ABSTRACT
The most common sensors used in active control applications are microphones. In modern digitally based control systems, the frequency response of the microphone used for providing either the reference signal or error signal to the controller is not very critical as any lack of flatness in amplitude or phase is taken into account in the system identification algorithms, and can be compensated in the weights used in the control filter. For this reason, it is common to find relatively inexpensive microphones used in active control systems. The two most common types are the piezo-electric microphone and the inexpensive prepolarised condenser (electret) microphone. The latter microphones may be purchased with power supply for tens of dollars or without cable or power supply for less than $5. Thus multiple error microphones (for which the outputs are summed) may be used economically to improve system reliability. The author has used these microphones in very dirty environments using the holder incorporating an air purge system as illustrated in Figure 5.1. The additional noise at the microphone location resulting from the air purge is dependent on the speed of the air flow over the microphone. For the case considered here it was about 85-90 dB, which was sufficiently below the duct noise that it was not a problem, as duct noise levels varied from 112 dB to 138 dB.
