Acoustics Transient BEM

You can use the Simcenter 3D Acoustics Transient BEM solver to solve acoustic and vibro-acoustic solutions in the time domain. Acoustic or vibro-acoustic excitations are functions of time.

You can include one or multiple scattering bodies in an Acoustics Transient BEM model that is excited by an acoustic monopole or an acoustic plane wave.

You can include time-dependent displacements, velocities, or accelerations of a structural model.

You can solve time-dependent BEM models using these solutions:

• Transient Acoustic — Solves for the pure acoustic response for a single excitation.

• Transient Vibro-Acoustic — Includes the vibro-acoustic structure fluid interaction and structural displacements, velocities, and accelerations.

Some examples of transient acoustic analyses are:

• Car door slam noise.

• Vibro-acoustic response from loudspeaker radiation.

• Time-dependent scattering of an acoustic monopole, or acoustic plane wave, around objects or absorbing barriers.

You can apply acoustic or vibro-acoustic excitations to an Acoustics Transient BEM model.

When you use the Acoustics Transient BEM solver you can define one acoustic load for a single load case. You define an Acoustic Monopole with a time-dependent volume flow rate, or an Acoustic Plane Wave with a time-dependent pressure.

• Acoustic excitations:An Acoustic Monopole can be created at a specified location with a time-dependent volume flow rate magnitude.An Acoustic Plane Wave can be created at a given location, with a given direction vector, and a time-dependent acoustic pressure magnitude.

• Vibro-Acoustic ExcitationsWhen you use the Transient Vibro-Acoustic BEM solver you can include structural excitations as displacement, velocity, acceleration, and force on a structural mesh, but you cannot combine displacements with velocities, and so forth.A single type of enforced vibration excitation can be applied from a Load Recipe to the structure in a subcase. You can apply:Structural DisplacementsStructural VelocitiesStructural Accelerations****Force on a Mode Set representation

You can include infinite planes, acoustic absorbers, transfer admittance, and panels in the model.

You can include panels and request panel contribution (PANCON) with the following restrictions

• You cannot include a Mode Set representation in the model.

• The excitation can be only enforced vibration.

You can request the following vibro-acoustic outputs:

• Microphone Output — Pressure

• Structural Output — Displacement, velocity, and acceleration. These requests are only valid for:A Mode Set representationForce excitation from a Load Recipe or individual forces.

In an Acoustics Transient BEM solution, the mesh is restricted to:

Element Types	Transient Acoustic Solution	Transient Vibro-Acoustic Solution
Acoustic mesh	TRIA3	TRIA3
Microphone mesh	TRIA3QUA4	TRIA3QUA4
Structural mesh	Not applicable	CBARCBEAMCBUSHCRODTRI3TRI6QUA4QUA8TET4TET8PYR5PYR13PEN6PEN15HEX8HEX20RBE2	CBAR	CBEAM	CBUSH	CROD	TRI3	TRI6	QUA4	QUA8	TET4	TET8	PYR5	PYR13	PEN6	PEN15	HEX8	HEX20	RBE2
CBAR	CBEAM
CBUSH	CROD
TRI3	TRI6
QUA4	QUA8
TET4	TET8
PYR5	PYR13
PEN6	PEN15
HEX8	HEX20
RBE2

Transient Acoustic model. 1) TRI3 acoustic mesh, 2) Acoustic monopole, 3) Microphone mesh

Acoustic Monopole transient excitation	Transient pressure response at a discrete time

Alternate Representations

In a Transient Vibro-Acoustic solution, you can edit the acoustic BEM mesh representation to use a Mode Set.

Solution control

You set solution parameters to control the transient solution.

Startup Smoothing Factor

The Start-up Smoothing Factor sets smoothing of the initial time segment (0, αT) of the transient analysis to help eliminate undesirable numerical artifacts. The factor is a sin2 function applied to the beginning of the analysis time.Start-up Smoothing Function applied to the solution time rangeYou set the Start-up Smoothing Factor α, which defines the initial transient part of the simulation duration T.

Step Definition Method

You set the Step Definition Method by setting the analysis time step Time Step (dT) or by setting the Courant, Friedrichs, Lewy (CFL) factor.

Time Step (dT)

You set the desired time step for the analysis.

Courant, Friedrichs, Lewy (CFL)

The Courant, Friedrichs, Lewy (CFL) factor controls the time step of the analysis and therefore the solution stability and accuracy. Numerical solution of transient phenomenon can be inherently unstable, so in most cases you should set the CFL factor to a value between 0.2 and 2 for an acceptable accuracy and solution time. CFL is related to the analysis time step by:dT = CFL x Hmax / c, where:dT = Time stepCFL = Courant, Friedrichs, Lewy (CFL) factorHmax = maximum element edge lengthc = Speed of sound

Duration Definition Method

You set the duration of the analysis using the Relaxation Factor or Solution Maximum Time (Tmax).

Relaxation Factor

The Relaxation Factor sets the solution duration indirectly. The Relaxation Factor corresponds to the number of times a residual acoustic impulse can travel, or reflect, between the excitation end time and the end of the solution. If you desire no acoustic output beyond the end of the excitation, set the Relaxation Factor to zero.Set the Relaxation Factor to a large number if:The excitation time is short. The acoustic domain is large.The Microphone mesh is far from the excitation.Set the Relaxation Factor to a small number, or zero, if:The excitation source is close to scattering objects in the domain.The scattering object is acting as a vibrating panel or source.Microphones are close to either the source or the scattering objects.

Solution Maximum Time (Tmax)

You set the Solution Maximum Time (Tmax) for the analysis.The total time of the simulation is:Tmax = Tsignal + Trelax, where:Tmax = Total duration of the simulationTsignal = Time duration of the excitationTrelax = Relaxation timeRelaxation time = Relaxation Factor x diam(object) / c, where:diam(object) = Maximum length of the acoustic object in the domainc = Speed of sound

Low Pass Filter

Low Pass Filter for Input and Response Signals

You can use the Low Pass Filter for Input and Response Signals to apply a low pass filter to the input excitation. Frequencies above the low pass filter setting are ignored.When you Request Maximum Frequency, the solver will compute:fmax = Low Pass Maximum Frequencyfmax = 1/ (4 x dT), where:dT = Time step

Learn more

Coupling for Indirect Vibro-Acoustic BEM and Acoustics Transient BEM

Acoustics Transient BEM workflow

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Acoustics Transient BEM, Simcenter 3D 2021.1 Series

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