Interior acoustic response analysis of armored vehicle

Guo-hong DENG，Wei LIU*，Jian OU，E-chuan YANG，Yong ZHANG

(1College of Vehicle Engineering，Chongqing University of Technology，Chongqing 400054，China) (2College of Mechanical Engineering，Chongqing University of Technology，Chongqing 400054，China)

Interior acoustic response analysis of armored vehicle

Guo-hong DENG1，Wei LIU1*，Jian OU1，E-chuan YANG2，Yong ZHANG1

(1College of Vehicle Engineering，Chongqing University of Technology，Chongqing 400054，China) (2College of Mechanical Engineering，Chongqing University of Technology，Chongqing 400054，China)

It has been a research top to study the vibration and noise control of armored vehicle in theory and engineer field since the internal noise directly affects the fighter’s operational level.So carrying on this research is a necessary task.First the armored vehicle inner cavity model and the acoustic-structure coupling model that considering the interaction between the structure and the air were built and analyzed.It was concluded that the body structure has some certain influence on the acoustic characteristics of the cavity by comparing the acoustic modal graphs.Then the pressure distribution at the driver’s right ear was gotten by analyzing the vehicle’s acoustic response of acoustic-structure coupling model using boundary element method.Finally the panel acoustic contribution of frequency was analyzed at peak pressure，which provides a reference for improving the acoustic characteristics of the vehicle.

Armored vehicle，Acoustic-structure coupling，Boundary element method，Acoustic contribution

1 Introduction

With the development of modern military equipment，new weapon technology is constantly upgrading.The higher requirements for armored vehicle interior noise are put forward.Noise level inside a car is not only an important index to evaluate the vehicle ride comfort，but also reflects an important indicator of the design and manufacture levels of vehicles.Especially for the armored vehicles，it will directly affect the fighter’s operational efficiency and ability in a war，so it’s of great importance to study the interior noise of armored vehicles.

Currently the armored vehicle interior noise pollution is still severe in China’s own production.The noise is mainly from the body panel vibration causedby the acoustic radiation which belongs to the low frequency noise between level 110 dB and 120 dB.According to the GJB50(allowed noise value of military operation)，the allowed noise level is 90 dB for daily continuous exposure of 8 hours.Noise inside the armored car exceeding the standard seriously will lead to a great damage to the armors［1］.Noise frequency between 20 Hz and 200 Hz is a special band notable inside the car，because 20 Hz is the lowest frequency to hear，and the noise caused by the vehicle body structure vibration is mainly in the 200 Hz.In this frequency range，the low frequency noise is mainly from engine and road roughness excitations，and the amplitude is high，so the general sound absorption and noise reduction has no effect［2］.To solve this kind of problem，it needs to take full consideration of the coupling effect between the structure and the air.The most direct way is to suppress the plate’s vibration that caused the noise.A lot of research has been ongoing in this aspect at home and abroad，and the results have been applied widely in the field of automobile engineering［3-4］.With the development of the modern acoustic technique，there has emerged some numerical analysis method to solve the interior noise of vehicle，like the finite element(FEM)and boundary element(BEM).The finite element analysis is to study low solid borne sound in car，and it provides a good solution for analysis of vehicle interior low frequency noise with fully considering the coupling effects between the air inside the car and the body plates in the calculation process.For the problem of internal noise of complex boundary，the boundary element can be used for solution considering the surface absorption of impedance［5］.

2 Establish the finite element model

2.1 Establishment of the cavity acoustic model

There is a relationship between mesh element size and the calculating frequency in the analysis using acoustic finite element or boundary element method.Generally there must be six elements for the minimum wavelength.The highest frequency is 400 Hz in this paper while the sound velocity in the air is c= 340 m/s，so the created acoustic unit size is:L≤c/ (6fmax)=141.67 mm.Considering the calculation time and accuracy，this paper used the finite element preprocessing analysis software HyperMesh to mesh and the target size is 100 mm.The small mesh size cannot improve the calculation precision for acoustic grid，because the acoustic calculation accuracy is controlled by the majority units，which is different from the analysis of structured grid［6］.So the model ignores the bolt hole and other shaping hole in the premise that it does not affect the car cavity shape and is also simplified for small chamfers to make the majority unit size close to the target size.Finally the automatic division of tetrahedron mesh was obtained through the outer surface of the closed.Considering the influence of seats on the cavity acoustic characteristics，seats were added to the model.The acoustic model is shown in Fig.1.The model contains 11434 nodes and 51877 gird.

