Rebecca Stewart

Historical page — please see Dr Becky Stewart's site

Contact Details

Title: Research Student
Tel: Internal: [13] 5528
National: 020 7882 5528
International: +44 20 7882 5528
Fax: National: 020 7882 7997
International: +44 20 7882 7997
Email: rebecca.stewart@elec.qmul.ac.uk
Room: Eng. 112

Research Group: Centre for Digital Music

Supervisor: Mark Sandler

Research Topic: Interactive Auralisation With Room Impulse Responses

Stereo playback introduced the controlled placement of a sound in a virtual space relative to a listener position. More recently, much due to video game and online culture, research has focused on interactive environments where users directly affect the placement of sounds in a contextually-driven virtual space. The context of the virtual environment may be derived from situations such as a video game plot or a musical performance, but it deems the space surrounding the sound sources to be important. Also, archeological studies of the acoustics of significant historical spaces are being examined and research is working to model the sonic environment of buildings and sites that no longer exist as originally built.

Various techniques have been developed to discretely pan a sound source to a specific location, often manipulating the amplitude and/or phase differences between multiple signals. These approaches seldom take into account a full acoustic sound field, but only aim to place a specific sound source at a discrete location. The approach here is to use the information collected from acoustic spaces to simulate a sound source at a location within a distinct space. Spaces are often sampled for convolution reverberation by measuring an impulse response. An impulse response is calculated for a specific source and receiver configuration which theoretically captures all acoustic information about the space. However, this information pertains only to the very specific setup used during the measurement.

This research looks to extrapolate information from existing impulse responses to create an impulse response with an arbitrary source and receiver position. An oversampled, measured sound field will be used as a basic data set for the analysis of the dependency of an impulse response on source and receiver positions and boundary conditions. The results will be validated through listening tests to determine that the placement of virtual sources is perceptually accurate. The intent is to recreate as realistically as possible the experience of freely moving and creating sound within a simulated room.

In 1995, Andrew Reilly and David McGrath presented a series of papers at successive AES conferences describing an interactive, real-time auralization system over both headphones and loudspeakers. They found sufficient accurarcy and decreased computation time when a room impulse response is split and only approximately the first 100 ms of the impulse response is updated according to source direction.

This approach has been updated, and with modern computing power, now runs on a PC. The subsection of the impulse response that updates with source or receiver movement is selected according to statistical parameters related to the mixing time of the room. A spatial average impulse response represents the reverberation tail for arbitrary source and receiver locations within the same space. This greatly decreases the computation time for convolution reverberation and provides a much more flexible system which employs multiple impulse responses to allow a source or receiver to move within a space. The system is blind to the source of the impulse responses, whether measured in an existing space or synthesised through software. High-quality impulse responses can then be calculated from modelling software in non-real-time and processed to allow real-time interactive auralization of virtual environments. The flexibility is further extended by using B-format impulse responses which allow for rendering the resulting sound field over multiple loudspeaker setups, or when combined with HRTFs, over headphones.

Publications

R. Stewart and M. Sandler. Statistical measures of early reflections of room impulse responses. In Proceedings of the DAFx'07. Bordeaux, France, September 10-15 2007.

R. Stewart and M. Sandler. Real-time panning convolution reverberation. 123rd AES Convention, Convention Paper 7296, New York, NY, USA, October 5-8, 2007.

Professional Activitites

Chair, Audio Engineering Society (AES) London Student Section