Fundamentals of Active Noise Control

Methods in eliminating noise ‘with noise’ were already described in the chapter on sound propagation. Namely, noise power emissions can more or less be reduced by placing two inversely phased sources at a small distance to one another. In other words, one can refer to one of such a source pair as ‘an additional sound source introduced for the purpose of noise reduction.’ This is the main idea behind what is known as active noise reduction, which is also sometimes referred ‘anti-noise.’ This term encompasses all major types of noise reduction based on the use of electro-acoustic converters (as described in Chapter 11). The disruptive, somehow unavoidable sound source is referred to here as the ‘primary’ source; additional electro-acoustic converters which are intended for use as noise reducers are referred to as ‘secondary’ sources.