Today, a multitude of applications for the simulation of sound propagation in room- and city acoustics exist.  Basically, these tools can be subdivided into three categories that are 1) commercial products, 2) internal developments and 3) open-source solutions.  Users, such as researchers or acoustic consultants, usually prioritize one of them which addresses their specific demands best possible. Although all solutions rely on the same standardized material descriptions for their simulation algorithms, a direct exchange of such data between different applications is practically impossible since most tools rely on their own proprietary database formats.

To overcome this issue, the open database project openMat was founded, which supports a detailed description of materials for the usage in acoustical simulation software. Numerical data as well as additional meta-information on the material are provided in an open Extensible Markup Language (XML) database format. Besides the absorption coefficients commonly used for the simulation of sound propagation, also scattering coefficients of the material are aimed at in the database explicitly. The conceptual design of openMat is being discussed with many interested developers and acoustic consultants all around the world. Thus, the database format is still under development and everyone is invited to join the discussion such that openMat will match all necessary user requirements in the future.

Supported Acoustical Property Descriptors

So far, an openMat material supports five different material descriptors: 1) impedance, 2) absorption coefficients, 3) scattering coefficients, 4) diffusion coefficients and 5) bidirectional transfer functions (BDTF).

Derived from the respective impedance Z, the angle-dependent reflection factor R describes the acoustical reflection properties of the material for an incident sound wave including magnitude and phase[Kuttruff2007] as function of incidence angle. The complex impedance at a surface, is mainly used for wave based simulation methods such as the Finite Element Method (FEM), the Boundary Element Method (BEM) and the Finite Difference Methods (FDM), and is mostly measured in an impedance tube[Cox2009]. For energetic simulation methods such as stochastic Ray Tracing, the sound absorption coefficient α rather than Z is used. It describes the energetic ratio of the sound energy absorbed by the material and can be computed from the reflection factor R, and, thus, from the impedance Z. For standardized computations, α has to be measured in a reverberation chamber according to ISO 354. Scattering and diffusion characteristics of a material and an object, respectively, are taken into account in openMat by supporting three different coefficient types: the scattering coefficient, the diffusion coefficient and the BDTF. The scattering coefficient [Vorländer2000] is measured in a reverberation chamber according to ISO 17497-1 , whereas a diffusion coefficient is defined for free-field conditions. According to Cox[Cox2009], the diffusion coefficient is a measure of the ability of diffusors to uniformly scatter in all directions, rather than just the ability of the surface to move energy away from the specular angles (scattering coefficient). Thus, the diffusion coefficient is a measure of quality, while the scattering coefficient is a measure of quantity. In contrast, the BDTF describes the propagation of the sound wave as a function of both the incidence angle and the angle of reflection[Siltanen2007].

All these descriptors are either angle dependent or are averaged over the complete angle range (random incidence) and can also be predicted by using appropriate simulation tools instead of measurements. For all descriptors, either measured or simulated, detailed information on the respective data acquisition can be stored separately. This information includes location, date and a responsible person for the measurement. In addition to measured or simulated data, interpolated values between the given values are supported, but they are indicated as such.

Additional Meta-Information for Materials

Besides the numerical acoustical description of the material, additional meta-information can also be added by the user. This includes images and textures for material visualization in different simulation environments as well as price information to achieve a rough estimation of installation costs. Also CAD files can be linked to each material, which could be used to even extrapolate scattering coefficients out of the geometrical structure of the material. In total, material databases in the openMat format can be applied during both the graphical and the acoustical planning stage. Finally, a detailed material description and the support of arbitrary additional files complete the multitude of meta-information.

The XML Schema

The XML schema applied in openMat is defined by a XML Schema Definition (XSD) file in compliance with the W3C recommendations. The XSD is — according to the W3C — a formal description pattern that 1) provides a list of elements and attributes in a vocabulary, 2) associates types, such as integer, string, etc., or more specific descriptors such as absorption coefficient, sample picture, etc., with values found in the documents, 3) constrains where elements and attributes can appear, and what can appear inside those elements, and 4) provides documentation that is both human-readable and machine-processable. A complete and comprehensive description of the openMat XSD schema is given in the manual.

C++ Library with Graphical User Interface

To use the openMat database format, a reference implementation in C++ has been created and published under LGPl3 to demonstrate the usage of the database. This library provides different functionalities for both exporting and importing openMat database files. When reading in a database, the provided importer validates the data against the openMat schema file before it gets further processed. The library provides functions for reading and writing complete databases, which aim at giving the interested user a reference implementation such that more optimized functions for personal use cases can be written quite easily. In addition, a graphical user interface (GUI) was created on top of this library for an easy usage of openMat. This GUI can be used to create, explore and modify database entries in openMat XML structure.

Python Library

For embedding openMat into other software, or to write scripts to convert data from other formats into openMat, a Python library is provided as download. This library contains classes and methods to read and write data in openMat-format and to access these objects. This source code is offered under LGPL that allows to use this library in other closed-source software.


[Kuttruff2007] H. Kuttruff. Acoustics: An Introduction. Taylor & Franscis, 2007.
[Cox2009] T. J. Cox and P. D’Antonio. Acoustic Absorbers and Diffusers. Taylor & Francis, 2nd edition edition, 2009.
[Vorländer2000] M. Vorländer and E. Mommertz. Definition and measurement of random-incidence scattering coefficients.
Applied Acoustics, 60(2):187–199, 2000.
[Siltanen2007] S. Siltanen, T. Lokki, S. Kiminki, and L. Savioja. The room acoustic rendering equation.
Journal of the Acoustical Society of America, 122(3):1624–1635, 2007.

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