), ambient noise in the background, and temporal stability would be the experimental variables in the proposed test.

The system meets a number of instrumentation standards that are beneficial to the use of acoustics: 

• a reduced microphone with minimal ambient noise that can detect low-level transmissions within a low-frequency passband; 

• a small, compact windscreen

• fast deployment of a microphone array throughout the field.

A data acquisition infrasound meter has been included in the system, which allows forbearing, real-time detection, and signature of a low-frequency object.

Infrasound, together with seismic, hydroacoustic, and atmospheric radionuclide monitoring, is used by the Preparatory Commission of the Comprehensive Nuclear-Test-Ban Treaty Organization. The 2013 Chelyabinsk meteor produced the loudest infrasound ever collected by the infrasound meter.