## Acoustics Module

### New App: Absorptive Muffler Designer

Mufflers are used to attenuate noise emitted by a combustion engine, for example, and should typically perform well in a specific frequency range. Attenuation is measured through transmission loss, which gives the damping in dB as a function of frequency.

The Absorptive Muffler Designer simulation app is used for studying and designing a simple resonant muffler with a porous lining. With the app, you can perform an analysis of both inductive and resistive damping in a muffler of your choosing.

The app enables you to study the results after changing the dimensions of the muffler, the ambient working conditions, and the material properties of the porous liner.

*User interface for the Absorptive Muffler Designer app.*

### New App: One-Family House Acoustics Analyzer

The One-Family House Acoustics Analyzer app is used to assess noise propagation in coupled rooms inside of a two-story house consisting of ten rooms. The app determines the sound pressure level (SPL) distribution in the house based on a number of sources that are interactively placed throughout the home.

It represents a classical room acoustics problem where engineers or architects want to determine the noise environment in a system of coupled rooms, such as homes, office spaces, or workshops. This is important when ensuring that an acoustic environment complies with noise and work regulations, for instance.

An engineer or architect can bring an app like this on-site and test different noise source scenarios and wall insulation characteristics. They can then compare the simulation results with actual measurements. The application lets you place, remove, and define multiple acoustic sources in different rooms of the house in order to determine the resulting SPL distribution.

The acoustics are modeled using the *Acoustic Diffusion Equation* physics interface in COMSOL Multiphysics, which is both fast and efficient at determining SPL distributions.

*User interface for the One-Family House Acoustics Analyzer app, showing the various noise source options.*

### New App: Organ Pipe Design

The Organ Pipe Designer allows you to study the design of an organ pipe, and then play the sound and pitch of the changed design in a user-friendly app. The pipe sound includes the effects of different harmonics with different amplitudes.

The organ pipe is modeled using the *Pipe Acoustics, Frequency Domain* interface in COMSOL Multiphysics. The simulation app allows you to analyze how the first fundamental resonance frequency varies with the pipe radius and wall thickness, as well as with the ambient pressure and temperature.

Using the app, you can find the full frequency response, including the fundamental frequency and the harmonics. With a Java® code written method, the app will detect the location and amplitude of all harmonics in the response, thus extending the analysis beyond the built-in functionality of the COMSOL Multiphysics user interface.

### New App: Acoustic Reflection Analyzer for a Water-Sediment Interface

Analyzing acoustic reflections at surfaces of various structures is important for many engineering disciplines. The Acoustic Reflection Analyzer for a Water-Sediment Interface app shows one such system where the analysis has relevance for underwater acoustics and sonar applications.

The app analyzes the reflection and absorption coefficients of plane acoustic waves, scattering off of a water-sediment interface at different frequencies and angles of incidence. Moreover, the random-incidence absorption coefficients are determined for the studied frequencies.

To describe the water-sediment system, the app uses the ability of the Poroelastic Waves interface in COMSOL Multiphysics to model the coupled acoustic and elastic waves in any porous substance (Biot's theory).

### Octave Band Plots

You can now use the newly dedicated acoustics plot, called the Octave Band Plot, to represent frequency-domain transfer functions, responses, sensitivity curves, insertion, and transmission loss. The plot has several built-in acoustics-specific features, such as predefined weighting (Z, A, C, and user-defined) and desired plot style (octave bands, 1/3 octave bands, or as a continuous curve). The band options correspond to plotting the average or integrated value of, for example, the squared pressure over a given frequency band defined by the midfrequency and the bandwidth.

The data input to an octave band plot is a frequency-domain solution. For example, it can be the acoustic pressure resulting from a Frequency Domain study or a parametric frequency sweep. The Octave Band Plot will automatically plot a given expression type on the dB scale, simplifying postprocessing, as it is no longer necessary to define the expression as a variable. The input geometric entity level of the plot can be global, point, edge, boundary, or domain. On the last three, an average is automatically performed, making it easier to define and plot the average power, for example, at the model's inlet.

There are three options for the plot input:

- Amplitude (e.g., the pressure measured at a point)
- Power (e.g., the incident intensity into a muffler)
- Transfer function (e.g., the electroacoustics transfer function between voltage and pressure in a microphone)

There are three plot styles for representing the response:

- Octaves
- 1/3 octave
- Continuous

You can also apply a weighting to the response:

- Z, A, and C weighting (complies with IEC 61672-1 standard)
- User-defined weighting (enter any user-defined weighting that can depend on the frequency)

*The sensitivity curve of the Loudspeaker Drive tutorial model plotted as a continuous curve and as 1/3-octave bands.*

The sensitivity curve of the Loudspeaker Drive tutorial model plotted as a continuous curve and as 1/3-octave bands.

*The transmission loss of the Absorptive Muffler tutorial model shown in 1/3-octave bands for two different liner configurations.*

The transmission loss of the Absorptive Muffler tutorial model shown in 1/3-octave bands for two different liner configurations.

