Virtual Acoustics for Heritage Spaces
by Damian Murphy and Jude Brereton
For a number of years, research at the AudioLab in the Department of Electronics, University
of York has had a strong focus on the measurement, study and modelling of acoustic spaces.
In particular this work has focused on heritage spaces – that is, buildings, either complete or
otherwise, that demonstrate some feature of architectural, archaeological, historical or
acoustic interest. Such examples to date range from Maes Howe passage tomb on Orkney,
through to The Reactor Hall, the subterranean remains of Sweden’s nuclear energy research
programme from the 1950s. Hence the term virtual acoustics refers to the computer based
reconstruction of the acoustic stimulus created by listening to a particular sound placed
within a particular building (either real or imaginary, complete or ruined). The desire is to
immerse the listener within the recreated acoustic field such that they perceive it as a reality.
Another common term used to describe this technique is auralization – defined as a parallel
to the more commonly understood term, visualisation.
This work has been based on developments in measuring the acoustic characteristics of
concert halls and opera houses that have resulted in standardised methods of impulse
response capture for a wide variety of auralization applications. The impulse response is a
short, sharp sounding audio recording (like a handclap) and is essentially the acoustic
fingerprint for a given space and uniquely defines its acoustic characteristics for a particular
sound source and listener combination. From these measurements objective acoustic
parameters (such as reverberation time) can be determined, and the impulse response is
central to the creation of virtual experiences based on an acoustic representation of the space.
This measurement process is an automated one-pass solution, designed to capture as much
information as possible in as short a time frame. A typical measurement session will take
about 3 hours, generally enabling three different source/listener combinations to be obtained.
Unfortunately, it isn't always possible to obtain measurements for a particular space.
Sometimes the space might be inaccessible, or it might not be practical (too much noise or
too many people). Alternatively the space might not exist - it might only be an architect's
concept design or a composer's ideal and specific performance space, or perhaps the space no
longer exists in its original form, as is often the case with heritage sites. In such
circumstances a virtual acoustic model must be developed. This has long been the practice in
architectural design of acoustically critical spaces where it is possible to develop "walk
through auralization" for clients, and these techniques are starting to gain acceptance in other
application areas.
Computer modelling can be used to construct a virtual representation of a site for a particular
set of circumstances. Such 3D graphical Computer Aided Design (CAD) techniques have
long been used in architectural and now archaeological practice, and such virtual 3D visual
walk-throughs are commonly used in heritage site interactives or popular documentary style
television shows.
Once the 3D computer model has been constructed graphically (often making use of high
resolution laser scanning of the site as shown in the example above) and used in an
appropriate visualisation the data can also be used for auralization.
There is some prior work in this area. Arup Acoustics are one of the leading advocates of
virtual acoustics, auralization and virtual reality applications in their acoustic research, design
and consultancy work with a network of dedicated auralization SoundLabs established at
their offices around the world. The CAHRISMA project explored the acoustics of certain
Mosques and Byzantine Churches, with a view to a better understanding of heritage sites
from the context of acoustic design. It included a significant amount of acoustic modelling
work, most (if not all) of which was completed using the leading commercially available
ODEON geometrical acoustic modelling and auralization software. However the user must be
aware of the limitations that such software imposes – there are many variables that have to be
fine-tuned to arrive at something that might be considered an accurate or truthful
representation of the space being studied. Hence novel wave-based approaches for accurate
low-frequency modelling are often more appropriate and research in the AudioLab also
focuses in this area through out own modelling and auralization system – RenderAIR.
St Michael's Cathedral - 'Old' Coventry Cathedral
York has had a long standing collaboration with The Dogrose Trust - "an innovative charity
which works to make all environments accessible to people with visual and other sensory
impairments [and] committed to the use of Universal and Inclusive Design and
Communication and the Multi- Sensory design”. In 2004 we were asked to collaborate on a
project they were leading that involved the development of a CD-based audio tour of St
Michael's Cathedral, Coventry - or as it is known, 'Old' Coventry Cathedral - that was mostly
destroyed in an air-raid on the night of 14th November 1940 during the Second World War.
