Noise reduction is typically a desired effect in any audio environment.
The most common approach is a passive approach--using physical sound-absorbing
or sound-blocking materials
Active Noise Reduction is then used to augument or replace passive systems
where they are not good enough, or impractical for the application. The
general concept of an active system is emitting a anti-noise signal to
null out ("cancel") the unwanted noise at the desired point.
The correct anti-noise signal can either be determined parametrically,
or by sampling and processing existing noise. A well-implemented noise
canceling system can easily outperform most practical passive systems
for similar applications.
Active noise canceling may be implemented many ways, generally classified
as open-loop, closed-loop, or adaptive.
In an open-loop implementation,
a microphone picks up only the unwanted noise (i.e. outside of a headphone
earcup), which is processed (inverted, adjusted for gain and delay) and
sent to the transducer(s). Open-loop systems are the easiest to implement,
but are limited in their accuracy and effectiveness.
In a closed-loop implementation, a microphone picks up both the wanted
audio and the unwanted noise (i.e. inside the earcup). The original audio
signal is then subtracted from the microphone signal, which is then
processed, and fed back to the transducer. While more complex than open-loop,
closed-loop implementations offer more accuracy, bandwidth and flexibility.
Adaptive noise canceling uses DSP algorithms to determine how best to
cancel noise picked up by one or more microphones. Comprehensive algorithms
paired with complex networks of microphones and transducers can be very
successful at eliminating noise and providing high quality audio--however,
these solutions are neceessarily expensive and laborious to implement.