## Acoustics Module Updates

For users of the Acoustics Module, COMSOL Multiphysics^{®} version 5.5 includes a new *Elastic Waves, Time Explicit Physics* interface, multiphysics couplings for acoustic-structure interaction with the time explicit formulation, and a *Port* boundary condition for the *Thermoviscous Acoustics, Frequency Domain* interface. Learn more about these and many more Acoustics Module updates below.

### New Elastic Waves, Time Explicit Physics Interface

The new *Elastic Waves, Time Explicit* physics interface is based on the discontinuous Galerkin time explicit method and enables efficient multicore computations of elastic wave propagation in solids. Features are included to provide realistic material data including anisotropy and damping. The interface is suited for modeling ultrasound propagation in solids, such as with transducers and sensors, and for nondestructive testing (NDT) applications, and is applicable to any acoustically large system that involves transient propagation over many wavelengths, which includes seismic wave propagation in soil and rock.

You can see the new interface used in the following models:

- Ground Motion After Seismic Event: Scattering off a Small Mountain (new model)
- Isotropic-Anisotropic Sample: Elastic Wave Propagation (new model)
- Angle Beam Nondestructive Testing (new model)
- Immersion Ultrasonic Testing Setup (new model)

*Elastic Waves, Time Explicit*interface shown here in a model of seismic waves propagating in the soil.

### Multiphysics for Acoustic-Structure Interaction with the Time Explicit Interfaces

For large transient acoustic-structure interaction simulations, a new *Acoustic-Structure Interaction, Time Explicit* multiphysics coupling is available. This coupling connects the *Pressure Acoustics, Time Explicit* and new *Elastic Waves, Time Explicit* physics interfaces. To take full advantage of the time explicit formulation, the use of nonconforming meshes is essential when coupling domains with different properties. This is achieved through the new *Pair Acoustic-Structure Boundary, Time Explicit* multiphysics coupling that handles geometric assemblies. The use of nonconforming meshes is a natural extension and use of the properties of the discontinuous elements. You can see this functionality used in the Immersion Ultrasonic Testing Setup model.

*Pair Acoustic-Structure Boundary, Time Explicit*multiphysics coupling feature.

### Material Discontinuity, Pair Conditions, and Dissipation for the Acoustic Time Explicit Interfaces

The fluid acoustics interfaces that are based on the discontinuous Galerkin time explicit method now have the option to include dissipation. Dissipation plays an important role when modeling high-frequency applications like ultrasound imaging and flowmeters. The new option exists for the *Pressure Acoustics, Time Explicit* and *Convected Wave Equation, Time Explicit* interface.

The *Pressure Acoustics, Time Explicit* interface now includes a *Material Discontinuity* (interior) boundary condition and a *Continuity* pair feature. These are used to handle jumps in material properties for a union with a conforming mesh, or an assembly using a nonconforming mesh, respectively. You can see the *Material Discontinuity* feature in the Isotropic-Anisotropic Sample: Elastic Wave Propagation model.

### Ports for Thermoviscous Acoustics

A new *Port* boundary condition has been added to the *Thermoviscous Acoustics, Frequency Domain* interface, used to excite and absorb acoustic waves that enter or leave waveguide structures in microacoustic applications. The port conditions provide a near-perfect, nonreflecting radiation condition for waveguide inlets/outlets, including the viscous and thermal boundary layers. In many cases, using the new *Port* condition provides superior ease of use and accuracy compared to an impedance condition or a perfectly matched layer (PML) configuration. When working with small acoustic subsystems, two *Port* conditions are used and combined to automatically compute the scattering matrix, transfer matrix, and impedance matrix relating the inlet to the outlet. These are all simplified lumped representations of subsystems typically used to efficiently analyze their integration into a full system. You can see this feature in the Wax Guard Acoustics: Transfer Matrix Computation model.

