The Ray Acoustics (rac) interface (
), found under the
Acoustics>Geometrical Acoustics subbranch (
) when adding a physics interface, is used to compute the trajectories, phase, and intensity of acoustic rays. Ray acoustics is valid in the high-frequency limit where the acoustic wavelength is much smaller than the characteristic geometric features. The interface can be used to model room acoustics in concert halls, theaters, class rooms, or car cabins, but also for underwater acoustics, atmospheric acoustics, and other outdoor environments.
The Release from Pressure Field and the
Release from Exterior Field Calculation features are used for coupling results form a wave-based
Pressure Acoustics, Frequency Domain simulation to the ray based
Ray Acoustics simulation. They allow users to set up realistic sources based on detailed multiphysics wave-based simulation of a loudspeaker or another sound radiating source.
When this physics interface is added, these default nodes are also added to the Model Builder:
Medium Properties,
Wall,
Material Discontinuity, and
Ray Properties. Then, from the
Physics toolbar, add other nodes that implement, for example, boundary conditions. You can also right-click
Ray Acoustics to select physics features from the context menu.
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern
<name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the
name string must be unique. Only letters, numbers, and underscores (_) are permitted in the
Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is
rac.
By default, the Allow frequency distributions at release features check box is cleared. Select this check box to model propagation of rays of different frequencies simultaneously. The ray frequency can be specified at release features by entering a value or expression, sampling the frequency from a distribution, or entering a list of values. If this check box is cleared, the frequency is the same for all rays and is specified in the settings for the
Ray Properties node.
Regardless of whether the Allow frequency distributions at release features check box is selected, properties at walls can depend on the frequency (variable
rac.f) of each ray released. Domain material properties, like the speed of sound, can also depend on the frequency. Entering a frequency distribution of the released rays also enables emitting more complex source signals (decomposed in their Fourier components).
The Maximum number of secondary rays prevents an inordinate number of rays from being generated by capping them at the number supplied in the text field. The default is 500. Rather than being produced directly by release features such as the
Release from Grid node, secondary rays are released when an existing ray is subjected to certain boundary conditions. For example, when a ray undergoes refraction at a
Material Discontinuity between different media, the incident ray is refracted and a reflected ray is created; the degrees of freedom for this reflected ray are taken from one of the available secondary rays, which are preallocated when the study begins.
By default the Only store accumulated variables in solution check box is cleared. If this check box is selected, then the degrees of freedom associated with individual rays (such as the ray position, wave vector, intensity, power, and number of reflections) will be solved for but not stored in the solution data. However, certain physics features define degrees of freedom in domains or on boundaries rather than on the individual rays, and these degrees of freedom will be retained in the solution. These quantities, called accumulated variables, are defined by the following features:
Enter a value or expression for the Speed of sound cext (SI unit: m/s). The default value is
343[m/s]. Then enter a value or expression for the
Density ρext (SI unit: kg/m
3). The default value is
1.2[kg/m^3].
When one of the Intensity Computation options is selected (see below), select an option from the
Attenuation type list:
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For Attenuation coefficient, Np per unit length (the default), enter a value or expression for the Attenuation coefficient αext (SI unit: Np/m). The default is 0.
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For Intensity attenuation coefficient, Np per unit length, enter a value or expression for the Intensity attenuation coefficient mext (SI unit: Np/m). The default is 0.
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For Attenuation coefficient, dB per unit length, enter a value or expression for the Attenuation coefficient α'ext (SI unit: dB/m). The default is 0.
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For Attenuation coefficient, dB per wavelength, enter a value or expression for the Attenuation coefficient  (SI unit: dB). The default is 0.
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Select an option from the Intensity computation list:
None (the default),
Compute intensity,
Compute power,
Compute intensity and power,
Compute intensity in graded media, or
Compute intensity and power in graded media. For
None the ray intensity and power is not computed.
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For Compute intensity, auxiliary dependent variables are used to compute the intensity of each ray. For a complete list of the auxiliary dependent variables that are defined, see Intensity and Wavefront Curvature in Theory for the Ray Acoustics Interface. This option is more accurate and is usually less computationally demanding than Compute intensity in graded media, but is only valid for computing intensity in homogeneous (constant speed of sound) media.
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For Compute power, the total power transmitted by each ray is defined as an auxiliary dependent variable, in addition to the auxiliary dependent variables that are declared when Compute intensity is selected. Like intensity, the power is affected by absorbing media and by reflection or refraction at boundaries. Unlike intensity, ray power does not change due to the convergence or divergence of a bundle of rays. The Sound Pressure Level Calculation subnode for the Wall feature, the Receiver, and the Release from Pressure Field are available. The Compute power option is necessary for the impulse response postprocessing, see Impulse Response Plot and Receiver Dataset.
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The option Compute intensity and power combines the dependent variables created by the Compute intensity and Compute power options.
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For Compute intensity in graded media, auxiliary dependent variables are used to compute the intensity of each ray. The intensity is affected by gradients in the speed of sound. Graded media may be present, for example, when modeling the effect of the saline climb in underwater acoustics. For a complete list of the auxiliary dependent variables that are defined, see Intensity and Wavefront Curvature in Theory for the Ray Acoustics Interface. This intensity computation method is valid for both homogeneous and graded media. If all media are homogeneous, meaning that the gradient of the speed of sound is zero everywhere except at material discontinuities, then it is recommended to select Compute intensity instead, since it is the more accurate intensity computation method for such cases.
