Port

The boundary condition is used to excite and absorb acoustic waves that enter or leave waveguide structures, like a duct or channel, in an acoustic model. A given port condition supports one specific propagating mode. To provide the full acoustic description, combine several port conditions on the same boundary. Make sure that all propagating modes in the studied frequency range are included (all modes that have a cutoff frequency in the frequency range). By doing this, the combined port conditions provide a superior nonreflecting or radiation condition for waveguides to, for example, the Plane Wave Radiation condition or a perfectly matched layer (PML) configuration. The same port boundary condition feature should not be applied to several waveguide inlets/outlets. The port condition supports S-parameter (scattering parameter) calculation but it can also be used as a source to just excite a system. The boundary condition exists for 3D, 2D, and 2D axisymmetric models.

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Duct with Right-Angled Bend: Application Library path

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Absorptive Muffler: Application Library path

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Muffler with Perforates: Application Library path

On a given boundary, a combination of ports will define the total acoustic field (sum of incident and outgoing waves) as

where the summation “i” is over all ports on the given boundary “bnd”, Sij is the scattering parameter, Ain is the amplitude of the incident field (at port “j”), ϕ is the phase of the incident field, and pi is the mode shape of the ith port. The mode shape pi is normalized to have either a unit maximum amplitude or a unit power (see the normalization option in the Global Port Settings section). This means that the scattering parameter Sij defines the amplitude of mode i when a system is exited at port j (with mode j). This corresponds to a multimode expansion of the solution on the given port. The scattering parameters are automatically calculated when an acoustic model is set up with just one port exciting the system. To get the full scattering matrix The Port Sweep Functionality can be used.


	Only one port should be excited at a time if the purpose is to compute S-parameters. The S-parameters are defined as acpr.S11, acpr.S21, and so on, and can be used in postprocessing. An evaluation group for the Scattering Coefficient exists as a Predefined Plot. When evaluating, change the Data Series Operation for the port sweep parameter (PortName) to Sum to get the proper data formatting. In other cases, having more than one port exciting the system might be wanted, but the S-parameter variables cannot be correctly computed. So when several ports are excited, the S-parameter output is turned off. If only two ports are added to the system and a port sweep is performed, the system transfer matrix and system impedance matrices are also computed.

Port Properties

Enter a unique . Only nonnegative integer numbers can be used as as it is used to define the elements of the S-parameter matrix. The numeric port names are also required for port sweep functionality. The port name is automatically incremented by one every time a port condition is added.

Select a : (the default), , , , or . Depending on the selection different options appear in the section (see below). Use the and for ports with the given cross section in 3D, the option in 2D axisymmetry, and the option in 2D. If the port has a different cross section, then either use the option or the port.

Port Mode Settings

Depending on the option selected for the (see above):

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For enter user defined expressions for the pn and the kn (SI unit: rad/m). The modes shape will automatically be scaled before it is used in the port condition. The normalized mode shape can be visualized by plotting acpr.port1.pn (here for Port 1, and so on). Use the user defined option to enter a known analytical expression or to use the solution from The Pressure Acoustics, Boundary Mode Interface. The solutions from the boundary mode analysis can be referenced using the withsol() operator.


	The User defined option can be used to easily define a plane-wave mode. Simply enter 1 for the mode shape and acpr.k as the mode wave number.

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The port option is used for waveguide cross sections that are neither circular nor rectangular. In this case, a boundary mode problem is solved on the port face to compute the desired propagating mode. This option requires the use of a step in the study. It should be placed before the step. In the study, add one step for each port and make sure to reference the proper in the study step.

Select the option to decide how the mode wave number kn is determined:

In general for a model with losses, use the default option; then the wave number is taken from the step. In this case, it is not possible to perform a frequency sweep in the study step. Only one frequency can be used and it should correspond to the entered in the step(s). One option is to add a and define a parameter for the frequency used in both the steps. In this case, care should be taken when setting up the search criteria in the mode analysis.

For a model without any losses, select the option. In this case, a frequency sweep is possible. The should only be carried out at the highest frequency in the sweep. The wave number is computed analytically for all the other frequencies.


	When the Numeric port option is used and the boundary mode analysis is run, the boundary conditions from the Pressure Acoustics model are automatically inherited in the boundary problem. For this automatic procedure, there is only support for the Sound Hard Boundary (Wall), Symmetry, Pressure, and Sound Soft conditions. If you need a more complex behavior, use the Pressure Acoustics, Boundary Mode interface in combination with the User defined port type.


	When running a frequency domain sweep, in a model that uses either the Circular or the Rectangular port options, you can get a solver warning: New constraint force nodes detected: These are not stored. This warning can be disregarded. The reason for the warning is that certain constraints, in underlying equations, only are activated when the frequency is above the mode cutoff frequency.

