Editing Security alarm (section)

=== Infrasound detectors ===

The [[infrasound]] detector works by detecting infrasound, or sound waves at frequencies below 20{{nbsp}}Hz. Sounds at those frequencies are inaudible to the human ear.<ref>{{Cite web|url=https://www.nasa.gov/larc/nasa-langley-researchers-nab-invention-of-the-year-for-infrasound-detection-system/#.VN2ml2TF874|title=NASA Langley Researchers Nab Invention of the Year for Infrasound Detection System|date=2014-07-25|website=www.nasa.gov}}</ref> Due to its inherent properties, infrasound can travel distances of many hundreds of kilometers.{{sfn|Chilo|Lindblad|2008|p=<!-- 18{{ndash}}21 -->}}

The entire infrasound detection system consists of the following components: a speaker (infrasound sensor) as a microphone input, an order-frequency filter, an analog-to-digital (A/D) converter, and an microcomputer to analyze the recorded signal.
[[File:Home Intruder Alarm system.JPG|thumb|Exterior alarm [[bell box]]]]<!-- <ref>{{Cite web|title=A Low Cost Infrasonic Recording System|last=Girisha Durrel De Silva & Kasun De Zoyza|website=http://www.comp.nus.edu.sg}}</ref> -->

If a potential intruder tries to enter into a house, they test whether it is closed and locked, uses tools on openings, or/and applies pressure, creating low-frequency sound vibrations. Before the intruder breaks in, the infrasound detector automatically detects the intruder's actions.

The purpose of such a system is to detect burglars before they enter the house to avoid both theft and vandalism. The sensitivity is dependent on the size of a home and the presence of animals.