Physical Microphone Research Articles

A novel method of designing stable feedback controllers for local control of sound

This paper investigates the performance and stability of a virtual microphone feedback controller for local control of sound through theoretical analysis, simulations and real-time implementation. In real-time implementation of practical systems we usually need to measure the frequency response of the controller before the controller is implemented, in order to ensure that the expected performance is achieved. If an unstable controller is used, we will not be able to measure the frequency response of the controller, which will make the implementation and maintenance of the controller more difficult. Therefore a novel method to design a stable feedback controller for a local control system is presented in this paper. The local control system is designed to attenuate a broadband disturbance at a point near the listener's ear, i.e. the virtual microphone, rather than at the control microphone, i.e. the physical microphone. The results showed that if a good performance was to be achieved the controller would be unstable. This is because the magnitude of the response of the virtual plant is lower than that of the physical plant. The feedback controller in this work is designed to minimize the noise at the virtual microphone, with a virtual plant response used in the performance objective. However the physical plant is used in the stability constraint. Therefore there is a trade-off between the performance and controller stability. The main contributions of the paper are to propose a novel method to design a stable feedback controller and analyze the performance and stability of a virtual microphone feedback controller for the local control of sound through simulations and real-time implementation.

Stability Analysis and Performance Evaluation of a Feedback Controller for Local Control of Sound

This paper investigates the performance and stability of a virtual microphone feedback controller for local control of sound through theoretical analysis and simulations. A novel method to design a stable feedback controller for the local control system is also presented in this paper. The local control system is designed to attenuate a broadband disturbance at a point near the listener’s ear, i.e. the virtual microphone, rather than at the control microphone, i.e. the physical microphone. The results showed that if a good performance was to be achieved the controller would be unstable. This is because the magnitude of the response of the virtual plant is lower than that of the physical plant. The feedback controller in this work is designed to minimize the noise at the virtual microphone, with a virtual plant response used in the performance objective. However the physical plant is used in the stability constraint. Therefore there is a trade-off between the performance and controller stability.

ACTIVE CANCELLATION OF ACOUSTIC PRESSURE AND PARTICLE VELOCITY IN THE NEAR FIELD OF A SOURCE

In a local active noise control system the pressure signal from a single microphone is usually taken as the error signal and cancelled by the action of a secondary source to create a zone of quiet. In this paper it is shown that the strategy of cancelling the acoustic pressure and a component of the particle velocity at a point in the near field of a secondary source considerably improves the acoustic performance of a local active noise control system with respect to the case in which only the acoustic pressure is cancelled. It is also shown that in the near field of a secondary source the active cancellation of the acoustic pressure and the particle velocity component due to only the secondary source produces similar near field zones of quiet to those obtained when the acoustic pressure and the total particle velocity component are cancelled instead. This suggests that an array of two loudspeakers having a fixed gain and phase relationship could be used as the single secondary source. The acoustic performance of a secondary source array formed by two loudspeakers is theoretically studied when the acoustic pressure and the secondary particle velocity component is cancelled at a point in its near field with and without a diffracting head present. The results show that this sort of secondary source array can produce larger near field zones of quiet than a conventional loudspeaker cancelling the acoustic pressure only. Finally, the acoustic performance of such a secondary source array in a local active noise control system with a virtual microphone arrangement that projects the zone of quiet further away from the secondary source than the position of the physical microphone is also studied. For this sort of arrangement, it will be shown that the cancellation of the pressure and the secondary particle velocity at two different points in the near field of the secondary source gives better performance than the cancellation of both acoustic magnitudes at the same field point.

Generation of zones of quiet using a virtual microphone arrangement

A local active noise control system is described which uses a virtual microphone arrangement. This arrangement is based on the assumption that the primary pressure at the physical and at the virtual microphone locations are similar. The implication of this assumption on the acoustic performance of a local system in a diffracted primary sound field is theoretically studied. The results show that the error at the virtual microphone position is lower when the virtual microphone arrangement is in the vicinity of a diffracting surface. A practical local active noise control system in a headrest has been built and used to measure the zone of quiet produced by a single and a dual channel system when the total pressure is canceled at one or two virtual microphone positions. It is shown that this type of arrangement is capable of projecting the zones of quiet further away from the secondary source than the position of the physical microphone. The measured zones of quiet produced by a single-channel system have been compared with the results produced with a theoretical model which includes the diffraction between two spheres, one representing the secondary source and the other the listener’s head. It has been found that, in the frequency range of applicability of a local active noise control system, the two-sphere model predicts the experimental results well. The effect that the diffracting head has on the performance of an adaptive feedforward controller using a physical and a virtual microphone is also explored and the results show that this effect is small provided the initial system identification is performed in the presence of the head.

Three-channel stereo playback of two tracks derived from three microphones

Playback of two-track stereo source material with a derived center channel offers accurate reproduction of the original stereo geometry, and requires very simple implementation. Essentially this two-track three-channel stereo depends on the principle that if two microphones are properly placed relative to each other and to the plane of the sound source, their combined output is that of a single microphone in the center, which output may be recovered by recombination of the two tracks. When a physical third microphone in the center is employed to feed the two tracks, its recovery from two tracks depends on relative polarity and amplitudes and in one recombination method the center microphone could be cancelled instead of being reproduced. An all-pass network may be used to shift the phase of one track so that on recombination the physical third microphone is always recovered, regardless of the manner in which it was mixed into the two tracks. The all-pass network produces 90° phase shift at only one frequency, but by choice of this frequency and the expectancy of additive polarity of original mixing, the center channel is recovered with excellent acuity, based on tests similar to those of Steinberg and Snow. Experimental recovery of a center output channel from a single center microphone feeding the two tracks, with flanking input signals zero, resulted in a center track output which was substantially indistinguishable from a normal monophonic reproduction.