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For Compute intensity and power in graded media, the total power transmitted by each ray is defined as an auxiliary dependent variable, in addition to the auxiliary dependent variables that are declared when Compute intensity in graded media is selected. The Sound Pressure Level Calculation subnode is available for the Wall feature, the Receiver, and the Release from Pressure Field are available. If all media are homogeneous then it is recommended to select Compute intensity and power instead.
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When the ray intensity or power is solved for (one of the options Compute intensity,
Compute power,
Compute intensity and power,
Compute intensity in graded media, or
Compute intensity and power in graded media is selected), a list for the
Reference pressure for the sound pressure level appears. Select
Use reference pressure for air (the default) to set the reference pressure to 20
μPa,
Use reference pressure for water to set the reference pressure to 1
μPa, or
User-defined reference pressure to enter a value or expression in the text field. The reference pressure is used to calculate the sound pressure level (variable
rac.Lp) and sound intensity level (variable
rac.LI) along rays.
If the ray intensity is solved for (one of the options Compute intensity,
Compute intensity and power,
Compute intensity in graded media, or
Compute intensity and power in graded media is selected), the
Compute phase check box is available. By default, the
Compute phase check box is cleared. Select this check box to compute the phase along each ray. An auxiliary dependent variable for the phase
rac.Psi is created. The phase changes along the ray trajectory based on its frequency and is also subjected to phase shifts that happen at walls with specular reflection and absorption.
When Compute intensity in graded media or
Compute intensity and power in graded media is selected from the
Intensity computation list, enter a
Tolerance for curvature tensor computation (dimensionless). This tolerance is used internally when computing the principal radii of curvature in a graded medium, and is used to define a criterion by which both principal radii of curvature are declared approximately equal. A larger tolerance makes the solution less accurate but more stable.
Select the Count reflections check box to allocate an auxiliary dependent variable for the number of reflections undergone by each ray, including reflections by the
Wall and
Material Discontinuity features. The auxiliary variable begins at 0 when rays are released and is incremented by 1 every time a ray is reflected at a boundary. The
Count reflections option is necessary for the impulse response postprocessing, see
Impulse Response Plot and Receiver Dataset.
By default, the Store ray status data check box is cleared. Select this check box to add new variables for quantities that cannot necessarily be recovered from the ray trajectory data alone. This is especially true if automatic remeshing has been used in a model. The following variables are created:
The global variable names in Table 8-1 all take the unreleased secondary rays into account. For example, suppose an instance of the Ray Acoustics interface includes
100 primary rays and
100 allocated secondary rays. At the last time step, suppose that
80 of the primary rays have disappeared at boundaries and that
40 secondary rays have been emitted, all of which are still active. Then the variable
rac.fac, the fraction of active rays at the final time step, would have the value
(20 + 40)/(100 + 100) or
0.3.
This section is only shown when Advanced Physics Options are enabled (click the
Show More Options button in the
Model Builder toolbar).
The Wall accuracy order sets the accuracy order of the time stepping used for time steps during which a ray-wall interaction happens. Select an order of
1 to use a forward Euler step and compute the motion both before and after the wall collision. Select an order of
2 (the default) to use a second-order Taylor method and compute the motion before the wall collision. After the collision a second-order Runge-Kutta method is used.
Select an option from the Arguments for random number generation list:
Generate unique arguments,
Generate random arguments, or
User defined. This setting determines how the additional argument to random functions is defined in features such as the
Wall boundary condition with the
Diffuse scattering wall condition. Typically the random numbers are functions of the ray index, time, a unique input argument for different variable definitions, and another argument
i, defined as follows:
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For Generate unique arguments the additional argument is based on the position of each node in the Model Builder. As a result, random numbers generated in different nodes are created independently of each other.
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For Generate random arguments the additional argument is randomly created each time the study is run.
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For User defined the additional argument is defined by a user input in the Settings window for each feature. Uncorrelated sets of random numbers can be obtained by running a Parametric Sweep for different values of i.
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Note that the random function is a pseudorandom number generator (PRNG), not a true random number generator (RNG) in the sense that an observer with perfect knowledge of the algorithm and a history of previously generated values could predict the ensuing values. However, PRNG is sufficient for most purposes.
The Allow multiple release times check box, which is cleared by default, allows an array of release times for the rays to be specified in any of the ray release features. If the check box is cleared, all rays are released at time
t = 0.
By default the Exclude Jacobian contributions from ray equations check box is cleared. If this check box is selected, then expressions for the time derivatives of the degrees of freedom on rays are not included in the Jacobian matrix. Excluding contributions to the Jacobian can significantly improve solution time. The drawback is that the Jacobian is not exact, so smaller time steps or path length intervals may be needed to obtain an accurate solution. This drawback is most noticeable when computing the ray intensity or power in attenuating media.
By default, the Treat unmeshed releasing entities as errors check box is selected. Certain ray release features, such as the
Release and
Release from Boundary features, compute the ray release positions based on a geometric entity, which must be meshed. While this check box is selected, failure to mesh the selections of such features will result in an error message when running any study in the model. If this check box is cleared, failure to mesh the selections of such features will result in them releasing zero rays, but will otherwise not interrupt the computation. Other physics features will still be able to release rays normally.
Enter a value for the Maximum number of wall interactions per time step. The default value is 1000. If a ray undergoes more than the specified number of boundary interactions in a single time step taken by the solver, the ray will disappear. This is included as a safeguard to prevent rays from getting stuck in infinite loops if the time between successive ray-wall interactions becomes infinitesimally small.
The dependent variables (field variables) are the Ray position components and
Wave number components. The name can be changed but the names of fields and dependent variables must be unique within a model.
