ABSTRACT
To appreciate the physical problems that limit the application of active noise control, it will be useful to first discuss the physical mechanisms that are responsible for the noise reduction achieved by an “anti-noise” source. In applications of active noise control, the cancelling signal is generated electronically and introduced into the system using transducers such as loudspeakers that convert the electronic signal to sound. In many cases, the physical mechanism responsible for the reduction of the unwanted noise is a little more subtle than mere cancellation. In cases where cancellation is the only control mechanism, the noise level may be reduced at some locations, but will be increased at others so that the total energy of the unwanted noise and the cancelling sound is conserved. This type of control is known as “local cancellation”. Examples of applications employing this mechanism are active headsets, (where the noise is cancelled at the entrance to the ear canal but increased at other locations) and noise cancelling headrests in aircraft and other transportation vehicles.
