The Electronic Controlled Acoustic Shadow System (ECASS), called the silence machine by its creator Selwyn Wright, an engineer at the University of Huddersfield in Yorkshire, UK, is the first device capable of blocking out a specified noise while leaving everything else audible. The silence machine works by sending out sound wave s that are the opposite of those being generated by the offending noise source.
Wright's sytem uses microphones to sample sounds and signal processors to measure the frequency of each part of the noise signal. A software application is used to create a noise that is the same frequency, but opposite in terms of phase . Each valley in the electronic opposite (sometimes called the anti-sound or the anti-noise ) meets a peak in the specified sound wave, and vice versa, so that the original sound is neutralized. Wright calls these areas where the sound waves cancel each other out sound shadows . The size of the shadow areas where the sound and anti-sound waves cancel each other out can be varied by changing the number (or position) of loudspeakers broadcasting the anti-sound.
The first version of Wright's machine is optimized for neutralizing continuous noise with predictable patterns, such as that generated by loud machinery. Silence machines capable of blocking other types of sound, such as music, snoring, and human speech, are expected to be available next year.
The concept of a silence machine was introduced in Arthur C. Clarke's 1957 short story, "Silence Please". Clarke intended it as a humorously improbable device - with dire consequences for the inventor.