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ACTIVE NOISE CONTROL
Active Noise Control
This section does not intend to cover what now is a broad subject. On the other hand, due to the numerous inquiries about the mysterious “antinoise” that happens in a day’s work, some explanations are in order.
On paper it is simple and straightforward: A noise is made of numerous sinusoidal waves; in order to cancel that noise, one simply has to generate the required opposite phase sinusoidal waves. The idea is so simple that it was already outlined in papers by the Persian physicist Al Farhabi in the 14th century. Later on, at the end of the 19th century, Helmholtz put the problem into equations. The actual experiment had to wait until the war preparations of 1933: At that time the detection of planes was mainly performed by acoustic means; thus, silencing them would give a clear advantage. Accordingly, the German physicist Olsen attempted active noise cancellation on a stationary source: a transformer. It quickly turned out that while a significant gain could be reached at a given point, amplification would be produced in other points too due to the impossibility of physically superposing the actual noise source and the counternoise source. With the development of radar, work on active noise control lost its immediate interest and was stopped for the time being.
Interest resumed again in the mid-1970s with its application on a single-dimension system, as there is no interference. In a ventilation duct the use of an active system helps avoid pressure loss and is economically viable when compared to a standard 3 m long dissipative silencer. Later on, the development of computational methods opened new perspectives in 3D.
The simplest active noise cancellation system calls for a back-feed loop where one checks that the overall resulting sound level value is minimized. The signal is picked when it arrives and phase reversed before being injected in a loudspeaker. In more elaborate systems, the signal is associated with several parameters (e.g., throttle setting), and a transfer function has been elaborated so as to produce the correct cancellation signal.
A few manufacturers now have headphones with in-built active cancellation to enhance the sound quality of the speech or music signal for the listener .
Silencers on Air Intake
Air intakes are important features of a building, as they allow fresh air inside; however, they can constitute a serious acoustic weakness in the fagade insulation. To minimize the potential complications of a long silencer, some manufacturers have introduced active cancellation inside the duct .
One has probably experienced active noise cancellation traveling on some airplanes (e.g., the ATR family or the B-777), where the sound levels inside the cabin are controlled using actuators on the walls.
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