Acoustics Module

New Physics Interfaces in Version 5.2a

Convected Wave Equation, Time Explicit

The Convected Wave Equation, Time Explicit is a completely new interface for modeling large acoustic problems in the time domain. The interface is based on the discontinuous Galerkin method (also known as DG-FEM or simply DG) and uses a time-explicit solver. This results in a very memory-efficient method.

The Convected Wave Equation, Time Explicit interface is found under the new Ultrasound subbranch. The interface is used to solve large transient linear acoustic problems containing many wavelengths in a stationary background flow. Application areas include ultrasound flow meters, ultrasound distance sensors, and other ultrasound sensors where time of flight is an important parameter. The applications are not restricted to ultrasound, but also include, for example, transient propagation of audio pulses in room acoustics or car cabins.

The interface is suited for time-dependent simulations with arbitrary time-dependent sources and fields. In general, it is suited for modeling the propagation of acoustic signals over large distances relative to the wavelength; linear ultrasound problems are example applications. The interface includes new Absorbing Layer nodes that are used to set up effective nonreflecting-like boundary conditions (sponge layers). The interface solves the linearized Euler equations assuming an adiabatic equation of state. The dependent variables are the acoustic pressure and the acoustic velocity perturbations. The background flow can be any stationary flow with small to moderate velocity gradients. No loss mechanisms are included in the interface. The physics interface exists in 3D and 2D geometries.

Thermoviscous Acoustics, Boundary Mode

The Thermoviscous Acoustics, Boundary Mode interface is used to compute and identify propagating and nonpropagating modes in waveguides and ducts. The interface performs a boundary mode analysis on a boundary, inlet, or cross section of a waveguide or duct of small dimensions. The thermal and viscous loss effects that are important in the acoustic boundary layer near walls are included.

The interface solves for the acoustic variations in pressure, velocity, and temperature, as well as the out-of-plane wave number of the modes. As mentioned, near walls, viscous losses and thermal conduction become important because a boundary layer exists. The thickness of these layers is known as the viscous and thermal penetration depth. For this reason, it is necessary to include thermal conduction effects and viscous losses explicitly in the governing equations. The Thermoviscous Acoustics, Boundary Mode interface is, for example, used when setting up sources in systems with small ducts like hearing aids or mobile devices. It can also be used to identify the propagating wave number and characteristic impedance of a duct cross section and use that information in the homogenized Narrow Region Acoustics feature of the Pressure Acoustics, Frequency Domain Interface.

The physics interface exists in 3D and 2D axisymmetric geometries and is applied at boundaries. It solves the equations defined by the linearized Navier-Stokes equations (linearized continuity, momentum, and energy equations), in quiescent background conditions, searching for the out-of-plane wave numbers at a given frequency.

Thermoviscous Acoustics: New Name for Thermoacoustics

In this release of COMSOL Multiphysics, all interfaces that used the Thermoacoustics term have been renamed to use Thermoviscous Acoustics instead. The interfaces have always been dedicated for detailed modeling of thermal and viscous acoustic losses in problems with small geometrical dimensions. That is, in problems where the losses in the thermal and viscous acoustics boundary layers are important. This is the case when modeling microphones, mobile devices, hearing aids, miniature transducers, and much more. Since Thermoacoustics is already a branch of acoustics, dealing with cooling or heating using acoustic waves, a more descriptive and correct terminology is now used: Thermoviscous Acoustics.

The following interfaces have new names:

•	Thermoviscous Acoustics, Frequency Domain

•	Thermoviscous Acoustics, Boundary Mode (new interface)

•	Acoustic-Thermoviscous Acoustic Interaction, Frequency Domain

•	Thermoviscous Acoustic-Structure Interaction, Frequency Domain