Active noise control

Active noise control reduces unwanted sound by generating sound waves that interfere destructively with it. Its effectiveness depends on the sound field, timing, and position of the control source.

Destructive interference: The reduction or cancellation of a wave when it combines with another wave out of phase. Active noise control uses destructive interference to reduce sound pressure at chosen locations.
Noise-cancelling headphones: Headphones that use microphones and generated sound to reduce ambient noise reaching the ears. They create a small, controlled listening region where cancellation can work effectively.
Passive noise control: Noise reduction through absorption, isolation, damping, or barriers without generating a cancelling sound wave. Passive methods need no powered cancellation loop and often work better at higher frequencies.
Quiet zone: A spatial region where sound pressure is reduced relative to the surrounding acoustic field. Cancellation is usually strongest in a limited zone rather than throughout a room.
Microphone: A transducer that converts sound into an electrical signal. Microphones measure unwanted sound so a controller can estimate what to cancel.
Active noise control in ducts: The use of microphones and loudspeakers to reduce propagating noise inside an air duct. A duct constrains sound propagation, making cancellation more predictable than in an open room.
Sound absorption: The conversion of acoustic energy into other forms, usually heat, as sound interacts with a material. Absorbers dissipate sound rather than creating an opposing wave.
Acoustic feedback: Unwanted coupling in which sound from a loudspeaker returns to a microphone and affects the system's signal. Feedback can destabilize a control system or make its cancellation inaccurate.
Loudspeaker: A transducer that converts an electrical signal into sound. The control loudspeaker emits the sound wave that interferes with the noise.
Active exhaust noise control: The reduction of engine or machinery exhaust noise using generated sound waves. Exhaust systems provide a confined flow path where low-frequency tones can be targeted.

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