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The port option is used for a port with a circular cross section. Enter the m and the n to define the mode captured by the port. In 3D, also right-click the condition to add the Circular Port Reference Axis when the port type is selected. The cutoff frequency of the mode can be evaluated in postprocessing using the variable acpr.port1.fc (here for Port 1, and so on).

Figure 2-1: The first 6 modes (m,n) of a waveguide with circular cross section.

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The port option, only available in 3D, is used for a port with a rectangular cross section. Enter the m and the n to define the mode captured by the port.

Figure 2-2: The first 6 modes (m,n) of a waveguide of rectangular cross section.

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The port option is only valid in 2D geometries. Enter the m to define the mode captured by the port.


	The cutoff frequency of the port modes can be evaluated in postprocessing using the variable acpr.port1.fc (here for Port 1, and so on).

Port Geometry

The settings in this section ensure that the correct port area (including symmetries) is used for computing important quantities like power, volume flow, and transfer matrix elements, associated with the port

Select the as (the default) or .

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When is selected, symmetry conditions adjacent to the port will automatically be taken into account if the is set to (the default); if set to , enter the area multiplication factor Ascale manually.

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When is selected, the port will have the area of the selected boundaries, without taking any symmetry conditions into account.


	The Port condition can only be applied to connected boundaries and the boundaries should typically be coplanar. The port condition can in principle be applied to a curved continuous boundary, but care should be taken that the setup is mathematically and physically consistent.


	The Port boundary conditions should in general be placed at least one waveguide diameter from other geometry features to ensure that only a pure fully developed propagating wave exists at the port. Note also that the port condition does not treat possible evanescent waves.

Incident Mode Settings

Activate if the given port is excited by an incident wave of the given mode shape. For the first condition added in a model, the is set to . For subsequent conditions added, the excitation is set to per default. If more than one port in a model is excited the S-parameter calculation is not performed.

When the is set to , then select how to define the incident wave. Set to (the default) or .

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For enter the amplitude Ain (SI unit: Pa) of the incident wave. This is in general defined as the maximum amplitude for a given mode shape.

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For enter the power Pin (SI unit: W) of the incident wave. This is in general defined as the RMS power of the incident wave.

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For both options enter the phase ϕ (SI unit: rad) of the incident wave. This phase contribution is multiplied with the amplitude defined by the above options. The input can be a complex number.

Note that when the option is selected at the physics level, the options in the section are deactivated. This is because this option automatically sends in a mode of unit amplitude, sweeping through one port at a time.


	All ports with incident wave excitation turned off have an arrow symbol, added in the Graphics window, that points outward. When incident wave excitation is turned on, the arrow symbol points inward.


	If a waveguide is cut with one or several symmetry planes, care should be taken when setting up the port modes and when postprocessing. For the Circular and Rectangular options make sure to only select modes that are actually symmetric according to the symmetry planes.


	The port condition is in general not compatible with the Background Pressure Field domain feature. Combining the two will generate unphysical results if placed next to each other.

Constraint Settings

To display this section, click the button (

) and select .

The Port Sweep Functionality

The port sweep functionality is used to reconstruct the full scattering matrix Sij by automatically sweeping the port excitation through all the ports included in the model. When the port sweep is activated, the options in the in the port conditions are deactivated and COMSOL controls which port that is excited with an incident mode.

The port sweep functionality is activated at the main physics interface level by selecting in the Global Port Settings section. Enter the , the default is PortName. Create a parameter with the same name under . This is the name of the parameter to be used in a parametric sweep, here it should represent the integer values (defined when adding the port conditions). Add a parametric sweep study step and run the sweep over the PortName parameter with an integer number of values representing all the ports in the model. Once the model is solved the full scattering matrix can be evaluated using the defined global variables acpr.S11, acpr.S21, acpr.S12, and so on. The transmission loss (TL) between two given ports is also computed, for example, the variable for the TL loss from port 1 to 2 is given by acpr.TL_12.

If only two ports are added to the pressure acoustics model, COMSOL also automatically computes the transfer matrix (two-port representation) of the system (variables acpr.T11, acpr.T12, acpr.T21, acpr.T22) and the impedance matrix of the system (acpr.Z11, acpr.Z12, acpr.Z21, acpr.Z22). These expressions are only true if plane wave modes are used.

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An evaluation group for the exists as a . The is used to evaluate the full scattering matrix acpr.S. When evaluating, change the for the port sweep parameter (typically PortName) to to get the proper data formatting.

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An evaluation group also exists for the . This plot evaluates the real and imaginary parts of the four matrix components separately. The output format can readily be exported to a file and used as interpolation data for subsequent use in other models, for example, in connection to a Lumped Port condition.