The University of Southampton
Engineering and the Environment

Research project: Local Control of Sound

Currently Active: 
Yes

Attenuation of sound throughout an entire enclosures, or global active control, is only possible at very low frequencies, where only a few acoustic modes contribute to the sound field in the enclosure. At higher frequencies, with more complex sound fields, global control requires an unreasonably large number of loudspeakers, and the preferred alternative is to control the sound at a smaller, restricted zone, usually surrounding the listener's ears. This approach is therefore called local active sound control, and can be implemented in a headrest of a seat, for example, as suggested by Olson and May back in 1953.

Project Overview

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Although the concept of local active control is not new, local control systems are not widely spread commercially, probably due to the inherent physical and system limitations. An extensive effort has been put in recent years in the ISVR to study the acoustics and control aspects of local control systems, mainly as implemented around a headrest.

The acoustics of a local active headrest system plays an important role. The extent of the zone of quiet was found to be limited to a tenth of a wavelength (Elliott et al, 1998) and is therefore of a practical extent only at frequencies below around 500Hz. The size of the zone of quiet usually depends on how "spatially flat" the secondary field be made (Garcia-Bonito and Elliott, 1995).

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A large non-directional loudspeaker and the existence of the listener head usually help to achieve this, thus extending the zone of quiet. An experiment on a laboratory headset with tonal disturbances and feedforward control has been performed to verify the acoustic limitations and validate the analytical models used in predicting the sound fields around a headset system (Garcia-Bonito, Elliott and Boucher, 1997). The use of a secondary source constructed from two loudspeakers showed the potential for larger zones of quiet (Garcia-Bonito and Elliott, 1999).        

Current studies in the ISVR are concerned with at the limitations imposed by the control system, mainly for broadband disturbance in a feedback control configuration (Rafaely and Elliott, 1999). Such a system is required to attenuate the noise in a jet aircraft, for example. It was found that good performance is achieved only if the secondary field is designed to cancel the pressure at the ear, using a "virtual microphone" design (Rafaely, Garcia-Bonito and Elliott, 1997), and that good performance is still achieved with head movements. The use of two loudspeakers in a feedback control configuration is the subject of a current study, where virtual microphone designs are integrated with MIMO feedback control and spatial sound minimization to provide best attenuation near the listener's ears.

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