Toward Reality Equivalence in Spatial Sound Diffusion

The diffusion of sound in space has always been a major topic interest to many composers working with music composed, produced, or performed using electronic means. Despite the rich possibilities inherent in current sound diffusion and spatialization systems, none of the current systems can fully mimic the spatial characteristics of natural sound systems. By examining the nature of our spatial hearing and the history of sound spatialization systems, this article attempts to map possible routes, taking us closer to full "reality equivalent" sound diffusion systems.

Since the remote transmission of sound became possible in the late 19th century, ways have been sought to enable sound systems to deal not only with the frequency and amplitude elements that make up a sound event, but also with the spatiotemporal ones. This is, naturally, of the greatest importance when the sounds being transmitted are musical in nature. Prior to the current era, all musical events were fixed in and wholly part of the acoustic location in which they were being performed. With the breaking of that age-old bond came enormous challenges for the audio engineer in finding ways of restoring the bond correctly after such space/time transmissions. Perhaps more importantly, enormous opportunities opened for composers explore and exploit the relatively little-known territory of space in music.

Edgar Varèse may be regarded as the first composer to grasp the importance of these new opportunities. When he began composing Intégrales (1925), he intended to explore something he first became aware of when listening to the scherzo of Beethoven's Seventh symphony:

Probably because the hall [Salle Pleyel, Paris] happened to be over resonant . . . I became conscious of an entirely new effect produced by this familiar music. I seemed to feel the music detaching itself and projecting itself in space. I became aware of a third dimension in the music. . . . [I]t gives a sense of . . . a journey into space.

(Varèse 1936)

The phrase "spatial music" appears to have evolved first in relation to Intégrales (Ouellette 1973) and the work itself can be regarded as ushering in the modern era of space in music. Varèse himself has said of the piece:

Intégrales was conceived for a spatial projection. I constructed the work to employ certain acoustical means which did not yet exist, but which I knew could be realized and would be used sooner or later. . . .

(Varèse 1959)

Since then, of course, many composers, particularly those involved in electronics-based genres, have ventured into this area, and engineers have been pushed to meet the demands for newer, better, and more interesting sound spatialization and diffusion technologies. As others have remarked in relation to the simulation of "real" or "natural" instrumental timbres, it is perhaps only by learning how to achieve a full simulation of the real and natural that we can appreciate the full extent of what is possible in the synthetic. It is therefore instructive to consider how far we have come today and where we might go in the future, in terms of building acoustic images in space that mimic reality.

Although much progress has been made, for instance with the development of HRTF-based binaural systems, cinema-style systems such as various 5.1 systems, ambisonics (Gerzon 1972,1975; Fellgett 1975), holophonics (Nicol and Emerit 1998), wave-field synthesis (Boone, Verheijen, and Jansen 1996), and hyper-dense transducer array technology (Malham 1999a), we are still a long way from having achieved performance that comes close to that of a natural soundscape. This article examines some of the issues involved as well as the history of our efforts to move toward this, and it uses that background to suggest possible future developments.

"Full Reality" Sound

For those people who do not to suffer from significant impairment of either visual or aural senses, the visual sense tends to be regarded as more important than the sense of hearing in the perception of what we call "reality," at least when we are not discussing music. When we think of a beautiful scene, it is the shape of the hills the...