*Port*feature in the

*Thermoviscous Acoustics, Frequency Domain*physics interface. There are three options for the port type:

*User defined*,

*Numeric*, and

*Circular*.

### Updated Port Feature in Pressure Acoustics

The *Port* condition is now available in 2D for the *Pressure Acoustics, Frequency Domain* interface, and has a *User defined* option and a *Slit* option to define the mode shapes. In general, when a port sweep is performed and two ports are used, one at the inlet and one at the outlet, the transfer matrix and the impedance matrix of the system are automatically computed. In COMSOL Multiphysics^{®} version 5.5, new transmission loss variables are automatically generated for the transmission between two or more ports. The port sweep functionality now also works when an inner sweep is performed over the port number. You can see this feature used in the Shape Optimization of an Acoustic Demultiplexer model.

### Background Fluid Flow Coupling and Mapping Study for Aeroacoustics

The new *Background Fluid Flow Coupling* multiphysics coupling and dedicated *Mapping* study features are added in version 5.5 to automate and simplify the coupling of a CFD model and a convected acoustic model. This includes the linearized Navier-Stokes, linearized Euler, and convected wave equation physics. The multiphysics coupling and mapping ensure that the computed CFD solution is correctly mapped from the fluid flow mesh to the acoustics mesh, while also taking care of different discretization orders. The mapping and interpolation are essential to avoid introducing numerical noise into the acoustic model, where the reactive terms are especially important to treat correctly.

You can see this functionality used in the following models:

- Helmholtz Resonator with Flow: Interaction of Flow and Acoustics
- Ultrasound Flowmeter with Generic Time-of-Flight Configuration
- Coriolis Flowmeter: FSI Simulation in the Frequency Domain

*Background Fluid Flow Coupling*multiphysics coupling and the

*Mapping*study, seen in the Model Builder window, to couple flow and acoustics, as seen in the Helmholtz Resonator with Flow tutorial model.

### Anisotropic Materials in Pressure Acoustics Interfaces

The new *Anisotropic Acoustics* feature for pressure acoustics makes it possible to define fluids with an effective anisotoropic density and a scalar effective bulk modulus. With this feature you can set up homogenized material properties for metamaterials and define effective fluid properties of porous and fibrous materials that have anisotropic structures. The effective density can be defined as having an *Isotropic*, *Diagonal*, or *Symmetric* structure. You can see this new feature in the Acoustic Cloaking model.

### Lorentz Coupling for Modeling Electroacoustic Transducers

The *Lorentz Coupling* feature supports a two-way coupling between the *Magnetic Fields* and *Solid Mechanics* interfaces. The Lorentz force is determined by computing the cross product of the current density, **J**, and the magnetic flux, **B**, in the volume of the domain, which is then applied on the mechanics side as a volumetric force. At the same time, the velocity is taken from the *Solid Mechanics* interface and applied in the *Magnetic Fields* interface, as a Lorentz velocity term. The feature automatically handles the frames and moving mesh effects.

This feature is intended for conductive, nonmagnetizable domains (typically, copper coils), and when combined with the *Acoustic-Structure Boundary* multiphysics coupling, enables you to model electroacoustic transducers. It is available in 2D, 2D axisymmetry, and 3D, for Time Dependent, Frequency Domain (Perturbation), and Eigenfrequency analysis. This functionality requires the AC/DC Module together with one of the Structural Mechanics Module, Acoustics Module, or MEMS Module. You can see this functionality used in the Loudspeaker Driver — Frequency-Domain Analysis and Loudspeaker Driver — Transient Analysis models.

*Lorentz Coupling*multiphysics feature for the electromechanical coupling on the voice coil in a loudspeaker driver.

### Acoustic-Pipe Acoustic Connection Multiphysics Coupling

With the new *Acoustic-Pipe Acoustic Connection* multiphysics coupling, you can couple the pressure acoustics interfaces to the pipe acoustics interfaces in both frequency and time domain simulations. The coupling is defined between a point in the pipe interface and a boundary in the pressure acoustics interface. You can see this functionality used in the Probe Tube Microphone and Acoustics of a Pipe System with 3D Bend and Junction models.

