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Modeling of Car Cabin Acoustics

AES 2017 International Conference on Automotive Acoustics

11:30 am – 12:30 pm, September 9, 2017

Mads Herring Jensen, Ph.D.

Technical Product Manger, Acoustics, COMSOL

mads@comsol.dk

COMSOL/Me • Mads Herring Jensen

– Technical Product Manger, Acoustics – Planning, specifications, development, teaching related to

the Acoustics Module

• COMSOL Multiphysics® – Multiphysics tools within most

areas of engineering and physics

• COMSOL A/S – I am located in Copenhagen,

Denmark

• Acoustics Team – Copenhagen, Stockholm, Boston

Outline

• Why simulate? • Car cabin acoustics overview • Simulation methods • CAD and Mesh • Boundary conditions • Material models • Source specification • Show model in COMSOL® • Other topics • Conclusions

speaker

Sedan Cabin of 3.3 m3

Why simulate?

• Optimize using virtual prototypes

• Fewer physical prototypes

• Virtual measurements and much more! – Understand cause and effect

– The full “sound” image

• Visualize sound field – Interference patterns and resonances

• Goal of the simulation/model – Absolute vs. relative

Car Cabin Acoustics

• Challenging from a computational point of view

• “Medium volume” – Audio range spans modal region,

medium frequency, and high frequency

• Complex listening space

• Complex boundary conditions – Porous, elastic structures, …

f = 2000 Hz

Simulation Methods

• Full field/wave methods – FEM or BEM – DG-FEM (transient) – Resolve wavelength with the computational mesh

• High frequency ray methods – “Local plane wave approximation”

• Lumped methods for sources (speakers) – Thiele-Small parameters

• Frequency versus time domain – Boundary conditions

Finite Element Method (FEM)

• Pros

– Versatile

– Fully multiphysics enabled

– Well suited for parallelization

– Sparse matrices

• Cons

– Domain mesh

– Resolving the wave: /6

– Memory consumption for very large problems

– DOFs f3

• Future

– Cheaper hardware and clusters

– Special Helmholtz preconditioners

Examples of FEM

f = 2000 Hz 4.1e6 DOFs, 16 GB RAM, 10 min 2 year old desktop with 32 GB RAM Solved on server for f = 4000 Hz >128 GB RAM

Boundary Element Method (BEM)

• Pros

– Ease of use of a boundary only mesh/CAD

– Memory consumption scales better than FEM for large pure acoustic models

– Multiphysics enabled (in COMSOL)

• Cons

– Dense matrix solver

– Computationally heavy assembly of matrices

– Somewhat heavy post-processing

– Better suited for open radiation problems

• Future

– Dedicated solvers for interior problems (with sharp resonances)

– Speedup of iterative solver BEM @ 2000 Hz

Ray Tracing

• Pros

– Fast and memory lean

– High frequency method

– Ease of use, coarse meshing of boundaries

– Frequency band data

• Cons

– High frequency method (only)

– < Lgeom and < Rgeom

– Boundary conditions

– Source definition

– Frequency band data

• Future

– Diffraction

– Synthetization of complex sources

Mathematically valid for f > 4-5 kHz Then there is “in practice” …

Time Explicit Methods

• In COMSOL Multiphysics we use Discontinuous Galerkin (DG-FEM)

• Pros – General transient sources – Full wave method – Very memory lean

• Cons – Difficult to include losses and realistic

impedance conditions – Mesh sensitive

• CFL and stability

– Large amounts of data to store

• Future – ODE/filter based boundary conditions

The right tool for the right job!

FEM

BEM

Ray Tracing

f

DG-FEM

0 Hz 2-3 kHz 5 kHz 20 kHz

60 Hz

95 Hz

98 Hz

1 kHz

2 kHz

CAD and Mesh

• CAD and Mesh are highly related

• Get a CAD specialist to work on the geometry!

• “Sloppy meshing” or Virtual Operations in COMSOL Multiphysics

• Boundary conditions

Geometry resolution controlled by mesh not by CAD

CAD and Mesh

51616 boundary elements 407249 domain elements

45746 boundary elements 365206 domain elements

f = 1000 Hz hmax = /6 = 5.1 cm

Validation

Boundary Conditions

• Quality of Boundary Conditions data dictates the quality results • Impedance data from complex structures

– In-situ impedance tube measurements – Acoustic holography

• Model liners and seats – Domains – Impedance conditions

• Vibrating panels/structures – coupling! • For ray models

– What detail is necessary? – Reflection coefficient R(f,) (detailed) – Absorption (fc) (band data?)

Boundary Conditions

f = 104 Hz The first mode of the shell (panel)

Material Models

• Porous materials – Used for seats and linings

– Equivalent fluid • Delany-Bazley-Miki

• Johnson-Champeaux-Allard (JCA)

• JCAL and JCAPL (low frequency corrections)

– Full poroelastic waves (Biot-Allard) • Coupling to structures (composite materials)

• Coupling to fluid

• Measurement/simulation conditions – Temperature, pressure, humidity

Source Specification

• FEM/BEM need to model geometry to get the directivity! – On Acoustical Modeling and Validation of Automotive Loudspeaker

Grilles by Martin Olsen, Peter John Chapman, and Michael Strauss ‐ Harman Lifestyle Audio and Harman Virtual Product Development

+ -

+ - coil

terminals

Rg RE LE(w)

V0

1

0

2 3 4

ic BLuD

RE(w) ’

Source Specification

Source Specification

Source Specification: Ray Tracing

• Spatial characteristics simulated/measured (phase and intensity)

• Point Source Assumption

– Spherical wave (spherical wave front)

– Wave front curvature important for intensity

– Source evaluated further than Rayleigh distance

– At the same time ray tracing is valid for

𝑅0 =𝑘𝑎2

2 = 𝜋𝑎2𝜆

𝐿𝑔𝑒𝑜𝑚 ≫ 𝜆

Source Specification: Ray Tracing f = 3000 Hz f = 6000 Hz

a = 3 cm a = 3 cm

a

a

2R0

2R0

2

2

Source Specification: Ray Tracing

• Consider

– Sources in proximity of reflectors

– Interference will generate a different source pattern

• The idea

– Synthetization of the source from FEM simulation • Intensity

• Local reflections

• Wave front curvature

Source Specification: Ray Tracing

FEM model

Ray model with point source from FEM model

Ray source

f = 3000 Hz

Source Specification: Ray Tracing

f = 3000 Hz Resulting SPL on exit surface

FEM

Ray tracing

Source Specification: Ray Tracing

Initialized with: • Direction • Intensity • Wave front curvature

More detailed models by combining methods!

Show Model in COMSOL®

Other topics

• Modal buildup times

• Structural couplings

• Panel contributions

• Thermoviscous losses in grills

• Non-linear effects at low frequencies

Conclusions

• Goal of the simulation – Absolute vs. relative

– Visualize and understand

• Choice of method – FEM/BEM/Ray

– Probably a combination!!

• Quality of input – Boundary conditions

– Sources

• Still open questions, research, development …

COMSOL News 2017 Special Edition Acoustics

Topics include: • Acoustics simulation • Virtual product development • High-precision microphones • Combustion instability • NVH performance • Transformer hum • Multibody-acoustics interaction • Acoustic cloaking • Infrasound-induced vibrations • Feedback reduction • Noninvasive acoustic technology • High-precision transducers • Guest editorial on computational acoustics www.comsol.com/offers/comsol-news-2017-special-edition-acoustics

COMSOL Multiphysics®

Acoustics in COMSOL Multiphysics®

All Industries Benefit from Multiphysics Simulation

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