York Science & Innovation Grand Tour May - Sept 2012

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University of York AudioLab researchers are using the latest 3-D audio modelling technology to recreate the sound of 16th century choral voices in the Abbey ruins.

Before its closure by Henry VIII in 1539, St Mary’s Abbey Church rivaled York Minster in both size and spectacle. What was it like 500 years ago to walk through its darkened aisles and hear soaring voices echo through the gothic vaults? Researchers in the AudioLab are bringing the sounds of the past to life using the technology that architects employ to design buildings of the future.

Images: Stephen Oxnard and Aglaia Foteinou.

The Acoustic Reconstruction of St Mary’s Abbey
Damian T. Murphy, Stephen Oxnard, Aglaia Foteinou
AudioLab, Department of Electronics, University of York, UK.

Auralization – An Introduction
Sound is often considered the poor relation to our visual sense, yet it plays a significant role in conveying information for rapid assimilation by a listener – we can hear, locate and understand the meaning of an ambulance siren long before we see it.

Sound working in combination with images is a key component for the successful perception of virtual reality environments – with perhaps the most obvious example being the cinema where good, integrated use of audio-visual material allows us to suspend disbelief and immerse ourselves in the story and characters of the film we are watching.

Auralization is therefore the audio equivalent of visualization, and visualization as an art form we have come to accept and appreciate in the modern use of computer graphics in film, television and video games. Computer visualizations are easy to comprehend and appreciate, and they can impart such a sense of quality that we accept them as some form of reality, be they based on actual real-world scenes or an imaginary subject or landscape. We are visual beings and computer rendered visualizations, whether static or dynamic, allow us to pause and appreciate, for instance, the beauty of detail, colour, or depth of field of the rendered scene. Recreating the equivalent with sound, however, is in many ways a much more complex process – it is a constantly changing, ephemeral experience with few fixed points of reference (unlike a visual landscape), and our perception and understanding of it can depend on many different aspects – our own personal sound experiences; the choices made by the designers in presenting the audio material to the listener; whether this is for personal listening (headphones) or shared experience (as in the cinema) – but the results can leave an impression or memory with us after images have long since faded.

The starting point for auralization is a model of a particular environment, usually enclosed, so a room rather than an outdoor space. The classic example, from where much of this work has originated, being a concert hall. Into this room, we place a sound source (perhaps an opera singer if we take the concert hall example further) at a particular location (on the stage) and a listener (in the best seat in the house). We then wish to recreate for this listener the binaural listening experience (that is, heard at both ears) of the opera singer on the stage of the modelled concert hall, as heard from the best seat in the house.

This requires acoustic knowledge about the sound source (what sound does our singer make? In what direction does the sound travel, and how do these properties vary over time or with audio frequency?); how the soundwaves so created propagate through the concert hall (how far do they travel before arriving at the listener’s ears? What happens when sound interacts with a wall, or an object in the room? What effect does air have on sounds travelling through it?); knowledge about the listener’s head and ears (the size and shape of the outer ears – the pinnae; does the listener move their head or remain static?). Will this modelled sound then be presented to the actual listener over headphones or over two (stereo) or more (surround-sound) loudspeakers, and if loudspeakers are to be used, will the listener be positioned in the middle of them – at the so called sweet-spot?

The acoustic impulse response is one of the direct outcomes of the auralization modelling process and can be considered as the audio fingerprint for a given space with a given source and listener position. Once this impulse response has been synthesized, it can be used to process any audio signal or sound recording. The result being that the recorded sound is heard as if it were being played back in the modelled space, at the position of the source from the perspective of the listener. This technique, the audio rendering of an acoustic space with impulse responses obtained from either a measurement or a model is known as convolution reverberation.
Auralization is most commonly used in Architectural Acoustics. As part of the design of any building, the characteristics of the building’s acoustics – its very sound – need to be specified, designed and tested. However, one of the most recent and interesting areas where auralization has been successfully used is in the understanding and preservation of heritage. Heritage buildings, sites and landscapes – our architectural and archaeological past – are all subject to change over time and so their audio/acoustic preservation is just as important for an understanding of the past by current and future generations of researchers as any of their other material or visual aspects or properties. With appropriate guidance from experts in archeology, architecture or history, we can piece together the collapsed stones of medieval buildings and listen again to the echoing words of the people who inhabited them and this is what has been done in the case of St Mary’s Abbey.

The Sound of St Mary’s Abbey
The Church of St Mary’s Abbey was mostly destroyed during the dissolution under the rule of Henry VIII and now only the ruins that can be found in the Museum Gardens adjacent to the river Ouse in York remain.

The particular project on which this work is based sought to build a 3D computer model of the church based on existing evidence, and produce auralizations to gain a better understanding as to what this building, which would have been comparable in size to York Minster, sounded like. In particular experiments were designed to test the effect that adding layers of architectural detail to the model had on the resulting sound, and whether listeners could perceive any difference in what they heard due to these changes being made.

The first version of the was based on a simple outline geometry consisting of large ceiling, floor and wall surfaces defined by 56 corner points and 40 surfaces. Interior columns, window bays and windows were included in the second model and the final phase saw the inclusion of interior archways and roofing structures. The second and third phase models consisted of 1144 and 2114 surfaces respectively.

Even with this stage of complexity, the level of detail is very basic compared with even the simplest graphical visualizations as the interior image shows, but it is sufficient to enable a simulation to take place demonstrating sound propagating and spreading out down the nave, aisles and transepts from as source placed at the east end of the church.

Finally using the simulation depicted on our exhibit it is possible to recreate and listen to the sound of a lone voice echoing once more through the darkened aisles and gothic vaults of St Marys using these acoustic reconstruction and auralization techniques.

Download the sound files

Acknowledgement:
With thanks to Dr Anthony Masinton from the Department of Archaeology, University of York for his guidance in the development of this model and the original audio recording used and Nigel Holdsworth for his singing performance used as the sound source for the auralization.
Selected References:
Pevsner, N., and Neave, D., Yorkshire: York and the East Riding (Pevsner Architectural Guides: Buildings of England), Yale University Press; 2nd New edition of Revised edition, 1995
Wilson, B., and Mee, F., St. Mary’s Abbey and the King’s Manor, York: The Pictorial Evidence, York Archaeological Trust, 2009. ‘St. Mary’s Abbey and the King’s Manor, York: The Pictorial Evidence’ by Barbara Wilson and Frances Mee
Kleiner, M., Dalenbäck, B.-I., and Svensson, P., “Auralisation – an Overview”, Journal of the Audio Engineering Society, (pp. 861-875), Vol. 41, No. 11, (Nov 1993).
Foteinou, A., Murphy, D.T., and Masinton, A., “Investigation of Factors Influencing Acoustic Characteristics in Geometric Acoustics Based Auralization”, Proceedings of the 13th Int. Conference on Digital Audio Effects (DAFx-10), pp. 178-181, Graz, Austria, Sept. 6-10, 2010.

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