GB782794A - Improvements relating to noise reduction applicable to electric induction apparatus and other sound sources - Google Patents

Abstract

782,794. Directing sound for noise reduction. GENERAL ELECTRIC CO. Dec. 30, 1955 [Feb. 9, 1955], No. 37444/55. Class 118 (2). An arrangement for reducing the noise which a primary source of sound having a comparatively large vibratory surface causes in a zone removed from said source comprises at least one auxiliary source of sound having a comparatively small vibratory surface located adjacent the vibratory surface of the primary source and means for driving the auxiliary source to produce therefrom a sound-wave having components at frequencies corresponding to at least the principal noise producing components of the sound from the primary source and of amplitude and phase such as to tend to cancel said noise producing components in said zone. The arrangement is described with respect to the reduction of noise from power transformers by attaching at least one loud-speaker 6 to the transformer tank wall 7, as shown in Fig. 1a, and feeding the loud-speaker from an energizing system 9 arranged for adjustment of relative phase and amplitude of harmonics of the power supply frequency. The loud-speaker is mounted about half-an-inch from the transformer wall 7 and is totally enclosed rearwardly by a box 6a, for example, of ¥-inch plywood. The energizing system comprises band-pass filters for three lower harmonics of the power supply frequency followed by adjustable phase-shifting networks and amplitude controlling potentiometers, the outputs of which are added, amplified and fed to loud-speaker 6 (Figs. 2 and 3, not shown). The input to the energizing system may be obtained from the supply itself via a suitable transformer and rectifying circuit, or by means of a vibration pick-up device or a microphone attached to the tank wall. Fig. 4 shows the sound levels found with the loudspeaker system adjusted for maximum cancellation of the main component only (i.e. twice the supply frequency) in an area adjacent the zero degree axis about 100 feet distant from the transformer, a beam of sound reduction of at least 6 db. and 23 degrees width being produced. The beam of sound reduction may be controlled in direction by suitable adjustment of the phase and amplitude of the loud-speaker excitation. By the use of two loud-speakers mounted about 80 inches apart on a transformer wall the beam width may be increased to 35 degrees (Fig. 8, not shown). If the transformer is surrounded by a masonry wall mounting the loud-speaker on top of the wall instead of on the tank may give better results. Fig. 10 (not shown) illustrates the sound levels obtained using a bank of four loud-speakers on a. scale model. The technique evolved for obtaining the required sound reduction in practice is also described.