We have come to accept and appreciate visualisation as an art form via the modern use of computer graphics in film, television and video games. Computer visualisations 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 visualisations, 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 auditory equivalent using auralisation, 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 over multiple loudspeakers (as in the cinema) - but the results can leave an impression or memory with us after images have long since faded. Our ears and brain are finely tuned interpreters of many competing streams of complex auditory information, and are sensitive to a broad range of acoustic sensations, both in terms of frequency (from bass to treble) and dynamic range (from silence to the threshold of pain).
One formal definition of auralisation is as follows:
'...the process of rendering audible by physical or mathematical modelling, the sound field of a source in space, in such a way as to simulate the binaural listening experience at a given position in the modelled space' (Vorländer 2008).
The starting point is a model of a particular environment, the classic example being a concert hall, from where much of this research has originated. Into this environment, we place a sound source (for instance, an opera singer, in the example of a concert hall) at a particular location (on the stage) and a listener (situated in what might be considered as the best seat in the house). We then wish to re-create for this listener the binaural listening experience of the opera singer on the stage of the modelled concert hall, as heard from their seat. More rigorously, we wish to re-create the acoustic pressure sensations at each of the listener's eardrums (hence binaurally). This requires acoustic knowledge about the sound source - the properties of the human voice when singing opera, the directions in which the sound travels, and how these properties vary over time or with audio frequency. Knowledge of how the sound waves so created propagate through the concert hall is necessary, including: the distance travelled before arriving at the listener's ears, changes imparted through interactions with a wall or an object within the room, and the effect of air as the medium on the sound waves passing through it. It is also important to have information about the listener's head and ears - the size and shape of the outer ears (pinnae); whether the listener moves their head or remains static. Finally, this modelled sound needs be presented to the listener - over headphones, or over two (stereo), or more (surround-sound) loudspeakers; if loudspeakers are to be used, will the listener be positioned in the middle of them – at the so called sweet-spot – or in a non-optimal seating position as part of a wider audience (as in cinema presentation). Auralisation separates the experience of listening to a sound within a given environment into the constituent acoustic elements, from sound source to listener's ear, and how this same effect can then be reproduced over an audio system. As a result this whole listening process can be better understood, and with understanding comes the ability to control, reshape and re-imagine the listening experience.
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