ACTIVE CONTROL OF ENVIRONMENTAL NOISE, V: THE EFFECT OF ENVIRONMENTAL CHANGE ON THE STABILITY OF FREE FIELD SOUND CANCELLING SYSTEMS

This paper considers the effect of environmental change on the stability of hardware implemented electronically controlled acoustic shadow (ECAS) systems. These systems generate quiet zones (acoustic shadows) from unwanted noise sources in unrestricted space. The acoustic transfer function of the system representing the acoustic environment is measured by using white noise impulse response techniques. The change in this function compared to the free field stationary propagating fluid value is determined. The effect of this change on the system stability is then expressed through the displacement of the stability region contours. This technique provides a method of measuring and correcting automatically for the displacement of the stability regions and thus maintaining the stability of the ECAS system through varying environmental conditions.