Different room acoustic modeling techniques have different advantages, and none of them is ideal for the whole frequency band and for the whole duration of an impulse response. Therefore it is often beneficial to build hybrid models that combine several methods such that each of them can operate in their optimal region. This approach has been first used in geometrical acoustics, where the image-source method is efficient in computing the early reflections whereas ray-tracing suits best for the late part of an impulse response [1]. In addition, it is typical to use the image-source technique to validate the specular reflection paths found by a ray-tracer.
This is the approach applied in most of the commercial room acoustic modeling software packages, such as in ODEON [2], and in CATT TUCT [3].
In recent years, the hybrids have become more complex, and there are proposals to include wave-based models such that the low-frequency modeling would become more accurate. Murphy et al. use 2D FDTD simulation to complement geometrical acoustics [4] whereas Arez et al. utilize FEM for low-frequency prediction in their hybrid model [5].
Our main contribution to hybrid modeling is in making full-bandwidth simulation that combines 3D FDTD simulation, beam tracing, and acoustic radiance transfer [2] .
Our work has concentrated on how different techniques should be applied to get reliable results. First, the different techniques operate in different domains as the FDTD and beam-tracing are able to handle directly the sound pressure whereas the the acoustic radiance transfer handles energies (which is a square of the sound pressure). This implies that the boundaries conditions should be formulated differently [6]. Another focus in our work is in calibration of the results provided by different techniques as it is not obvious how the sound sources should be implemented such that the obtainedresults would be easy to merge into one full-bandwidth impulse response [6].
[1] M Vorländer: Simulation of the transient and steady-state sound propagation in rooms using a new combined ray-tracing/image-source algorithm. Journal of the Acoustical Society of America 86(1):172–178, 1989. BibTeX / Info
@article{Vorlander_1989,
author = {Vorl\"{a}nder, M},
journal = "Journal of the Acoustical Society of America",
number = 1,
pages = "172--178",
title = "Simulation of the transient and steady-state sound propagation in rooms using a new combined ray-tracing/image-source algorithm",
volume = 86,
year = 1989
}
[2] G Naylor: Odeon - another hybrid room acoustical model. Applied Acoustics 38(2-4):131–143, 1993. BibTeX / Info
@article{Naylor_1993_Odeon,
author = "Naylor, G",
journal = "Applied Acoustics",
number = "2-4",
pages = "131--143",
title = "Odeon - another hybrid room acoustical model",
volume = 38,
year = 1993
}
[3] B -I Dalenbäck: Room acoustic prediction based on a unified treatment of diffuse and specular reflection. Journal of the Acoustical Society of America 100(2):899–909, 1996. BibTeX / Info
@article{Dalenback_1996,
author = {Dalenb\"{a}ck, B.-I.},
journal = "Journal of the Acoustical Society of America",
number = 2,
pages = "899--909",
title = "Room acoustic prediction based on a unified treatment of diffuse and specular reflection",
volume = 100,
year = 1996
}
[4] D Murphy, M Beeson, S Shelley and A Moore. Hybrid room impulse response synthesis in digital waveguide mesh based room acoustics simulation. In Proc. Int. Conf. Digital Audio Effects. 2008, 1–8. BibTeX / Info
@inproceedings{Murphy_2008,
author = "Murphy, D and Beeson, M and Shelley, S and Moore, A",
booktitle = "Proc. Int. Conf. Digital Audio Effects",
pages = "1--8",
title = "Hybrid room impulse response synthesis in digital waveguide mesh based room acoustics simulation",
year = 2008
}
[5] M Aretz, R Nöthen, M Vorländer and D Schröder. Combined broadband impulse responses using FEM and hybrid ray-based methods. In Proc. EAA Auralization Symp. Espoo, Fin. 2009. BibTeX / Info
@inproceedings{Aretz_2009,
author = {Aretz, M and N\"{o}then, R and Vorl\"{a}nder, M and Schr\"{o}der, D},
booktitle = "Proc. EAA Auralization Symp.",
title = "Combined broadband impulse responses using {FEM} and hybrid ray-based methods",
volume = "Espoo, Fin",
year = 2009
}
[6] A Southern, S Siltanen and L Savioja. Spatial Room Impulse Responses with a Hybrid Modeling Method. In Proc. 130th Audio Eng. Soc. Conv., preprint no. 8385. 2011. URL BibTeX / Info
@inproceedings{Southern_2011,
address = "London, UK",
annote = "The synthesis of an arbitrary enclosures room impulse response (RIR) may be performed using acoustic modelling. A number of acoustic modelling methods have been proposed previously each with their own advantages and limitations. This paper is concerned with mixing the RIRs from different modelling methods to synthesise a hybrid RIR. Low frequencies are modelled using the finite difference time domain method (FDTD), high frequencies are treated with geometric methods. A practical implementation...",
author = "Southern, A and Siltanen, S and Savioja, L",
booktitle = "Proc. 130th Audio Eng. Soc. Conv., preprint no. 8385",
keywords = "Auralization,Geometrical acoustics,Wave-based models",
title = "Spatial Room Impulse Responses with a Hybrid Modeling Method",
url = "http://www.aes.org/e-lib/browse.cfm?elib=15852",
year = 2011
}