MAXIS - Music Alternative and X-Disiplinary Approaches in Sound
Researchers
- Peter Lennox
- Bruce Wigggins
- Iain Paterson-Stephens
This project involves the design and development of a multi-channel system for experimentation and display of artificial soundscapes. The system comprises 40 speakers at discrete locations around the surface of a nominal sphere, or part thereof. The radius of the sphere can be up to 20 metres, though in practice this may often be nearer to 10 metres, depending on local facilities. Generally, this virtual sphere will be used in truncated form, with the floor of the listening area intersecting the lower part, resulting in 50% to 60% of the sphere's volume being generated. The estimated maximum comfortable capacity is likely to be 200 persons. The intention is to produce and control the salient spatial features of the sound field within this listening area, for greater than 90% of the listeners, which in practice means that spatial 'accuracy' must be discernible at any position within approximately 70% of the part-spherical sound field.
One of the research aims is to eventually closely specify what is meant by "spatial accuracy" in large-scale listening circumstances such as these, for a wide variety of expert and non-expert listeners, and to look for variations between such groups. From this, and in accord with a developmental aim, the intention is (funding permitting) to feed results back into system development in an iterative fashion, to improve control-and-display of "spatial accuracy". This is facilitated by the nature of the speaker-feed decoding software, which inherently is capable of running several different coding regimes (mono, stereo, quadraphonic, 5.1 and ambisonics, for instance), simultaneously. Comparisons between codecs, and more importantly, optimised hybrid codecs, can be auditioned and compared with minimal delay.
The other significant aim in designing this system is facilitation of collaborations with composers who have a significant interest and/or expertise in the area of "spatial sound fields" - whether as music or sound effect (such as accompaniment for televisual material). This requires audiences, of course, and so the system must be constructed with health-and-safety and aesthetic considerations properly attended to. Demonstration '3-d sound' material has been kindly contributed by composers, engineers and production companies from around the world.
Considerable interest has been expressed in this aspect of the system's capabilities, from organisers of festival and conference events; nevertheless this remains a highly specialised niche, largely consisting of professionals in use-of-sound applications. Because the system is intrinsically scaleable, down to quite small listening rooms, there is scope for flexibility in proposed usage. The system will first be demonstrated at the "Maxis" event in April 2002, at Sheffield Hallam University; this is a symposium of about 200 people, dedicated to the experimental uses of sound.
Submissions include a variety of multi-channel audio formats, which this system can comfortably accommodate. Discussions are in progress with several other specialised-event organisers. Nevertheless, as this is a research-and-development initiative with a stated aim of pushing back boundaries beyond what is considered feasible in a business sense, this system will not be commercially available in wider markets.
Technically, the basis of the unusual flexibility which this system enjoys is the principle whereby data-channels are uncoupled from speaker-channels. This is so because "3-dimensionality" can be expressed mathematically and hence 3-d manipulations can be carried out comfortably by current generations of processors available in off-the-shelf computer technology. This mathematical system, and the technology which realises it, is called "ambisonics", and was invented by Micheal Gerzon in the 1970s; however, software realisations could not be extensively explored until quite recently, with the maturation of computer capabilities. More recently, the mathematical bases for various different approaches to spatial sound rendition has been investigated and has resulted in considerable convergence; even where technologies are based on quite different psycho acoustic principles, 'interchangeability' is becoming feasible.
There are several properties of this type of system which, though not originally intended, have been observed by workers in the field. One is the issue of system headroom, or practical dynamic range of the system-as-a-whole. In a conventional public address system, where moderate to high sound pressure levels are sometimes required, many components contribute 'compression' artefacts when operating near the upper boundaries of their performance design. These components in a complex system can sum in unpredictable ways, resulting in theoretically sub-optimal system performance. The type of system under construction here, which is a development of previously realised 12- and 16-channel designs is well known to display a dynamic range which appears to be beyond the capabilities of the individual components. That is, distortion-free performance is apparent at sound pressure levels not envisaged by component designers.
Another opportunity arises out of the nature of the codec: low-frequency management can be individually attended to within the central codec, to optimise SPLs and spatial depiction. Whilst considerable dissent is evidenced in the public-sound industry as to the value of directional accuracy of low-frequency sounds, we have found strong psycho acoustical evidence in support of the notion that mechanisms for high and low frequency directional hearing should agree where possible.
A final property which has recently been observed is that, under certain circumstances, the coherent rendition of 'distance information' is better than had been predicted, or has been observed in other systems. The robustness of this type of property is under investigation.
The system also offers improved support for visual material and especially for increased viewing angles such as in experimental cinema or even 'surround vision'.
