Active Control of Environmental Noise, Iii: Implementation of Theory Into Practice

Active Control of Environmental Noise, Vii: Performance of Multi-Frequency, Multi-Channel, Freefield Sound Cancelling Systems

“…For free"eld conditions, it is found that K has a series of instability peaks (maxima) as a function of the acoustic frequency, given by equation (75) of reference [5]. These peaks should be avoided if the adaptive system is to remain stable.…”

“…For each propagation path, one can (a) Measure N RD automatically using, for example, white noise impulse testing techniques described in reference [5] (here it is necessary to perform frequency-domain analysis to obtain the phase response). (b) Physically measure or calculate from geometry or use white noise impulse response to measure r QK .…”

“…in the delayed LMS algorithm for each propagation path, according to equation (34) in reference [5] i.e.,…”

Active Control of Environmental Noise, V: The Effect of Environmental Change on the Stability of Free Field Sound Cancelling Systems

“…In reference [4] the e!ect of three-dimensional primary sources, out-of-phase primary sources, re#ecting surfaces and steady wind have been considered, again by using wave theory computations. It has been shown in reference [3] how these wave systems can be implemented into practical hardware and how the stability of the adaptive process can be maximized. This paper considers the e!ect of acoustic environment change on the stability of these implemented systems, compared to free "eld, stationary propagating #uid conditions.…”

“…These individual examples are not important; they serve only to show, in general, how the environment a!ects the acoustic system transfer function compared to that in free "eld conditions. Figure 2 shows the schematic diagram of a practical control system considered in detail in reference [3]. The functions H D , H A and H K represent the transfer functions of the signal conditioning "lters, secondary cancelling sources (speakers) and detection microphones, respectively.…”