Creating Virtual Soundfields

As stated in the introduction, our aim is to create “virtual soundfields” which envelope the listener while only utilising few loudspeakers.
A Virtual surround system must trick the auditory mechanism into believing that a sound is emanating from a location where it is not. There are several ways that this has been attempted as described in the following sections.

Application of Head Related Transfer Functions

HRTF measurements contain all of the information necessary to deduce the approximate signals at the ear canal corresponding to a source anywhere in the soundstage. Using HRTF measurements it is simple to convert a multichannel signal into a two channel signal; one containing the signal to be delivered to the left ear, one containing the signal to be delivered to the right ear.

These signals must then be delivered to the ears for the listener to perceive the correct image. The delivery of the signals is difficult. Playing the signals through conventional loudspeakers is not effective since the signals are not delivered exclusively to each ear. From the left ear the listener is able to hear the right speaker and vice-versa. This effect is known as crosstalk.

The most direct way to reproduce the signals at the ears without crosstalk is to use headphones. The HRTF approach is often the basis of virtual surround systems using headphones [6]. However, delivery with headphones is not ideal, the experience is solitary and the listener is isolated from the actual acoustic environment in which they are located. There is also some imaging performance degradation, when locating a sound a subject will subconsciously adjust their head position modifying the HRTF in order to refine the location. With headphones it is clear that moving the head will have no effect on the signals at the ears, this can result in a "sound in the head" sensation.

A more effective way of delivering the signals is by use of loudspeakers and a crosstalk cancellation system. Crosstalk cancellation requires a network of filters which are the inverse of the transfer function between loudspeaker and listeners ears. This allows the signals arriving at the entrance to the listener’s ear canals to be controlled. As this method is based on system inversion, problems can occur if the acoustic paths are badly conditioned [7]. The Optimal Source Distribution (OSD) [8] approach uses a speaker array designed to aid the filter inversion. When used with binaural recordings this system is remarkably convincing.

More commonly, systems have been designed to work with normal loudspeakers. [7] In both cases the approach used relies on the listener being located exactly between correctly positioned speakers and close enough for the direct sound to be louder than the listening rooms’ reverberant sound.

Application of Local Boundary Reflections

A direct method of producing an image of a source in a different location to the loudspeaker is to use reflection. When a sound is reflected from a smooth large flat surface an image source is produced at a location determined by the angle of incidence.

To produce a virtual source by reflection the sound must be directed mainly at the wall with the amplitude of the direct signal kept to a minimum. Figure 2

Application of a Diffuse Field

A single conventional loudspeaker is a very coherent radiator and is relatively easy to localise. This is, of course, not desirable if we wish to disguise where the loudspeaker is. In order to achieve our goal of enveloping the listener while not surrounding them with speakers it is clear that the diffuse fields have some usefull attributes.

A diffuse field is not directional and is not localised by the listener. A diffuse field generated in front of the listener will consequently give a sensation of spacious envelopment.

Real environments produce ambient sound fields having diffuse characterstics largely determined by size, shape, and materials of the environment. Reproducing this ambient field involves mimicking their diffuse field. In a small room the presence of strong specular reflections makes this a difficult task. One approach is to use dipole loudpspeakers with electronic decorrelation achieved by frequency shifting [5].

The use of diffuse sources for surround channels is especially effective where the surround signal is mainly reverberant.

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