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Localisation in virtual choirs: outcomes of simplified binaural rendering

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A virtual choir would find several uses in choral pedagogy and research, but it would need a relatively small computational footprint for wide uptake. On the premise that very accurate localisation might not be needed for virtual rendering of the character of the sound inside an ensemble of singers, a localisation test was conducted using binaural stimuli created using a simplified approach, with parametrically controlled delays and variable low-pass filters (historically known as a ‘shuffler’ circuit) instead of head-related impulse responses. The direct sound from a monophonic anechoic recording of a soprano was processed (1) by sending it to a reverb algorithm for making a room-acoustic diffuse field with unchanging properties, (2) with a second-order low-pass filter with a cut-off frequency descending to 3 kHz for sources from behind, (3) with second-order low-pass head-shading filters with an angle-dependent cut-off frequency for the left/right lateral shadings of the head, and (4) with the gain of the direct sound being inversely proportional to virtual distance. The recorded singer was modelled as always facing the listener; no frequency-dependent directivity was implemented. Binaural stimuli corresponding to 24 different singer positions (8 angles and 3 distances) were synthesized. 30 participants heard the stimuli in randomized order, and indicated the perceived location of the singer on polar plot response sheets, with categories to indicate the possible responses. The listeners’ discrimination of the distance categories 0.5, 1 and 2 meters (1 correct out of 3 possible) was good, at about 80% correct. Discrimination of the angle of incidence, in 45-degree categories (1 correct out of 8 possible) was fair, at 47% correct. Angle errors were mostly on the ‘cone of confusion’ (back-front symmetry), suggesting that the back-front cue was not very salient. The correct back-front responses (about 50%) dominated only somewhat over the incorrect ones (about 38%). In an ongoing follow-up study, multi-singer scenarios will be tested, and a more detailed yet still parametric filtering scheme will be explored.

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