Near Field Zones of Quiet

Abstract

This paper examines the consequences of driving a single secondary loudspeaker to cancel the pressure due to some primary source at a point in its near field. This simple technique has been applied to the sound field in a highly reverberant room to produce zones of quiet in the vicinity of the loudspeaker, which have diameters that are typically equal to one-tenth of the acoustic wavelength, within which the sound pressure level is attenuated by at least 10 dB. The principal advantage gained with this strategy over other active techniques for controlling the sound field in rooms is that the sound pressure level well away from the control point is largely unaffected, an increase of only a small fraction of one dB being typical. Such a loudspeaker-microphone configuration could be located, for example, in the head rests of cars or aeroplanes, or indeed anywhere where the listener is seated for significant lengths of time and subjected to high ambient noise levels such that auditory comfort may be disturbed. Measurements are presented of the near field quiet zone in a reverberant room at frequencies well above the Schroeder frequency. These experimental findings, which represent a space averaged result over source position, indicate good agreement with the simple theory developed in this paper. It is demonstrated theoretically that the diameter of the quiet zone formed at any arbitrary point and direction in the near field of the loudspeaker is numerically proportional to the specific acoustic impedance at that point and direction. In general terms, the 10 dB quiet zone is observed to increase as the diameter of the secondary loudspeaker increases and the microphone is moved increasingly further from the loudspeaker. Close to the loudspeaker the pressure due to the secondary source is predominantly governed by the directly radiated near field and so the quiet zone formed in this region is therefore insensitive to the nature of the primary sound field. This important feature makes it highly suitable for producing reductions in the sound pressure level within enclosed spaces at frequencies where the modal density is high such that global control strategies are rendered ineffective.