In a feedforward Active Sound Cancellation (ASC) system, the acoustic delay between a primary source and an error microphone must be greater than the delay between the measurement of that source by the reference microphone and the arrival of the secondary source's wave at the error microphone. Such a configuration is called a causal configuration. For periodic disturbances, cancellation can still be achieved in non-causal configuration. Since the waveform is periodic, each cycle of the waveform is identical, and the cycle being canceled is not the measured part of the waveform, but a subsequent cycle in the waveform. Non-periodic sources cannot be cancelled by a non-causal ASC system, and convergence of the Least Mean Squares algorithm is not as effective in a non-causal configuration as in a causal configuration. The ASC system was implemented to create a local zone of silence inside a reverberant enclosure. The primary source was a 125 Hz sinusoid generated outside of the enclosure. System delays were calculated and a causal component configuration was chosen. System performance under both causal and non-causal component configurations was examined. The system was able to create a maximum attenuation of >18 dB in both the causal and non-causal configurations. However, it was discovered that in the non-causal configuration, the computation of the optimal inverse signal was much slower than in the causal configuration.
Anderson, Eileen and Wright, Andrew B.
"Study of Causal Component Placement in an Active Sound Cancellation System,"
Journal of the Arkansas Academy of Science: Vol. 60
, Article 5.
Available at: http://scholarworks.uark.edu/jaas/vol60/iss1/5