*Acoustics-Pipe Acoustic Connection*multiphysics coupling in the Acoustics of a Pipe System with 3D Bend and Junction tutorial model.

### Acoustic-Structure Couplings for Layered Shells

The multiphysics couplings between acoustics and structures have been extended to support the *Layered Shell* physics interface. With this functionality, you can model vibroacoustic problems involving composite materials and other layered structures. Note that you need the Composite Materials Module to enable this functionality.

The *Layered Shell* interface is now supported for the following multiphysics couplings:

*Acoustic-Structure Boundary**Thermoviscous Acoustic-Structure Boundary**Aeroacoustic-Structure Boundary**Porous-Structure Boundary*

### Improved Acoustophoretic Force

The *Acoustophoretic Force* feature has been renamed *Acoustophoretic Radiation Force*. This feature has new force expressions that are more accurate, because they account for the viscous and thermal boundary layers that form around particles in an acoustic pressure field. You can now specify whether the particles are solid or liquid. Then you can choose a *Thermodynamic loss model*: *Ideal*, *Viscous*, or *Thermoviscous*. The feature can be combined with both pressure acoustics and thermoviscous acoustics to model particle sorting and other acoustofluidic applications. You can see this new feature in the Acoustic Levitator and Acoustic Streaming in a Microchannel Cross Section models.

### News in Ray Acoustics

#### Preview Grid Release Positions

When you release particles from a grid of points using the *Release from Grid* feature, you can now preview the initial particle positions in the *Graphics* window. In the *Initial Coordinates* section of the *Settings* window, click the *Preview Initial Coordinates* button to view the initial particle coordinates as a grid of points. Click the *Preview Initial Extents* button to view the spatial extents of the initial coordinates as a bounding box. These buttons allow you to check the initial particle positions before running a study.

In addition, when you right-click a *Study* node and click *Get Initial Value*, you can preview the initial particle positions and velocities for all release types.

#### New Release Type: Hexapolar Cone

When you release rays in a cone, a new type of *Conical distribution* is available: *Hexapolar*. For the *Hexapolar* cone option, rays are released at uniformly distributed angles from the cone axis, with each ring having six more rays than the previous one.

#### Isotropic Scattering Wall Condition

You can now select *Isotropic scattering* as the wall condition when particles hit boundaries in the geometry. Like the *Diffuse scattering* condition, the *Isotropic scattering* condition causes particles to be reflected with randomly sampled velocity directions around the surface normal. However, whereas the *Diffuse scattering* condition uses a probability distribution based on the cosine law, the *Isotropic scattering* condition follows a probability distribution that gives equal flux across any differential solid angle in the hemisphere.

#### Renamed Ray Release Features

Ray release features have been renamed in COMSOL Multiphysics^{®} version 5.5. The *Inlet* is now called *Release from Boundary*, and the *Inlet on Axis* (in 2D axisymmetric models) is now called *Release from Symmetry Axis*.

### Improvements to Iterative Solver Suggestions in Acoustics

The auto-generated solver suggestions have been improved for models that include interfaces from the Acoustics Module together with multiphysics couplings. Additionally, the Lagrange multiplier variables are handled correctly by the Vanka preconditioner when necessary. Common iterative solver suggestions are now set up for the following couplings and combination of couplings:

*Acoustic BEM-FEM Boundary**Acoustic-Structure Boundary**Thermoviscous Acoustic-Structure Boundary**Acoustic-Thermoviscous Acoustic Boundary**Aeroacoustic-Structure Boundary**Piezoelectric Coupling**Solid-Shell Connection*

Other default and solver suggestion improvements include an iterative solver suggestion for the *Compressible Potential Flow* interface. A new *Stationary-Frequency* and *Stationary-Transient* solver configuration is available when coupling *Compressible Potential Flow* and *Linearized Potential Flow* in a convected acoustics simulation. A second iterative solver suggestion is now added for models to couple *Pressure Acoustics* to *Solid Mechanics* with the *Acoustic-Structure Boundary* multiphysics coupling. Lastly, a better default solver has been added for the *Linearized Euler* interfaces. You can see some of these improvements used in the Loudspeaker Driver in a Vented Enclosure model.

