car cavity acoustics using ansys car cavity acoustics ......c is the speed of sound in air, l is the...
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ANSYS, Inc. Proprietary© 2006 ANSYS, Inc.
Car Cavity Acoustics using ANSYSCar Cavity Acoustics using ANSYS
Muthukrishnan AAssistant ConsultantTATA Consultancy Services185,Lloyds Road, Chennai- 600 086INDIA
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Introduction
• The study of vehicle interior acoustics in the automotive industry has gained importance due to
– Legal restrictions – Growing demand for comfort– To reduce the number of prototypes– High performance of modern computers
• The objectives of efficient automobile design are– Safety– Fuel consumption– Interior comfort
• The challenges are– Lower weight improves the fuel efficiency but
increases the vibrational sensitivity– Increase in weight generally improves the safety
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Objectives
• To predict the individual effect of low frequency noise (50-200Hz) on the car cavity due to sound absorption of
– Floor carpet– Roof lining – Seat– Front window open– Combined effect of all the above
• In all the above cases the front wall is excited with constant displacement of 1mm
• Location of interest– Drivers right ear (DRE) located at (1.50, 0.8, 1.25)
metre in the global X, Y, Z directions respectively – Passengers right ear (PRE) at the back of the driver
located at (0.6, 0.8, 1.25) in the global X, Y, Z directions respectively.
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Classification of Noise inside the Vehicle
Major source of low frequency noiseRoad excited vibrationMay be noticeableFan noiseNot importantEngine inletNot importantEngine exhaust
Major source of high frequency noiseEngine airborne noise and itstransmission
Major source of low frequency noiseEngine vibration
Noise inside the vehicleOrigin of noise
Noise based on frequency:Low frequency noise (50-200Hz)High frequency noise(200-4000Hz)
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Flow Chart for Finite Element Analysis
DIMENSIONAL MEASUREMENT OF ACTUAL CAR
FE MODEL
MODAL ANALYSIS
HARMONIC ANALYSIS
SOUND PRESSURE LEVEL
BOUNDARY CONDITIONS
EXCITATION FORCES
FREQUENCY RANGE
MATERIAL PROPERTIES
NATURAL FREQUENCY
END
ENDTIME HISTORY POST PROCESSING
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Case Studies
• The following five cases were analysed for a typical Indian car (Ambassador Mark IV)
– Car cavity with no absorption– Car cavity with roof absorption– Car cavity with seat absorption– Car cavity with full absorption– Car cavity with full absorption and window open
condition
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Element Specification
MATERIAL PROPERTIES (AIR)1.2 Kg/m3Density343 m/sVelocity of sound in air
Sound pressure Level in dBSound pressure Level Average pressurePressure
ELEMENT OUTPUTS
• Fluid structure interface• Impedance
Surface loads
#4 (Ux,Uy,Uz,PRES)-if Fluid structure interaction is present#1 PRES – if no structure at the interface
Degrees of freedom (D.O.F)8Number of nodesFluid 30 (3D volume element)ELEMENT NAME
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Ambassador Car Cavity - Dimensions
1.98 0.210.32
0.30
0.50
0.30
0.30 1.58 0.52 0.11
0.42
0.23
0.06
0.39
Overall dimensions of cavityLength = 2.51mWidth = 1.40mHeight = 1.10m
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Different Finite Element Views Of Car Cavity
Isometric view of interior elements
Isometric view of car cavity
Interior rear view of outer elements
Interior side view of outer elements
Hexagonal MappedMesh typeFluid 30 (Volume element)Element type 27411Number of nodes24570Number of elements
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Modal Analysis Of Car Cavity
• Modal analysis is done for the car cavity and the natural frequencies are obtained using theoretical calculation and ANSYS.
• The frequency range of interest is 0-200 Hz.
• Theoretical calculation is made considering a rectangular box with the overall dimensions. This enables a quick verification of computed frequencies.
f = (c/2)Where, c is the speed of sound in air, L is the length of the car cavity, W is the width of the car cavity, H is the height of the car cavity
• In ANSYS, the equation of motion for an undamped system, expressed in matrix notation as [M]{ } + [K] {u} = {0} .For harmonic vibration {u} = {Φ}i COS ωi t and the solution is given by [K] - ωi
2[M]= 0
/H)(r /W)(q/L)(p ooo ++
u••
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Ambassador Car Cavity-Mode Shape
At 74.99Hz At 122.69Hz
At 137.20Hz At 145.65Hz
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Ambassador Car Cavity-Mode Shape
At 162.10Hz
At 188.73Hz
At 175.02Hz
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Comparison-Modal Frequencies
198.36NA(0,1,1)
183.6188.73(2,1,0)
170.29175.02(1,0,1)
155.97162.1(0,0,1)
140.32145.65(1,1,0)
136.71137.2(2,0,0)
122.55122.69(0,1,0)
68.3574.99(1,0,0)
THEORITICAL RESULTS
ANSYS RESULTSMODESMODAL FREQUENCY
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Harmonic Analysis-Solver Options
SteppedStepped or Ramped Boundary condition
Sub-steps of 5HzFrequency sub-steps
50Hz to 200HzHarmonic frequency range
Time/FrequencyLoad step options
Real + ImaginaryDOF Print out format
FullAnalysis type-Solution Method
HarmonicAnalysis options
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Pressure Plot-No Absorption
No impedance-Pressure plot
At 75 Hz At 125 Hz
At 135 Hz At 145 Hz
No impedance-Pressure plot
At 175 HzAt 165 Hz
At 190 Hz
NO IMPEDANCE SOUND PRESSURE LEVEL
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130
140
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160
50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220
Frequency in Hz
Soun
d pr
essu
re le
vel i
n dB
NO impedance DRE NO impedance PRE
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Roof Absorption
Impedance value is set to 1 on all the six faces of the element.
