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A noise-cancellation speaker emits a sound wave with the same amplitude but with an inverted phase (also known as antiphase) relative to the original sound. The waves combine to form a new wave, in a process called interference , and effectively cancel each other out – an effect which is called destructive interference .
The amplifying devices in audio and radio equipment are vulnerable to a variety of feedback problems, which can cause distinctive noise in the output. The term motorboating is applied to oscillations whose frequency is below the range of hearing, from 1 to 10 hertz , [ 3 ] so the individual oscillations are heard as pulses.
Noise generated on board ships and submarines can have far-reaching effects on the ability of the vessel to operate safely and efficiently. Military vessels in particular need to be quiet to avoid detection by sonar , so many methods have been used to limit a vessel's noise signature.
Radiated noise is the noise generated by a ship that has an effect on other ship's sensors, especially submarines. The Masker-Prairie countermeasure systems are key elements in this program to reduce radiated noise and self-noise that are not reduced by controlling the sounds at their source or their transmission to the hull. [2]
Adaptive feedback cancellation originated during the evolution of the hearing aid. The hearing aid became digital, and as such feedback cancellation was needed. In 1980 a directional microphone was introduced in the digital hearing aid, and adaptive feedback cancellation was created to block external noise that the microphone picked up. Today ...
The first submarine decoys were the German Bold fitted to U-boats of World War II. These were a pellet of calcium hydride in a simple metal container. On contact with sea water, the calcium hydride decomposed to produce a trail of hydrogen gas bubbles that acted as a bubble curtain and reflected ASDIC impulses to produce a false target.