This invention relates to a method of attenuating a given sound field by means of a radiation pattern which interferes with and reduces the effect of the original field. Up till now, most attempts to achieve a reduction in noise level have been confined either to reducing the efficiency of the source generating the sound or to passive methods of control, e.g. by lagging with absorbing material or by deliberate impedance mismatching to prevent efficient transmission of the sound.
A method of generating sound waves which interfere destructively with the sound field to be attenuated was described by Jessel and Mangiante in the "Journal of Sound and Vibration" 1972 in a paper under the file "Active Sound Absorbers in an Air Duct". However the method described is inefficient and the reasons for this inefficiency are fully described later in this specification.