Surface load
0.13 (50 Hz -125 Hz)0.53 (130Hz –200Hz)
Sound absorption coefficient (MU)
2400 m/sVelocity of sound (SONC)
1100 Kg/m3Density (DENS)
Material properties
3D-Model
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Roof Absorption – Pressure plot
Roof impedance only-Pressure plot
At 125 Hz
At 75 Hz
At 135 HzRoof impedance only-Pressure plot
At 145 Hz At 165 Hz
At 190 HzAt 175 Hz
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No Absorption and Roof Absorption: SPL at DRE and PRE
Roof Impedance comparison chart
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50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220
Frequency in Hz
Soun
d pr
essu
re le
vel i
n dB
NO impedance DRE NO impedance PRERoof Absorption coeff 0.13/0.53 DRE Roof Absorption coeff 0.13/0.53 PRE
98.6095.06Minimum SPL133.31132.95Maximum SPL
PRE in dBDRE in dBDescription
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Floor Absorption
Impedance value is set to 1 on all the six faces of the element.
Surface load
0.2 (50 Hz -125 Hz)0.55 (130Hz – 200Hz)
Sound absorption coefficient (MU)
343 m/sVelocity of sound (SONC)
55 Kg/m3Density (DENS)
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Floor Absorption – Pressure plot
Floor impedance only-Pressure plot
At 135 HzAt 125 Hz
At 75 Hz
Floor impedance only-Pressure plot
At 145 Hz
At 175 Hz At 190 Hz
At 165 Hz
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No Absorption and Floor Carpet Absorption: SPL at DRE and PRE
Floor carpet impedance comparison chart
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50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220Frequency in Hz
Soun
d pr
essu
re le
vel i
n dB
NO impedance DRE NO impedance PREFloor impedance 0.2/0.55 DRE Floor impedance 0.2/0.55 PRE
107.78104.18Minimum SPL125.91125.45Maximum SPL
PRE in dBDRE in dBDescription
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Seat Absorption
Impedance value is set to 1 on all the six faces of the seat element.
Surface load
0.1/0.5 (50Hz –200Hz)
Sound absorption coefficient (MU)
343 m/sVelocity of sound (SONC)
80Kg/m3Density (DENS)
Material properties
3D Model
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Seat Absorption – Pressure Plot
Seat impedance only-Pressure plot
At 75 Hz
At 135 Hz At 145 Hz
At 125 Hz
Seat impedance only-Pressure plot
At 190 HzAt 175 Hz
At 165 Hz
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No Absorption and Seat Absorption: SPL at DRE and PRE
Seat impedance comparison chart
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50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220
Frequency in Hz
Soun
d pr
essu
re le
vel i
n dB
NO impedance DRE NO impedance PRE Seat impedance 0.1 DRE
Seat impedance 0.1 PRE Seat impedance 0.5 DRE Seat impedance 0.5 PRE
93.10102.8772.2592.88Minimum SPL124.32130.71118.40135.53Maximum SPL
PRE in dBDRE in dBPRE in dBDRE in dB
0.5 absorption0.1 absorptionDescription
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Full Absorption
3D Model
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Full Absorption – Pressure plot
Full impedance -Pressure plot
At 75 Hz
At 135 Hz At 145 Hz
At 125 Hz
Full impedance -Pressure plot
At 190 HzAt 175 Hz
At 165 Hz
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No Absorption and Full Absorption: SPL at DRE and PRE
Full impedance comparison chart
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50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220
Frequency in Hz
Soun
d pr
essu
re le
vel i
n dB
NO impedance DRE NO impedance PRE FULL impedance DRE FULL impedance PRE
70.5091.32Minimum SPL101.66122.89Maximum SPLPRE in dBDRE in dBDescription
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Full Absorption with window open
3D Model
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Full Absorption with window open –Pressure plot
Full impedance –Window open-Pressure plot
At 75 Hz
At 135 Hz At 145 Hz
At 125 Hz
Full impedance –Window open-Pressure plot
At 190 HzAt 175 Hz
At 165 Hz
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No Absorption and Full Absorption Window Open: SPL at DRE and PRE
Full impedance window open comparison chart
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50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220
Frequency in Hz
Soun
d pr
essu
re le
vel i
n dB
NO impedance DRE NO impedance PREFULL impedance DRE FULL impedance PREFULL impedance-window open-Impedance=1 DRE FULL impedance-window open-Impedance=1 PREFULL impedance-window open-Pressure=0 DRE FULL impedance-window open-Pressure=0 PRE
MinimumMaximum
Description
54.8173.4363.0083.3997.40114.3296.38114.14PRE in dBDRE in dBPRE in dBDRE in dB
Pressure =0Absorption Coefficient =1
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Conclusions
• In each case, the SPL was compared with that of no absorption and found to decrease considerably.
• The major individual contributors were found to be seat and window open condition.
• It has been found that the reduction in SPL predominantly occurs at modal frequency.
• Interior acoustics can be improved by having seat with more sound absorption coefficient suitably placed.
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References
• Kopuz et al(1995).Analysis of interior acoustic fields using the finite element method and the boundary element method. Applied acoustics 45 pp 193-210.
• T.Priede (1971) Origins of automotive vehicle noise. Journal of sound and vibration 15, pp 61-73
• Utsuno, et al., Analysis of the sound field in an automobile cabin by using the boundary element method. SAE Paper No. 891153, 1990, pp. 1147-52
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Acknowledgement
• I sincerely acknowledge the valuable guidance given by Prof.Chandramouli, IITM, Chennai in completing this project.
• I acknowledge the support and sponsorship given by my organization TCS for encouraging me to present this paper