ABSTRACT
Active sound control exploits the destructive interference which can exist between the sound fields generated by an original ‘primary’ acoustic source and a controllable ‘secondary’ acoustic source. In order for this destructive interference to be effective, the waveform of the sound field generated by the secondary source must be very close to that of the primary source, and also the spatial distributions of the two sound fields must be well matched in the region where the sound is to be controlled. The need for close spatial matching between the interfering sound fields
means that active sound control is best suited to the control of lowfrequency sounds, for which the acoustic wavelength is large. Conventional passive methods of sound control can struggle to attenuate the noise in this frequency region and so the two methods of sound control can be complementary in many applications. Although the principle of active sound control has been known since the
1930s (Lueg 1936; Figure 8.1), and single-channel analogue control systems were developed in the 1950s (Olsen and May 1953; Conover 1956), it was not until the development of modern digital signal processing (DSP) devices in the 1980s that adaptive digital controllers enabled the technique to be used in many practical problems (Chaplin 1983; Roure 1985). Since then, there has been considerable interest in the commercial application of active sound control, and this has led to a more detailed investigation of its fundamental acoustic limitations (Nelson and Elliott 1992). At the time of writing, the most successful applications of active sound
control are probably in active headsets, which are widely used in both military and civil applications (Wheeler 1987; Rafaely 2000) and in the control of sound inside aircraft (Billout et al. 1995; Ross and Purver 1997). Although the active control of both engine noise and road noise at low frequencies in cars was demonstrated some time ago (Elliott et al. 1988b; Sutton et al. 1994), the automotive application has taken some time to reach mass production, mostly because of cost constraints. Similarly, the
active control of low-frequency sound in air-conditioning ducts has been well studied in the laboratory, but it has been difficult for this technology to compete economically with passive absorbers, except in specialist applications where weight or size are of prime consideration. In this chapter, the physical mechanisms and limitations of active sound
control will be illustrated, initially for plane, freely propagating waves in a duct, and then for three-dimensional sound fields in free space. The different strategies of controlling enclosed sound fields will then be illustrated in a one-dimensional duct before the more realistic problem of controlling sound in a three-dimensional enclosure is considered. Finally, the local control of sound is described, which can generate zones of quietness in specific regions of a space.
