ABSTRACT
In free space, the complexity of the sound field combined with the difficulty of obtaining a reference signal prior to the noise arriving at the control sources means that it is not generally practical to attempt control of broadband or transient noise from large noise sources, although feedback control can be used to obtain noise reductions in local areas at the expense of increased noise levels elsewhere. Tonal noise radiating into free space can be controlled, and a good example of this is the control of the hum from electric power transformers, which is characterised by three or four major tones.
