Experiments on the active control of transformer noise

This paper describes an experiment which demonstrates how a useful degree of active noise control can be achieved with ordinary sound amplification and reproduction equipment. A loudspeaker positioned next to a large pair of noisy electricity transformers was made to mimic their noise but in antiphase. The aim of the study was to investigate the degree to which the “antisound” would cancel a disturbing noise heard in a nearby office. Some 20 decibels of control was achieved very easily for the 100 Hz component of the noise but the higher frequency sound could only be controlled in localized patches. The experiment suggests that sounds of discrete frequencies of less than 100 Hz are relatively easily controlled with unsophisticated audio equipment, but that useful control of higher frequency elements is much more difficult.