*Suggested Iterative Solver*in the Loudspeaker Driver in a Vented Loudspeaker Enclosure tutorial model.

### New Solvers for Large Acoustic Problems

For frequency-domains simulations modeled with the *Pressure Acoustics, Frequency Domain* interface, two specialized iterative solver methods have been introduced for simulating finite element method models at high frequencies. First, the domain decomposition method now supports the use of absorbing boundary conditions for the domain boundaries, which is important for cluster computations using domain decomposition for frequency-domain acoustics. Second, the new complex Shifted Laplacian (SL) method can be used for both the normal multigrid preconditoner and the domain decomposition method. The multigrid alternative is the best option for large models when not using a cluster.

With this new functionality, you can solve significantly larger models in acoustics than before. For example, a car cabin interior acoustic model can now be solved up to 7 kHz, solving 83.5 million DOFs using 105 GB of RAM, whereas it would only converge for up to about 3 kHz in earlier versions of the software. This corresponds to an order of magnitude larger simulation due to the fact that frequency domain acoustics scales with approximately the cube of the frequency. You can see this functionality used in the Car Cabin Acoustics — Frequency Domain analysis model.

### Restructure of the Model Wizard Tree and Application Library

With the introduction of the new *Elastic Waves, Time Explicit* interface, the physics interface locations in the *Model Wizard* tree have been updated with two new branches: *Elastic Waves* and *Pipe Acoustics*. To get a better overview of the existing and the many new tutorial models, the Application Library categories have also been updated with new categories:

- Elastic Waves
- Tutorials, Pressure Acoustics
- Tutorials, Pipe Acoustics
- Tutorials, Thermoviscous Acoustics

### Important Enhancements in the Acoustics Module

- In the
*Exterior Field Calculation*feature- The location of an infinite symmetry and antisymmetry plane can be specified by entering an offset value

- Postprocessing news
- The
*Octave Band*plots have the option of using 1/6 octaves - The reference direction can be set in
*1D Radiation Pattern*plots of 2D models - The
*Directivity*plot comes with a true logarithmic axis and has moved to 1D plot groups

- The
- For the
*Exclude Edges*and*Exclude Points*options- Added for all constraint type boundary conditions (Dirichlet conditions) in the following interfaces:
*Thermoviscous Acoustics*interface*Linearized Navier-Stokes*interface*Linearized Euler*interface

- Handle over-constrained problems and simplify certain combinations of boundary conditions
- Available when the
*View Advanced Physics*option is selected

- Added for all constraint type boundary conditions (Dirichlet conditions) in the following interfaces:
- The unit "rayl" used for the specific acoustic impedance
- Available in the SI unit:
`[rayl]`

- Available in the cgs unit:
`[rayls_cgs]`

- Available in the SI unit:
- Surface stress variables exist on both exterior and interior boundaries in the following interfaces:
*Thermoviscous Acoustics*interface*Linearized Navier-Stokes*interface

- A new
*Absorption Coefficient*option is available for the impedance boundary condition- Simplifies the input of certain measured surface impedance data
- Useful in the higher frequency range

- Available for all pressure acoustics interfaces

- Simplifies the input of certain measured surface impedance data
- The
*Characteristic specific impedance*condition in pressure acoustics interfaces- Works for waves propagating at a given angle toward the boundary
- A priori knowledge of the solved problem can improve the simple nonreflective conditions significantly

### Extended Support for Jiles–Atherton Hysteresis

The nonlinear *Magnetostrictive Material* has been extended to include the Jiles–Atherton model of magnetic hysteresis. The model is suitable for investigating the hysteretic loss effects in applications such as power transformers and rotating electric machines. The model parameters are related to microscopic physical effects in magnetic materials and they can also be estimated based on experimental data.

