Active acoustic absorbers for low‐frequency hum

Previous attempts to cancel the 120‐Hz hum of large power transformers by active means have relied only upon Young's principle [e.g., W. B. Conover, Noise Control 2, 78–82 (1956)]. M. Jessel established that Young's principle of interference can be combined with Huygens' principle of virtual sources [Brevet Français 1, 494–967 (1966)]. This new theory suggested a technique by which the sound field may be cancelled completely outside of a Huygens' surface surrounding a primary source, by employing a number of secondary sources at the Huygens' surface, which act as active sound absorbers. The theory has been realized experimentally for sound in a duct [M. Jessel and G. Mangiante, J. Sound Vib. 23, 383–390 (1972)] and in three dimensions for a small simple source [G. Mangiante, D.E.S. Fac. Sciences, Marseilles (1968)]. This technique has been applied by the authors to a small acoustic model of a large distribution substation transformer in a free‐field over a reflecting plane. Six‐ten dB attenuation of the fundamental frequency has been achieved everywhere in the farfield using only a limited number of secondary sources. Cancellation of two hum components simultaneously has been achieved also. It also has been found that attenuation over a limited horizontal angle does not increase the radiation outside this angle significantly.