Fig.1 Acoustic cavity’s FEMmodel considering the seats

2.2 Establishment of acoustic-structure coupling model

Typically，the body panels’vibration at the outside excitation will oppress the air around the body panels thus changing the interior sound pressure，which in turn amplifies or suppresses the vibration somewhat［7］.The interaction between the structure and the air forms the structure-acoustic coupling system.

After the body-cavity grid model was built in HyperMesh，Nastran module was select to export a BDF file which was opened with Notepad，and then“ACMODL IDENT”was added below the keyword"BEGIN BULK".This can combine the external nodes of the cavity model with the correspond nodes in the body，ensuring they move together during the analysis and finally complete the structural-acoustic model.We can only get the view of structure modal shape above the coupling surface from the model calculated by LMS.Virtual.Lab Acoustics，and it cannot display the acoustic cavity modal shape after coupling.Therefore，this paper uses the complex eigenvalue method based on modal superposition in Nastran.The coupled modal is formed by the interaction between the structure and the cavity air，so the modal shape results are part of the structure deformation and part of the sound pressure change，respectively corresponding to the structure and cavity modal［8］.The coupled model is shown in Fig.2 after hiding doors and glass.

Fig.2 FEM model of the acoustic-structural system

3 Acoustic modal analysis

3.1 Modal analysis only containing acoustic element

When analyzing the cavity’s acoustic modal it is assumed that the cavity wall plates are rigid to get the cavity’s inherent acoustic modal.Then the acoustic gird created by HyperMesh was imported in BDF format to MAC.Nastran，and the analytic type was set to carry on modal analysis.Finally the modal vibration shape characterized by sound pressure distribution was gotten.The first seven order acoustic natural frequencies are shown in Table 1.The first and second orders modal vibration shapes are shown in Fig.3.

Table 1 Acoustic modal frequency and vibration shape

Fig.3 Acoustic cavity’s inherent modal shape

In order to verify the accuracy of the acoustic modal simulation，it can be estimated by the empirical formula［9］.Generally the first-order modal frequency of acoustic system in the longitudinal，lateral and vertical can be estimated by the half speed of sound divided by the corresponding acoustic system length，width，and height.In this paper the armored vehicles’length，width and height are 4.83 m，2.23 m and 1.4 m respectively.The velocity of sound is c= 340 m/s.So the estimated first-order modal frequencies in the longitudinal，lateral and vertical are 35.19 Hz，76.23 Hz and 121.43 Hz.They are consistent with the simulation result，which verifies the accuracy of the simulation analysis.

3.2 Analysis of the acoustic cavity’s modal after coupling

The complex mode solver SOL110 in Nastran was used to solve the coupled mode，and then the acoustic vibration shape was gotten after coupling.The first two orders are mainly changed in sound pressure，as shown in Fig.4.

Fig.4 Acoustic modal shape in coupled system

Through the comparison of the modal shape diagrams，the coupled modal shape changes mainly in sound pressure，the corresponding cavity’s inherent modal has changed，and the modal frequency has also slightly changed.This suggests that the body structure has certain influences on the acoustic characteristics of the cavity.So the analysis above has the role of guiding predicting and avoiding acoustic resonance.

4 Analysis of the acoustic response in the vehicle

In order to get the car’s certain point(the driver’s right ear)acoustic pressure，the vibration response analysis of the model was taken firstly.Vertical excitation force was applied on three mounting point of the engine，and the amplitude is 1 N，the frequency range was of 20-200 Hz，stepping 2 Hz.The vibration response calculation was used to compute the interior acoustic pressure as a boundary condition.

Considering the large scale of the model and the impedance acoustic problems，the acoustic boundary element method was used to solve it to get a higher computational efficiency.In order to ensure that the vibration response data fully transfer to the acoustic boundary element model，the structure shell element around the cavity was taken as acoustic boundary element mesh，and the size was coarsened properly to meet the requirements of the minimum wavelength containing 6 elements.The boundary element model is shown in Fig.5.

