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Infrasound - The Infrasonic Sound Of Death
Sound Measuring Tools
Infrasound Monitoring Arrays
Infrasound could be the last sound you never hear. Working outside the normal human hearing range, infrasound consists of the spectrum of frequencies below the lower human hearing threshold of 20Hz down to about one thousandth or .001 Hz. That range of sound is extremely interesting for various reasons. One reason is that infrasound can travel very large distances (even encompassing the globe multiple times) and is very adept at maneuvering past obstacles with negligible dissipation while doing so. Infrasound is emitted in all sorts of natural disasters from volcanic eruptions to snow avalanches to earthquakes, and therefore infrasound is an important indicator for scientists wishing to monitor and predict such geologic disasters. However, there are a plethora of man-made infrasound sources as well. Early infrasound pioneers used infrasonic traces to track enemy artillery on a battlefield. Space shuttles and rockets also present an infrasound signature if one has the proper tools to listen in. After the Comprehensive Nuclear Test Ban Treaty was set forth in 1996, the United States (who has signed but not ratified the treaty) and the international community began developing Defense Treaty Inspection Readiness procedures which includes a CTBTO International Monitoring System. The IMS consists of 337 monitoring facilities (inspection stations and labs) around the world, and a portion of these are designated infrasound monitoring arrays that can detect a nuclear weapon explosion signature and triangulate the source. With so many dangerous sources of infrasonic activity, that is why infrasound just might be the last sound you never hear.
Traditional microphones and standard amplifiers are generally not capable of detecting and recording infrasound waves. This can be due to the microphone preamps and amps which can have trouble with ac coupling at a frequency below 20Hz. Also, condenser microphones generally have a vent hole to keep a stable atmospheric equilibrium and that may in turn act as a high-pass filter which eliminates sub-20Hz sound. The International Monitoring System is made up of high-tech infrasound detector arrays that rely on techniques other than the well-known pressure microphone. The microbarometers and a complex discernment of circular waves, plane waves, near field effects and particle motion velocities is necessary to obtain reliable data and locate the source. Infrasound can often behave in quite strange ways. There have been instances of volcanic eruptions where the sensors at the base of the volcano did not record anything while those half the world away picked up the signature. Infrasonics is the field of science tasked with deciphering all of the metrics.
So the network of nuclear treaty monitoring sites consists of seismic stations, radionuclide stations, infrasound monitoring stations, and hydroacoustic stations all organized in a manner to verify the compliance of nations to the terms of the treaty. The systems is quite widespread with every continent having stations in and around including at various spots in the oceans. It would seem quite impossible for anyone to get in a nuclear weapon test explosion without being detected by multiple stations. At the infrasound detection locations, microbarographs are equipped to measure the small changes in air pressure. The tolerances of the monitoring microbarographs are quite high, but it is possible for an amateur to build an inexpensive homemade microbarograph for experimental purposes. A phenomenon known as "voice of the sea" or microbaroms was discovered in 1939 and is one source that must be filtered out by the international nuclear explosion monitoring system. Microbaroms are created by marine storms and can travel thousands of kilometers because of the low frequencies involved. The accuracy and filters of the government microbarographs are cutting edge should any nation think of devising a conspiracy and trying a low yield nuclear detonation test while blaming the alarms on a passing hurricane. Amateurs can build a microbarometer based off of online schematics and use data logging seismic software for microbarometers to record the results. Obtaining measurements in units of pacals and microbars is possible with a well designed version as opposed to standard meteorological barometers which record air pressure in hecto-pascals. Another important factor to be accounted for in infrasound measurement is wind. Wind noise reducers induce a veritable signal gain by reducing the effect of the wind in the measurement. In the event of a nuclear disaster, infrasound might be the last sound you never hear, but it is a certainty that something will record it.