ANTI-NOISE （1）

Published:2011/8/17 22:20:00 Author:Phyllis From:SeekIC

The sound of silence

Noise has a serious impact on the environment as well as on human beings. Besides classical sound reduction techniques such as insulation, researchers are trying to attack the root of the problem using electronic means. They have already had some success.

Sometimes it happens that an underlying physical process is well understood on paper, but the job of translating the theory into the real world takes a long time. One example of this is the patent awarded to the German physicist Paul Lueg in 1933. His idea was astonishingly simple and elegant. In his original patent document (Figure 1) he describes how an incoming sound wave can be detected using a microphone, converted to an electrical signal, given a 1 80; phase shift, and mixed with the original signal using a loudspeaker. The result is silence (Figure 2). Well, at least in theory the result is silence; the frequency response of the microphone, loudspeaker and electronics along with other effects such as temperature and humidity mean that in practice things are harder than that.









Anti-noise in headphones

Noise has a serious impact on the environment as well as on human beings. It causes tiredness, loss of concentration, nervousness and irritability, and can lead to permanent hearing damage. Passive ear protection reduces noise especially in the middle and upper frequency ranges, but is not adequate at lower frequencies (Figure 3).




Is it possible to use Paul Lueg’s patent in the battle against noise? Indeed it is: the most widespread and well-known application is in air travel, where active noise compensation is used in headphones for crew and passengers (first class only, of course!). The system is known as ANR (active noise reduction) or ANC (active noise cancellation). In the USA at least, there is considerable consumer demand for ANR headphones to allow music to be heard undistorted in noisy surroundings. The market fulfils this demand, with units available from thirty dollars from such well-known brands as Philips, Sony, Aiwa, Maxell, Bose, Shure, Koss, AltecLansing and Jensen. Panasonic also offers lnear headphones with ANR. Sennheiser (www.sennheiser.com) is among the leading suppliers of headsets and headphones featuring ANR, with their NoiseGard™ system. An eleclret microphone capsule is fitted in each headphone unit, along with a negative-feedback circuit and a transducer system. The sound received by the microphone, which includes the wanted signal as well as the interference, is amplified and the wanted signal filtered out. The interference signal is processed, phase shifted by 1 80 D, and added to the wanted signal. The result is sent to the transducer system. Superimposing the anti-phase signal considerably reduces the amplitude of the interference, whereas the wanted signal, which has not passed through the compensation circuit, is unaffected.





NoiseGard™ headsets and headphones are available in closed and open versions. The open versions are suitable for compensating for the noise level and spectrum found in business jets or commercial aeroplanes. More than 10 dB of compensation is achieved over the range from 400 Hz to 1000 Hz (Figure 4). This allows the volume level of speech radio communications to be reduced, without compromising their intelligibility. Thanks to the open form of the headphones, voices and alarm signals in the cockpit can be heard practically without distortion. Noise, on the other hand, is considerably attenuated. Lightweight open headphones offer excellent comfort to the wearer, which is particularly important for long journeys.

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