modeling of car cabin acoustics - aes€¦ · grilles by martin olsen, peter john chapman, and...
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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
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
Product Suite – COMSOL® 5.3