This technique involves recording impulse responses using a sound field microphone in the middle of a room recording four consecutive impulses from speakers placed on the room’s extremities in P-format configuration. P-format (Spatial PCM Sampling) uses FLU (front, left up), FRD (front, right down), BLD (back, left down), and BRU (back, right, up) corresponding to the impulse response’s speaker positions. In this technique the anechoic A-format sound is encoded to P format: FLU = (W+X+Y+Z)/2, FRD = (W+X-Y-Z)/2, BLD = (W-X+Y-Z)/2 and BRU (W-X-Y+Z)/2 and each of the four encoded channels is then sent to a four-channel bus with a multi-channel convolution reverb plug-in containing the corresponding sound field impulse response. The four-channel buses are then converted back into B-format where they are added to the B-format final mix.

This technique delivers an empirically derived reverberation track that changes depending on the position of the sound source resulting in a realistic and spatialized sounding reverberant mix. Compromises for this technique include the lack of available four-channel convolution reverb plug-ins, the need to record multiple impulse responses generating a total of 16 files per room, and the computationally demanding task of processing 16 convolution reverb.  When using this technique accuracy decreases as the order of ambisonic (spatial resolution) increases. As a result, audio sources originating from areas between the original impulse responses will return a combination of the two closest impulse response recordings generating spatially inaccurate early reflections such as the A-format convolution reverb method.

Proposed Method

A more reliable alternative to Bruce’s method is to use ray tracing for the early reflections and a single four-channel ambisonic convolution reverb for the reverberation tail. Ray tracing is a technique that involves modeling the geometry of the original environment and simulating the reflections bouncing on the walls of the model in order to calculate the angle of incidence of each reflection and the distance traveled. The simulated data can then inform the delay time, volume, and imaging needed to generate each reflection. The problem with ray tracing is its high computational cost which prevents real-time playback on consumer computers. For that reason, a combination of convolution reverb and ray tracing is an ideal compromise between fidelity and processing demand.

In order to create a plug-in that uses the combined ray trace method, it is necessary to capture ambisonic impulse responses as well as 3D geometric data of the recreated environment (which can be done by imaging or by modeling a floor plan). The ray tracing element of the plug-in is then incorporated into the spatializer plug-in (which converts monophonic sound sources into ambisonic) where it will generate the early reflections. The late reflections are then created by a convolution reverb plug-in placed on the ambisonic track at the end of the signal flow.