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Using Acoustic Ray Tracing to Model Speaker-Room Performance

At lower frequencies, the wave-based FEM method discussed in the previous part is essential for modeling a speaker's response in a room. When frequencies are high enough for the acoustic wavelength to be much smaller than the characteristic geometric features, acoustic ray tracing is the most suitable technique for numerical simulations. The lower frequency bound for applying this technique in a room, known as the Schroeder frequency, can be estimated using the expression , where is the reverberation time for a 60-dB decay and is the volume of the room. Once above the Schroeder frequency, one can consider using the ray acoustics functionality included in the Acoustics Module to model a speaker's response in the room.

You can add the Ray Acoustics physics interface to your model in COMSOL Multiphysics^® from the Acoustics>Geometrical Acoustics subbranch, to release rays based on an acoustic source's directivity and compute the trajectories, phase, and intensity of acoustic rays. Rays can be released from points, edges, surfaces, and volumes. We will release our rays from points or surfaces, which is the most common setup for real speaker sources. Over the next few parts, we will set up a ray tracing analysis of the FEM room acoustics scenario from Part 1 using point and surface sources.

The geometry requires slight modifications depending on whether point or surface ray releasing methods are used. These adjustments will be illustrated in detail when we set up the ray acoustics model.

Ray Acoustics Features

The Ray Acoustics interface provides several features—covered in subsequent sections—that enable you to define either a point ray source with the radiation directivity of a speaker or a surface ray source to model a real speaker. The point source option is well suited for cases where the speaker is small compared to the room, effectively approximating a far-field source. In contrast, a surface source can capture interactions between the speaker and its immediate surroundings, making it appropriate for modeling near-field behavior.

Methods using Point Ray Sources

1. Release from Exterior Field Calculation - This option is used when the source speaker's radiation characteristics are computed from a wave-based FEM simulation conducted within the same model where ray tracing is set up. It enables you to specify a point ray source by directly adopting the speaker's far-field radiation pattern, described through an exterior field variable defined in the FEM model. This is considered a direct FEM-Ray coupling because it requires no user-defined expressions beyond calling the exterior field variable.
2. Source with directivity - This option is used when the speaker's radiation pattern data are obtained from either measurement or a simulation conducted in a separate model. These data are typically stored in a text file and brought into the ray acoustics model through interpolation functions, which are then used to define the source directivity in user-defined expressions.

Methods using Surface Ray Sources

3. Release from Pressure Field - This option, available in versions 6.2 and up, is applicable when the intensity on the release surface is computed from a FEM source model using the Pressure Acoustics, Frequency Domain interface. The ray acoustics model should be constructed within the same model component as the source model, with the source field in the Release from Pressure Field feature explicitly referencing the pressure acoustics interface. This provides a direct FEM to ray coupling without a need for any user-defined expressions.
4. Release from Boundary - This option is useful when intensity data are obtained from measurements or simulations conducted in a separate model or a different component within the same model. It requires manually entering user-defined expressions to specify the ray direction and power on the release surface. These expressions can utilize the acoustic intensity vector variables from the study solution of a FEM source model built within the same model, whether in the same component or a different one. Alternatively, they can use surface intensity vector data stored in a text file, sourced from measurements or simulations carried out in a different model. The data in the text file should be imported into the ray model using interpolation functions.