Testing VR Spatialization Plugins

At last year’s AES International Conference on Immersive and Interactive Audio, I presented a poster on a paper examining the differences between two spatialization plugins. This research was carried out as part of my work at the University of Edinburgh as described in my post about my work at The University of Edinburgh. The two spatialization plugins we examined was Google Resonance and Steam Audio.

The research examined the acoustics of St Cecilia’s Hall in the past and present. St Cecilia’s Hall is the oldest purpose-built concert hall in Scotland, opened in the 18th century, and was home to the Edinburgh Musical Society. The hall was a unique oval-shaped space which had a number of particular acoustic characteristics. By the end of the 18th century, the building had been sold off and the hall made rectangular.

In the mid-20th century St Cecilia’s Hall was bought by the University of Edinburgh and became a museum for their collection of historic musical instruments. The university decided to restore the hall back to its original oval shape, but the building regulations have matured somewhat in the intervening 250 years. Gone are the rows and rows of wooden benches and candle lights and in are electric lights and fire escapes.

3D modelling of the space

The hall was measured and mapped using LIDAR scans which was able to provide me with 3D mesh models, albeit with a very high number of polygons. The first step was to rebuild the model in 3DS Max to reduce the number of polygons into a manageable number. This template was used to create models of the hall in the present day and in the year 1769.

As stated, the hall was furnished very differently in the past and we used the following books as references as to what the hall would have looked like:

Blackie, J., A New Musick Room, The Friends of St Cecilia’s Hall, 2002

Rock, J., Hillman, M., and Bunch, A., The Temple of Harmony, Friends of St Cecilia’s Hall, 2011.

 

VR model of St Cecilia’s Hall in 1769

 

VR Model of St Cecilia’s Hall in present day

 

3D Model of St Cecilia’s Hall in present day in Odeon

 

The acoustic properties of the materials used for the hall in the past and present and the settings used for the different simulation software is given in the paper.

Impulse Responses

The method we used to compare the different software models was to obtain impulse responses (IRs) for each simulation. In addition, we measured the IR of the space itself by using a dummy head with microphones in the ears and playing a sine sweep through a loudspeaker. The actual IR was measured using Matlab software developed by Dr Michael Newton.

To obtain the IRs of the virtual spaces in the past and present, the same sine sweep from the Matlab software was used as an audio source, with an audio listener to capture the output. The position of the source and receiver were identical to those used in the actual hall.

A further IR was captured using Odeon acoustic simulation software, a commercial software package used to measure the acoustic properties of spaces. This gave a total of 7 IRs to compare:

  • The actual measured space
  • The virtual space in the present day, modelled by Google Resonance
  • The virtual space in the present day, modelled by Steam Audio
  • The virtual space in the present day, modelled by Odeon
  • The virtual space in 1769, modelled by Google Resonance
  • The virtual space in 1769, modelled by Steam Audio
  • The virtual space in 1769, modelled by Odeon

Findings

The results were evaluated objectively and subjectively. The objective measurements for each space were the T30, C80 and EDT. T30 is a reverberation time and is the time the IR signal decays from -5dB to – 35dB. The ratio of the acoustic energy arriving before and after 80ms (C80) is referred to as clarity. A positive value signifies more energy is in the early sound whereas a negative value signifies more energy in the late sound. The early decay time (EDT) is evaluated from the slope of the decay in the first 10ms. The EDT is subjectively important to the perceived reverberance.

Listening tests were carried out during the 2018 Manchester Science Festival where 25 participants were invited to listen to a reference audio clip then select the identical one from two choices presented.

Results are published in the AES paper for the conference from which it can be seen that there is a variation between statistics for the different IRs. Our research certainly highlights the need for improvements in spatialization software enabling it to capture the properties of the materials used in the spaces. The more control over acoustic parameters given by the software increases the accuracy of the models. In turn, this allows us to get closer to what we believe music in the historic spaces will have sounded like, giving an important insight into Scottish heritage.

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