### Dissipated Energy Variable in Pressure Acoustics

For Pressure Acoustics, Poroacoustics, and Narrow Region Acoustics, you can now simulate dissipated power density for all fluid models. The variable is called acpr.Q_pw and is located under the Heating and losses section in the Add/Replace Expressions menu when working with results (see the image linked below). The expression is valid in the plane wave limit for traveling waves. The predefined variable is used in the Focused Ultrasound Induced Heating in Tissue Phantom tutorial model, where dissipated acoustic energy heats a tissue phantom.

### Normal Velocity and Normal Displacement Boundary Conditions in Pressure Acoustics

In the *Pressure Acoustics* interfaces, the *Normal Acceleration* boundary condition is now supplemented by two new boundary conditions for prescribing a normal velocity or a normal displacement. This simplifies the modeling procedure when modeling sources in acoustics. An example of this can be found in the Generic 711 Coupler – An Occluded Ear-Canal Simulator tutorial model, where the source is defined via the *Normal Displacement* boundary condition.

### Additional Features in the Poroelastic Waves and Elastic Waves Physics Interface

The *Poroelastic Waves* and *Elastic Waves* physics interfaces are updated and improved, with several features and boundary conditions. They now also include the following features:

- Domain Features:
- Spring Foundation
- Added Mass

- Boundary Features:
- Symmetry
- Rigid Connector
- Thin Elastic Layer
- Spring Foundation
- Added Mass

- Edge Features:
- Fixed Constraint
- Prescribed Displacement
- Edge Load
- Spring Foundation
- Added Mass

- Point Features:
- Fixed Constraint
- Prescribed Displacement
- Spring Foundation
- Point Load
- Point Load on Axis
- Ring Load

### Updated Intensity Variables in All Acoustics Interfaces

All of the acoustics interfaces include updates to the intensity variables, which are now consistent across physics interfaces and study types. Intensity is defined in the frequency domain (averaged values over one period) and the so-called instantaneous intensity is defined in the time domain. The intensity variables in the *Thermoacoustics* and *Linearized Navier-Stokes* interfaces now include the viscous stress contributions. The variables are available in the results when clicking on the Add/Replace Expression buttons.

### Full Mass Matrix Input in Added Mass

The *Added Mass* feature has been extended so that it is possible to enter a full mass matrix.

### Prescribed Velocity/Acceleration Interpretation in Stationary Analysis

When the *Prescribed Velocity* or *Prescribed Acceleration* nodes are present in your model, you can define how these boundary conditions should be interpreted in a stationary analysis. They can either be treated as a constraint (constrained), or ignored (free). This is particularly useful in models and apps with multiple mixed-analysis types, including frequency-domain, time-dependent, and stationary types.

### Minor Updates and Bug Fixes

The new release of COMSOL Multiphysics includes several minor updates and bug fixes:

- The PARDISO solver, used in the Thermoacoustics and Linearized Navier-Stokes interfaces, now uses the Multithreaded forward and backward solve option by default. This provides minor speedup in general, with the most noticeable effect in eigenfrequency problems.
- The Pipe Acoustics interface includes updated model inputs.
- The Thermoacoustics interface has a new entropy variable, ta.s_entropy, for use in postprocessing, as well as updated dissipated thermal and viscous power density variables.
- The Acoustic Diffusion Equation interface has updated user-defined band structure behavior.
- To avoid locking on curved boundaries, the Thermoacoustics, Frequency Domain, and Linearized Navier-Stokes interfaces have updated slip conditions.

### New Tutorials in the Application Gallery

Four new tutorial models have been added to our online Application Gallery.

*The acoustics of an apartment, analyzed using the acoustic diffusion equation.*

Vibrating Plate in a 2D Viscous Parallel Plate Flow

- This simple 2D tutorial model couples the Linearized Navier-Stokes, Frequency Domain, Solid Mechanics, and Creeping Flow physics interfaces to model the vibrations of a plate located in a 2D viscous parallel plate flow.

Apartment Acoustics Analyzed Using the Acoustic Diffusion Equation

- This tutorial model computes the sound distribution from a TV in a one-bedroom apartment. The simulation demonstrates the use of the Acoustic Diffusion Equation interface to get a quick and simple estimate of the local sound pressure level. For increased accuracy, an analytical expression for the direct sound is added in the living room.

Acoustic-Solid Interaction with Two Perfectly Matched Layers (PMLs)

- This simple tutorial model shows how to set up a model with two perfectly matched layers (PMLs), one for a pressure acoustics domain and one for a solid mechanics domain.

Shape Optimization of a Tweeter Waveguide

- This tutorial model illustrates how to use the optimization capabilities of COMSOL Multiphysics to automatically develop novel designs, satisfying critical design constraints. The simulation optimizes a simple speaker geometry. Examples of constraints could include the radius of the loudspeaker or a desired minimum achievable sound-pressure level.