Receiver
Use the Receiver feature to define the boundaries of a receiver sphere in a Ray Acoustics model. The receiver collects information (arrival time and power) about the intersecting rays during the simulation. This information is then used for computing the impulse response in the Results analysis. Details are found in the Impulse Response Plot and Receiver Dataset section.
Create parameters under Global Definitions>Parameters for the receiver radius and number of released rays. These two quantities are linked by the expected error in a given time interval Δt of the impulse response, see Ref. 6 for details. With the room volume V, the receiver radius R, and for an expected error of 1 dB, the number of released rays N needed is
The receiver radius can be set to give a certain spatial resolution (like the width of a seat in a concert hall), and a standard time interval is Δt = 0.01 s. For small rooms or car cabins it can be useful to set a receiver radius that represents the actual size of a microphone or slightly larger. The value obtained from this calculation should be rounded up to ensure an integer number of released rays. A higher value can also be chosen and will result in increased resolution.
The receiver should be defined as a sphere with Object Type selected to be Surface in the Geometry. The receiver should also be meshed adequately. Typically a mesh size of R/3, where R is the receiver radius is a good choice. The receiver is transparent to rays and is only used for detection. The receiver feature is only available when Compute power or Compute intensity and power is selected in the Intensity Computation section. Count reflections should also be enabled in the Additional Variables for a correct impulse response computation.
Coordinate System Selection
By default, Global coordinate system is selected from the Coordinate system list. Other user defined coordinate systems can also be selected from the list. However, boundary coordinate systems are not included in the list.
The coordinate system is used to define local built-in variables for the azimuthal and polar angles. For a physics interface with the default name rac and a Receiver node with the default name rec1, these variables are called rac.rec1.phi (azimuthal angle) and rac.rec1.theta (polar angle). These angles are defined with respect to the selected coordinate system; for example, if the Global coordinate system is used, then rac.rec1.theta is the angle between the ray direction vector and the positive z-axis, while rac.rec1.phi is the angle of the projected ray direction vector in the xy-plane, measured counterclockwise from the positive x-axis. A default expression showing the variables is set up for the Spatial directivity input for a dB based expression.
Receiver
Select the Receiver radius as Automatic (the default) or User defined. The automatic option will work well for a spherical receiver geometry defined in COMSOL. For an imported CAD geometry it can be useful to enter the radius rrec manually. The radius is used for computing the receiver volume necessary for computing the detected signal intensity.
Select the Directivity type as Omnidirectional (the default), User defined (dB), or User defined (linear). For the two user defined options enter an expression for the Spatial directivity of the receiver. Built-in polar angle variables, that are defined in relation to the Coordinate system, exist to simplify setting up the receiver directivity. The Spatial directivity D(φ,θ) can be an analytical expression or based on an interpolation function.