ACTIVE CONTROL OF ENVIRONMENTAL NOISE, VI: PERFORMANCE OF A FUNDAMENTAL FREE-FIELD SOUND CANCELLING SYSTEM

Before the adaptive cancellation performance of multichannel free-field acoustic systems could be evaluated in detail, it was considered prudent to establish first the performance of a single channel free-field cancelling system. The adaptive theory for this basic system, including the stability process, is described. Measured adaptive performance is given confirming the theory. ncept of stability bands and their prediction is considered. The bands are a consequence of phase differences between the primary source and secondary cancelling field, generated by the transfer functions of the control system. At the edge of these stability bands satellite pole frequencies “beat” with the cancelling frequency, generated by the zeros, to produce side bands. The relation between the stability bandwidth and the adaptive speed, in terms of reference signal strength and adaptive step size, are investigated. und generation and cancellation performance of this basic canceller are then considered. The acoustic sound field directivity characteristics are similar to that of a dipole, tripole or quadrupole type source, depending on the primary–secondary source separation distance. The acoustic shadow characteristics are established in detail and compared with measurement. Good agreement is obtained.