Only the original walls and spire of this medieval building now remain. The intended
audience for the CD included those with visual impairments and so it was decided that the
quality of the soundtrack was critical and this was to include a reconstruction of what the
building would have sounded like at the height of its importance around 1500. The project
included the recording of an appropriate narrative and choral plainchant for use as source
material.
ODEON was used to develop the acoustic model, and architect Peter Howell from the Trust
did the hard work of interpreting appropriate plans and various sources to develop a
reconstructed and simplified "stripped down" plan and section view that could be used as the
basis for the virtual acoustic simulation. The wireframe model produced in ODEON is shown
below (Left). As can be seen it is much simplified (e.g. no spire) from what the original
building would have been like, but is sufficient for an acoustic simulation where much of the
architectural detail (probably) has minimal influence on the final acoustic result given the
size, shape and scale of the overall space. Peter also gave us guidelines on what materials
would have been used on the walls and bounding surfaces to help us in our acoustic design
work. The internal 3D view is also shown below (Right).
Two sound sources were placed - the plainchant choir in the quire of the church (visible at the
far end of the internal view) and the narrator positioned in the centre of the nave (the
approximate viewpoint in the internal view). The listener/receiver was placed next to the
narrator. A surround- sound (first-order ambsionic B-format) response was generated for the
receiver position and from this a standard stereo response was derived. As an indication as to
the acoustic properties of the virtual space, the reverberation time (as defined by ISO3382T30) was calculated with the results shown in the table below.
It is of course impossible to say whether these results are correct given that the building no
longer exists in its original form. Also, geometric acoustic techniques are well known to be
unreliable in the low frequency (up to about 1kHz) region, but the general result was
considered to be appropriate for the space given what was known about its design and how
this was actually interpreted for the simulation. Soundfile examples are available here.
By way of comparison, the results for York Minster (a much larger space), with the sound
source placed directly under the centre of the Tower (or Crossing) with the listener placed
23.5m further down the centre of the Nave are as follows:
Soundfile examples for York Minster are available here.
Sound and the heritage experience
Although the auralization of ‘Old’ Coventry Cathedral described above was produced as part
of an audio-based tour, particularly with visually impaired people in mind, it is worth
remembering that sound plays an essential part in everyone’s experience of heritage spaces.
Very often visitors to heritage sites come with an underlying assumption that the best way to
experience the site is during is in silence and contemplation. York Minster, for example, is
often experienced by visitors in respectful quiet, but is at its best when filled with glorious
sound. Visitors often come to the Museum Gardens today for a ‘bit of peace and quiet’
whereas throughout its history the site of the Abbey precinct was not always the peaceful
haven it may be today.
The use of digital technology to deliver visualisation and auralization of spaces is still in its
infancy, and software and hardware possibilities are improving rapidly. For example, it is
now possible to deliver user-appropriate audio-guides directly to the visitor’s own mobile
phone. In an outdoor space the audio content can be delivered to the user automatically
depending on the their actual location around a site through the use of GPS tracking
technology, which is becoming increasingly more widespread in hand-held devices and
mobile phones. York has an abundance of heritage spaces, churches, parks, factories, sites
and landscapes. Some are well preserved, while others are ruined and many, whilst still
extant, are used for purposes much different than those originally intended. This gives great
scope for the use of state of the art audio- guides and the kinds of auralization techniques
described here, since sound is key in enhancing the visitor experience whether to reconstruct
the acoustic of a now ruined church building, capture the noise an industrial site in its heyday
or to preserve the sound of today’s cherished spaces for future generations.
Hence sound and in particular acoustic modelling has an important part to play in the
interpretation, understanding and recreation of heritage spaces through the powerful,
immersive and believable audio based virtual reality experiences they can create, especially
when used in combination with high quality visual imagery.