Protection of a 3-dimension zone requires many microphones and speakers, making it more expensive. The term '1-dimension' refers to a simple pistonic relationship between the noise and the active speaker (mechanical noise reduction) or between the active speaker and the listener (headphones). Several commercial applications have been successful: noise-cancelling headphones, active mufflers, anti- snoring devices, vocal or center channel extraction for karaoke machines, and the control of noise in air conditioning ducts. Protection of a '1-dimension zone' is easier and requires only one or two microphones and speakers to be effective. Periodic sounds, even complex ones, are easier to cancel than random sounds due to the repetition in the wave form. the passenger compartment of a car) global noise reduction can be achieved via multiple speakers and feedback microphones, and measurement of the modal responses of the enclosure.ДЄpplications can be '1-dimensional' or 3-dimensional, depending on the type of zone to protect.

Noise cancellation at other locations is more difficult as the three-dimensional wavefronts of the unwanted sound and the cancellation signal could match and create alternating zones of constructive and destructive interference, reducing noise in some spots while doubling noise in others. This requires a much lower power level for cancellation but is effective only for a single user. Alternatively, the transducer emitting the cancellation signal may be located at the location where sound attenuation is wanted (e.g. In this case it must have the same audio power level as the source of the unwanted sound in order to cancel the noise. This effectively reduces the volume of the perceivable noise.ДЄ noise-cancellation speaker may be co-located with the sound source to be attenuated. This inverted signal (in antiphase) is then amplified and a transducer creates a sound wave directly proportional to the amplitude of the original waveform, creating destructive interference. Adaptive algorithms are designed to analyze the waveform of the background aural or nonaural noise, then based on the specific algorithm generate a signal that will either phase shift or invert the polarity of the original signal.

Modern active noise control is generally achieved through the use of analog circuits or digital signal processing. The waves combine to form a new wave, in a process called interference, and effectively cancel each other out вЂ“ an effect which is called destructive interference. A noise-cancellation speaker emits a sound wave with the same amplitude but with inverted phase (also known as antiphase) relative to the original sound. Sound is a pressure wave, which consists of alternating periods of compression and rarefaction.

Acoustic echo cancellation head phones