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Diffraction Modelling

In geometric acoustics, diffraction can be modelled using diffraction paths that travel from the source to the receiver via edges of the room geometry. Based on the geometry of the edge and relative position of the source and receiver, the frequency response can be calculated. When the source is in the shadow zone, as shown in Figure 1, diffraction exhibits a low-pass filter response with a roughly 3db/octave roll-off.

Zones around a diffracting edge
Figure 1: Zones around a diffracting edge.

Models

RAC implements multiple diffraction models:

  • Attenuate: Attenuates the sound based on the length of the diffraction path. It is a simple model that does not consider the angle of incidence or frequency-dependent effects.

  • LPF: Applies a 1st order low-pass filter at 1kHz to diffraction paths when the source is in the shadow zone. It is a simple model that does not consider the angle of incidence.

  • UDFA: The Universal Diffraction Filter Approximation1 uses multiple 1st order shelving IIR filters to model the low-pass filter response of diffraction paths.

  • UDFA-I: A simplified version of UDFA that only models diffraction in the shadow zone.

  • NNBest and NNSmall: Neural network-based models2 that approximate the frequency reponse of diffraction paths using a 2nd order IIR filter.

  • UTD: The Uniform Theory of Diffraction3 (UTD) model implemented using a Linkwitz-Riley filterbank4.

  • BTM: The Biot-Tolstoy-Medwin-Svensson5 (BTMS) model is a physically accurate model implemented using an FIR filter.

Currently, only 1st order diffraction (paths that include a single diffracting edge) is supported.

  1. Kirsch C, and Ewert S. "A universal filter approximation of edge diffraction for geometrical acoustics." IEEE/ACM Trans. Audio, Speech, and Lang. Proc., 31:1636-1651, 2023 ↩

  2. Mannall J, et al. "Efficient diffraction modeling using neural networks and infinite impulse response filters." J. Audio Eng. Soc., 71:566-576, 2023 ↩

  3. Kouyoumjian R, and Pathak P. "A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface." Proc. IEEE, 62:1448-1461, 1974 ↩

  4. Schissler C, et al. "High-order diffraction and diffuse reflections for interactive sound propagation in large environments." ACM Trans. Graphics, 39:1-12, 2014 ↩

  5. Svensson P, et al. "An analytic secondary source model of edge diffraction impulse responses." J. Acoust. Soc. Am., 106:2331–2344, 1999 ↩