Introduction

Multichannel audio is now widely available to the consumer. These multichannel formats demand numerous loudspeakers. While five or more loudspeakers in the room may be acceptable to the enthusiast, to the normal consumer this is an undesirable compromise. A more widely acceptable compromise, however, is likely to be a small reduction in the surround performance in exchange for fewer 'boxes' around the room.

In order to realise a virtual surround system it is necessary to devise some means of tricking the human auditory system into thinking that a sound is coming from somewhere that it is not. Our aim is to create “virtual soundfields” which envelope the listener while only utilising few loudspeakers.

Perception

Comparison of the signals at each ear allows the brain to determine the spatial characteristics of the soundfield in which the subject is located.

If the two signals are sufficiently coherent, sources may be localised without too much difficulty. In this context, the degree of coherence is a measure of how perfectly these signals can destructively interfere . The coherence of two signals follows from how well correlated the two signals are [1] A conventional loudspeaker is a very coherent radiator of sound. It is relatively easy for the auditory system to determine the direction from which a loudspeaker radiates. With multiple loudspeakers the location of ‘virtual sources’ produced by amplitude panning is similarly clear. [2]

Where the sound at each ear is incoherent the sound tends to be attributed by our cognitive processes to the environment. This leads to the perception of ‘spaciousness’ that embodies a sense of awareness of the surrounding space. [3]

Multiple Loudspeakers, together with appropriate signals, are able to produce spatially incoherent sound-fields. It is common for multiple loudspeakers to be used for the rear channels in cinema audio systems in order to give a more spacious surround effect. Spaciousness can also be experienced in a similar way, albeit to a lesser degree, with stereo.

Localisation

An important feature of our anatomy is that we have two ears spaced some distance apart. The physical arrangement of the ears, on either side of the head, along with the anatomy of the torso, head and pinnae, provide a difference in the two signals arriving at each ear.

The difference in distance between source and ears contributes an azimuthally level difference and arrival time for incident sound. The relative difference in level & arrival time between the ears is commonly referred to as the inter-aural level difference (ILD) and the inter-aural time difference (ITD).

These two effects, ITD & ILD, are at the root of the mechanism that allows us to determine the direction of coherent sounds.

One effect of the full pinnae, head and torso geometry is to introduce direction varying frequency response differences. This information is also used during localisation. It is believed that these differences, together with head movement, allow the perception of the height of a source.

The full transfer function between a source in the far field and the ear canal is referred to as the Head Related Transfer Function (HRTF). Due to the variation of shape and size of the pinnea, head and ear canal the HRTF varies between individuals.

Spaciousness

A diffuse field has the property of random incidence. Acoustic waves travel in every direction with equal probability. In this situation we could expect the soundfield at our ears to be totally uncorrelated and the localisation mechanisms discussed above cannot function.

To illustrate this consider the situation shown in figure 1 where the subject is surrounded by an applauding crowd. In circumstances such as these the signals arriving at the ears are uncorrelated. The normal spatial mechanism based upon signal arrival times and signal level differences cannot function. Instead of a perception of localisation a perception of spaciousness is experienced. The subject is aware that the soundfield is around them, they are enveloped by it, but they are unable to determine the locations of the individual sources.

This sensation is often experienced in a large reverberant space, particularly with continuous sounds. Localisation becomes difficult and is replaced by a sense of spaciousness. In such a space one is much more likely to be in a diffuse field, particularly if the source is distant and continuous.

In structural acoustics the inter-aural correlation coefficient [4] is one commonly used metric for the degree of diffuseness of a reverberant field. A low IACC is desirable for concert halls since it indicates a diffuse sound which will be percieved as spacious.

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