An active 
noise cancellation 
system includes a series of features for more effective cancellation, greater reliability, and improved stability. A particular feature adapted for 
headset systems includes locating a residual 
microphone radially offset from the center of a sound generator to detect a 
signal more similar to that incident upon the 
eardrum of the user. In addition, an open back 
headset design includes perforations on the side of the 
headset instead of the back, so that the perforations are less susceptible to inadvertent blockage. The 
system also includes a mechanism for detecting changes in the acoustic characteristics of the environment that may be caused, for example, by pressure exerted upon the earpieces, and that may destabilize the cancellation 
system. The system automatically responds to such changes, for example, by reducing the 
gain or the 
frequency response of the system to preserve stability. The system further includes other methods for detecting imminent 
instability and compensating, such as detecting the onset of signals within enhancement frequencies characteristic of the onset of 
instability, and adjusting the 
gain or 
frequency response of the system or suppressing the enhanced signals. The system further includes a mechanism for conserving battery life by turning the system off when sound levels are low, or adjusting the power supply to the system to correspond to the current power requirements of the system.