Car Cavity Acoustics using ANSYS

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1 Car Cavity Acoustics using ANSYS Muthukrishnan A Assistant Consultant TATA Consultancy Services 185,Lloyds Road, Chennai INDIA

2 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

3 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.

4 Classification of Noise inside the Vehicle Origin of noise Engine vibration Engine airborne noise and its transmission Engine exhaust Engine inlet Fan noise Road excited vibration Noise inside the vehicle Major source of low frequency noise Major source of high frequency noise Not important Not important May be noticeable Major source of low frequency noise Noise based on frequency: Low frequency noise (50-200Hz) High frequency noise( hz)

5 Flow Chart for Finite Element Analysis DIMENSIONAL MEASUREMENT OF ACTUAL CAR FREQUENCY RANGE FE MODEL MODAL ANALYSIS MATERIAL PROPERTIES NATURAL FREQUENCY END HARMONIC ANALYSIS BOUNDARY CONDITIONS EXCITATION FORCES TIME HISTORY POST PROCESSING SOUND PRESSURE LEVEL END

6 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

7 Element Specification ELEMENT NAME Number of nodes Degrees of freedom (D.O.F) Surface loads ELEMENT OUTPUTS Pressure Sound pressure Level MATERIAL PROPERTIES (AIR) Density Velocity of sound in air Fluid 30 (3D volume element) 8 #4 (Ux,Uy,Uz,PRES)-if Fluid structure interaction is present #1 PRES if no structure at the interface Fluid structure interface Impedance Average pressure Sound pressure Level in db 1.2 Kg/m m/s

8 Ambassador Car Cavity - Dimensions Overall dimensions of cavity Length = 2.51m Width = 1.40m Height = 1.10m

Different Finite Element Views Of Car Cavity

Number of elements Number of nodes Element type

9 Different Finite Element Views Of Car Cavity Isometric view of car cavity Interior side view of outer elements Interior rear view of outer elements Isometric view of interior elements Number of elements Number of nodes Element type Mesh type Fluid 30 (Volume element) Hexagonal Mapped

10 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 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, (po/l) + (q o/w) + (ro /H) 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]{ u } + [K] {u} = {0}. For harmonic vibration {u} = {Φ} i COS ω i t and the solution is given by [K] - ω i2 [M]= 0

11 Ambassador Car Cavity-Mode Shape At 74.99Hz At Hz At Hz At Hz

12 Ambassador Car Cavity-Mode Shape At Hz At Hz At Hz

13 Comparison-Modal Frequencies MODES ANSYS RESULTS MODAL FREQUENCY THEORITICAL RESULTS (1,0,0) (0,1,0) (2,0,0) (1,1,0) (0,0,1) (1,0,1) (2,1,0) (0,1,1) NA

14 Harmonic Analysis-Solver Options Analysis options Analysis type-solution Method DOF Print out format Load step options Harmonic frequency range Frequency sub-steps Stepped or Ramped Boundary condition Harmonic Full Real + Imaginary Time/Frequency 50Hz to 200Hz Sub-steps of 5Hz Stepped

15 Pressure Plot-No Absorption At 75 Hz No impedance-pressure plot At 125 Hz At 165 Hz No impedance-pressure plot At 175 Hz At 135 Hz At 145 Hz At 190 Hz NO IMPEDANCE SOUND PRESSURE LEVEL Sound pressure level in db Frequency in Hz NO impedance DRE NO impedance PRE

16 Roof Absorption Material properties Density (DENS) Velocity of sound (SONC) Sound absorption coefficient (MU) Surface load 1100 Kg/m m/s 0.13 (50 Hz -125 Hz) 0.53 (130Hz 200Hz) Impedance value is set to 1 on all the six faces of the element. 3D-Model

17 Roof Absorption Pressure plot Roof impedance only-pressure plot At 75 Hz At 125 Hz At 135 Hz At 145 Hz Roof impedance only-pressure plot At 165 Hz At 175 Hz At 190 Hz

18 No Absorption and Roof Absorption: SPL at DRE and PRE Roof Impedance comparison chart Sound pressure level in db Frequency in Hz NO impedance DRE NO impedance PRE Roof Absorption coeff 0.13/0.53 DRE Roof Absorption coeff 0.13/0.53 PRE Description Maximum SPL Minimum SPL DRE in db PRE in db

19 Floor Absorption Density (DENS) Velocity of sound (SONC) Sound absorption coefficient (MU) Surface load 55 Kg/m m/s 0.2 (50 Hz -125 Hz) 0.55 (130Hz 200Hz) Impedance value is set to 1 on all the six faces of the element.

20 Floor Absorption Pressure plot Floor impedance only-pressure plot At 75 Hz At 125 Hz At 135 Hz At 145 Hz Floor impedance only-pressure plot At 165 Hz At 175 Hz At 190 Hz

21 No Absorption and Floor Carpet Absorption: SPL at DRE and PRE Floor carpet impedance comparison chart Sound pressure level in db Frequency in Hz NO impedance DRE NO impedance PRE Floor impedance 0.2/0.55 DRE Floor impedance 0.2/0.55 PRE Description DRE in db PRE in db Maximum SPL Minimum SPL

22 Seat Absorption Material properties Density (DENS) Velocity of sound (SONC) Sound absorption coefficient (MU) Surface load 80Kg/m m/s 0.1/0.5 (50Hz 200Hz) Impedance value is set to 1 on all the six faces of the seat element. 3D Model

23 Seat Absorption Pressure Plot At 75 Hz Seat impedance only-pressure plot At 125 Hz At 135 Hz At 145 Hz Seat impedance only-pressure plot At 165 Hz At 175 Hz At 190 Hz

24 No Absorption and Seat Absorption: SPL at DRE and PRE Seat impedance comparison chart Sound pressure level in db Frequency in Hz 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 Description 0.1 absorption 0.5 absorption DRE in db PRE in db DRE in db PRE in db Maximum SPL Minimum SPL

25 Full Absorption 3D Model

26 Full Absorption Pressure plot Full impedance -Pressure plot At 75 Hz At 125 Hz At 135 Hz At 145 Hz Full impedance -Pressure plot At 165 Hz At 175 Hz At 190 Hz

27 No Absorption and Full Absorption: SPL at DRE and PRE Full impedance comparison chart Sound pressure level in db Frequency in Hz NO impedance DRE NO impedance PRE FULL impedance DRE FULL impedance PRE Description Maximum SPL Minimum SPL DRE in db PRE in db

28 Full Absorption with window open 3D Model

29 Full Absorption with window open Pressure plot Full impedance Window open-pressure plot At 75 Hz At 125 Hz At 135 Hz At 145 Hz Full impedance Window open-pressure plot At 165 Hz At 175 Hz At 190 Hz

30 No Absorption and Full Absorption Window Open: SPL at DRE and PRE Full impedance window open comparison chart Sound pressure level in db Frequency in Hz NO impedance DRE NO impedance PRE FULL impedance DRE FULL impedance PRE FULL impedance-window open-impedance=1 DRE FULL impedance-window open-impedance=1 PRE FULL impedance-window open-pressure=0 DRE FULL impedance-window open-pressure=0 PRE Description Maximum Minimum Absorption Coefficient =1 DRE in db PRE in db Pressure =0 DRE in db PRE in db

31 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.

32 References Kopuz et al(1995).analysis of interior acoustic fields using the finite element method and the boundary element method. Applied acoustics 45 pp T.Priede (1971) Origins of automotive vehicle noise. Journal of sound and vibration 15, pp Utsuno, et al., Analysis of the sound field in an automobile cabin by using the boundary element method. SAE Paper No , 1990, pp

33 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

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