Sound moves from its source to the ear by wavelike fluctuations in pair pressure, something like the crests and troughs of ocean waves. One way to keep from hearing sound is to use ear plugs. Another way is to cancel out the sound with anti-sound. Using a noisemaker controlled by a microprocessor, engineers have produced sound waves that are half a wavelength matched to a trough, and vice versa. Once the researchers have recorded the offending sound, a microprocessor calculates the amplitude and wavelength of sound that will cancel out the crests and troughs of the noise. It then produces an electric current that is a amplified and fed to a loudspeaker, which produces anti-sound and wipes out the noise. If the anti-sound goes out of synchronization, a microphone picks up the leftover sound and sends it back to the microprocessor, which changes the phase of the anti-sound just enough to cause complete signals
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