Generation of zones of quiet using a virtual microphone arrangement

Abstract

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.

Design of Remote Quiet Zones Using Spot-Type Sound Reducers

“…The following section presents simulation results which reproduce previous work on the diffraction effect of the listener's head modeled by a rigid sphere [7], and on the effect of the diaphragm surface area of the secondary sources on the size of the quiet zone [4]. Figure 6 (left) illustrates the effect of a rigid diffraction sphere of 8.75 cm radius in the middle of the quiet zone.…”

“…In the early 1990s, Elliott and David proposed using a virtual microphone arrangement that can move the zone of quiet so that it is further away from the secondary sources than the physical error microphone [21]. This arrangement is based on the assumption that, at low frequencies, the spatial rate of change of the primary sound field is small at the physical and at the virtual microphone locations [7]. In the early 2000s, Kestell et al showed that the zone of quiet could be further extended using a virtual energy density sensor from the pressure and pressure gradient [22], considering that the weighted summation of pressure and velocity is much more spatially uniform than pressure only.…”

A Kalman filter approach to virtual sensing for active noise control

“…Note that the implementation illustrated in Figure A.1 was adopted in previous research into virtual sensing methods for active noise control [2][3][4][5][6][7][8][9][10][11][12]. The plant in this figure can be described by the following standard state-space model [14] …”

“…The resulting zones of quiet are usually centered at the error sensors, and are often too small to extend from the error sensors to the observer's ears [1]. To overcome these practical limitations, a number of virtual sensing methods for local active noise control systems have been suggested [2][3][4][5][6][7][8][9][10][11][12]. These methods can be used to obtain estimates of the error signals at locations remote from the physical locations of the error sensors.…”

Modeling local active sound control with remote sensors in spatially random pressure fields

“…In the development of local active controllers there has also been considerable interest, for practical reasons, in monitoring the sound field within the region of control with sensors outside this region, as reviewed by Moreau et al 13 Originally a virtual microphone technique was proposed for the problem, [13][14][15][16][17] where the primary pressure was assumed to be the same at the sensor and in the control region. Later, systems were developed using the remote microphone technique, 13,18 which assumes a given transfer response between the sensor and control region.…”