Fluid Dynamics and Acoustics Group




By Professor Philip A. Nelson


The introduction of the compact disc into widespread use during the early 1980's marked the start of a new era in the technology of sound reproduction. The ability to capture and store acoustic signals in digital format produced a massive advance in the quality with which sound could be reproduced. At the Institute of Sound and Vibration Research (ISVR) at the University of Southampton, the latest research has shown that digital technology can also be used to produce a further vast improvement in the quality of sound reproduction systems. The work has concentrated on improving the ability of audio systems to produce "images" of sound sources perceived by the listener. In short, we try to produce the illusion in a listener of being in a "virtual" acoustic environment which is entirely different from that of the space in which he (or she) is actually located. We are thus attempting to achieve the long sought-after goal of making a listener in his living room hear sound as if he were in a concert hall. The availability of modern electronic technology for processing acoustic signals digitally has Anechoic chamber / ISVRtransformed our ability to generate this illusion, almost irrespective of the environment (living room, office or automobile interior) which surrounds the listener. The approach that we take is to process acoustic signals prior to their transmission by loudspeakers. We undertake this processing in order to generate the illusion in the listener that sound is coming from a number of "virtual" sources in well defined spatial positions relative to the listener. Of course, the intention of conventional "stereo" sound reproduction by loudspeakers is to produce just such an illusion, but two channel stereophony is capable only of producing acoustic virtual source images over a very narrow range of spatial positions, these being restricted to positions in the plane of, and in between, the two loudspeakers used for reproduction. The use of modern signal processing techniques can remove this restriction, even when only two loudspeakers are used for reproduction. A number of approaches to "3D Audio" have been developed in recent years, but few have correctly tackled the basic signal processing problem that has to be solved. This is the design of a processing scheme that ensures that the correct signals are produced at the listener's ears. In order to achieve such a goal, the processing scheme has to account for the effect on the signal of the loudspeakers, of the transmission path (including room reflections), and of the effect of the listeners head and torso on the propagation of sound to the ears. The central problem to be tackled is one of "inversion" where all these effects have to be "turned upside down" (and thus compensated for) before the signals are transmitted by the loudspeakers. This is a problem with many technical subtleties, but by tackling it correctly, it's solution can produce remarkable results.

Below we describe some of the ISVR research in this field, much of which has been undertaken jointly with Professor Hareo Hamada's group at Tokyo Denki University (TDU) in Japan. In particular we highlight the development of a very specific virtual imaging technique based on the "Stereo Dipole" which uses a pair of very closely spaced loudspeakers to produce virtual images. The major advance associated with this work is associated with the precise form of the acoustic field produced by the loudspeakers which makes it an ideal means for producing virtual images. The "Stereo Dipole" can be spectacularly convincing and we expect this technology to come into widespread use in the not too distant future.

Other topics addressed will include a description of the physical form of the acoustic fields produced by such imaging systems, the signal processing technology required and the influence of the environment in which the imaging is undertaken. There are many factors controlling the success of these systems, not least of which is the complex processing of acoustic signals that occurs in the auditory system of the listener. However, many of the features of the performance of virtual acoustic imaging systems can be explained in terms of the known behaviour of the human auditory system in localising acoustic sources. In addition to this, we are very much aware of commercial developments associated with multi-channel "surround sound" systems for "home cinema", and we are working on multi-channel signal processing techniques which could one day greatly enhance our ability to record and reproduce sound fields with multiple loudspeakers. We hope you enjoy visiting our web site and that you can share some of the fascination which makes this multi-disciplinary research such an enjoyable field in which to work.