Additionally, the *Jiles–Atherton* material model for magnetic hysteresis has been extended to support parametric stationary studies (in addition to the previously available Time Dependent analysis). Ferromagnetic hysteresis is for low-to-moderate frequencies, rate-independent, and can be analyzed using a parametric stationary study, for example when studying magnetization and demagnetization. This functionality requires the AC/DC Module together with one of the Structural Mechanics Module, Acoustics Module, or MEMS Module.

### Visualization of Loads

Applied mechanical loads are now available as default plots in all structural mechanics physics interfaces. The loads plots are solution dependent, so both arrow directions and colors are updated when a dataset is updated with a new solution. Even abstract loads, such as forces and moments applied to rigid connectors and rigid domains are plotted at their true point of application. A new arrow type, used for plotting applied moments, has been introduced for this functionality. More than 100 models are updated with this new functionality.

### New Tutorial Models

Version 5.5 brings several new and updated tutorial models.

#### Sound Transmission Loss Through a Concrete Wall

**Application Library Title:** *sound_transmission_loss_concrete*

#### OW Microspeaker: Simulation and Correlation to Measurements

**Application Library Title:** *ow_microspeaker*

#### Wax Guard Acoustics: Transfer Matrix Computation

*Port Sweep*functionality and the

*Port*boundary condition. The results show the acoustic velocity fluctuation contours inside the microperforates of the wax guard.

**Application Library Title:**

*wax_guard_acoustics*

#### Head and Torso HRTF Computation

**Application Library Title:**

*head_torso_hrtf*

#### Shape Optimization of an Acoustic Demultiplexer

**Application Library Title:**

*demultiplexer_shape_optimization*

#### Ground Motion After Seismic Event: Scattering off a Small Mountain

*Elastic Waves, Time Explicit*physics interface.

**Application Library Title:**

*ground_motion_seismic_event*

#### Isotropic-Anisotropic Sample: Elastic Wave Propagation

**Application Library Title:**

*isotropic_anisotropic_sample*

#### Head and Torso Simulator Acoustics

#### Angle Beam Nondestructive Testing

#### Immersion Ultrasonic Testing Setup

#### Probe Tube Microphone

*Acoustic-Pipe Acoustic Connection*multiphysics coupling, and consists of an external acoustic domain, an elastic probe tube, and a cavity in front of the microphone diaphragm.

**Application Library Title:**

*probe_tube_microphone*

#### Acoustics of a Pipe System with 3D Bend and Junction

*Acoustic-Pipe Acoustics Coupling*multiphysics coupling. The results show the pressure in the pipes, T-junction, and bend.

**Application Library Title:**

*acoustics_pipe_system*

#### Impedance Tube Parameter Estimation with Data Generation

**Application Library Title:**

*impedance_tube_parameter_estimation_data*

#### Topology Optimization and Verification of an Acoustic Mode in a 2D Room

#### Acoustic Mean Free Path in a Room

#### Modeling Piezoelectric Devices as Both Transmitters and Receivers

#### Spherical Scatterer: BEM Benchmark

*Pressure Acoustics, Boundary Elements*interface with the analytical solution for several frequencies, and they show very good agreement.

**Application Library Title:**

*spherical_scatterer_bem_benchmark*

#### Acoustic Cloaking

*Anisotropic Acoustics*functionality. The results show the SPL of the scattered acoustic field for several configurations. The homogenized anisotropic model shows the lowest scattered field level and best performance.

**Application Library Title:**

*acoustic_cloaking*