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Once the wave-based and the geometrical acoustics solutions are successfully calculated, Treble hybridizes the results of both solvers into a broadband impulse response. The wave-based solution is low-pass filtered to remove the frequency content above the transition frequency, and, similarly, the geometrical acoustics solution is high-pass filtered below the transition frequency. The two impulse responses are then summed to generate the hybrid response. For this process, both solvers are first calibrated to output a sound pressure level of 94 dB at a distance of one meter for an omnidirectional point source in free field (which amounts to a sound power of 105 dB).

The room acoustics parameters displayed in the result panel of Treble are computed for each individual solvers, depending on whether the considered frequency band is lower or higher than the transition frequency. If the transition frequency falls inside a frequency band, the parameters for this particular band are calculated based on the hybrid response. Note that for the geometrical acoustics solver, the energy-based parameters are extracted directly from the energy histogram. This can result in slightly different results from computing the parameters from the squared impulse response due to additional filtering and overlapping band effects [1].


[1] G. Marbjerg, J. Brunskog, C.-H. Jeong, V. Zapata-Rodriguez. The influence of overlapping band filters on octave band decay curves, Acta Acustica united with Acustica, 104:943-946, 2018.