Active systems for global attenuation of noise

Abstract

Theoretical approaches to active noise absorption [G. A. Mangiante, J. Acoust. Soc. Am. 61, 1516–1523 (1977)] provide criteria that may prove useful for the design of any system for active noise attenuation, especially one which must produce attenuation in all directions around a complex sound source. (1) Topological fitness: a proper net of “anti‐sources” ought to surround completely either the primary noise source(s), or the space to be silenced. (2) Directivity: each anti‐source ought to radiate only towards the space to be silenced. (3) Concentration: practical system design requires substituting the theoretical continuous distributions of anti‐sources by discrete ones. This approach has been successfully applied to reducing the audible hum of an actual electric substation transformer. The active abatement method is easier for discrete frequencies, such as the spectrum of transformer hum, and is most effective at the relatively low frequencies—120, 240, 360, and 480 Hz—most prominent in the transformer hum spectrum. A relation has been found experimentally for the attenuation achievable versus the number of sources for each of these hum frequencies. The active attenuator interferes less with the normal ventilation and cooling of a transformer than do passive methods. Active abatement also allows attenuation differing in different directions.