THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

404 14. Machinery Noise Controlmaterials. Dissipative silencers are more effective over a wider range of frequenciesand in silencing fluctuating machinery noises. In a number of situations, in orderto obtain the greatest noise reduction over a wider frequency range, both reactiveand dissipative principles are combined in a single silencer.14.15 Active Noise ControlRelatively recent advances in digital electronics have made it more economicallyfeasible to apply active noise control in cutting down on noise in aircraft andautomotive interiors, pumps, compressors, electric motors, transformers, and soon (Piraux and Nayroles, 1980; Nelson and Elliott, 1992, 1997). Earlier activenoise control techniques were first used in the study of noise in fan ducts. Theadvent of inexpensive digital processors enabled the conversion of analog audiofrequency signals into digital form, then processing them through a digital filterand then converting back into an analog signal with very little time delay. Thebasis of active noise control is to duplicate the noise that is the same but 180 ◦ outof phase, so that when the offending noise and the duplicated out-of-phase noiseare combined, a cancellation will occur.Figure 14.21. An example of an active noise control system for a communication headset.