Fig.5 BEM model of interior cavity

The seats and the ceiling have many porous sound absorption materials，and the impact of which on the vehicle interior sound field is not negligible.In order to consider their role in absorption，seats and ceiling need to be added to sound absorption properties.The acoustic response condition in LMS.Virtual.Lab was established，and the vibration response was transferred to the acoustic boundary elements as boundary conditions.Next the driver’s right ear was set as the field，and the body panel was divided.The resulted sound pressure-frequency curve is shown in Fig.6. From the figure above，the peak pressure at the driver’s right ear is found in the frequency of 64 Hz and 180 Hz.For these frequencies，some of them are caused by the resonance of the coupled acoustic cavity’s natural frequency and excitation frequency coincident.

Fig.6 Driver’s right ear sound pressure-frequency curve

5 Panels’acoustic contribution analysis

To get a better understanding on which structure’s vibration causes the peak pressure，there is a need to carry out a contribution analysis of acoustic cavity around the panels.An obvious peak response emerges at the frequency of 64 Hz，so it is necessary to get the contribution at this frequency.The panels’acoustic contributions are shown in Fig.7.

Fig.7 Acoustic contributions of the panels at 64 Hz

Panels located in the axis above give a positive contribution and they play the roles of amplification effect on sound pressure，rather than suppressing the sound pressure.When a panel’s acoustic contribution is relatively small，whether positive or negative，it can be taken as a neutral area.Because it has little effect on the pressure value，it is generally not modified as acoustic design object.It can be seen from Fig.7 that the hatchback door and the side panels belong to the positive contribution regions at this peak frequency and shall be treated as the main noise sources.It can be achieved to reduce noise through reducing the structural vibration.The front windshield glass belongs to the negative contribution region，it suppresses the sound pressure，and properly amplifying the vibration can also suppress the noise.Other panels’contributions are relatively small，and they can be regarded as neutral zone.The total contributions of each panel are positive.

6 Conclusions

Firstly，the acoustic model of the cavity and the acoustic finite element model considering the coupling between car’s body structure and the air were built.Then the modal analysis was taken and the acoustic modal and frequency before and after coupling were compared.These works get the result that body structure has certain influence over the acoustic modal of the cavity.Next the calculated value of the A weighted sound pressure level at the driver’s right ear was gotten through putting vertical excitation on three engine mounting points.Finally the plates’sound pressure contribution was analyzed at a higher frequency of peak pressure，and the large contributions of the back door and the side wall were found，which provide the reference for the acoustic design in the future.

Acknowledgements

This paper is supported by Fund Project of The NaturalScienceFundofChongqing(CSTS，2008BB6338)，and 2013 Chongqing University InnovationTeamBuildingProgramfundedprojects (KJTD201319).

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裝甲車車內(nèi)聲學響應分析

鄧國紅1，劉偉1*，歐健1，楊鄂川2，張勇1

1.重慶理工大學車輛工程學院，重慶 400054
2.重慶理工大學機械工程學院，重慶 400054

裝甲車的振動噪聲控制一直是理論和工程領域不斷研究的課題，其車內(nèi)噪聲直接影響作戰(zhàn)人員的作戰(zhàn)水平，進行裝甲車內(nèi)部噪聲的研究十分必要。首先分別建立并分析了裝甲車車內(nèi)空腔模型以及考慮結構和空氣之間相互作用的聲固耦合模型，通過對比耦合前后的聲學模態(tài)振型圖，得出車體結構對空腔聲學特性有一定的影響;然后使用邊界元法對整車聲固耦合模型進行聲學響應分析，得到駕駛員右耳處的聲壓分布情況;最后對聲壓峰值的頻率點進行板件聲學貢獻量分析，為改善車內(nèi)的聲學特性提供了參考依據(jù)。

裝甲車;聲固耦合;邊界元法;聲學貢獻

10.3969/j.issn.1001－3881.2015.06.013 Document code:A

U461.4

Hydromechatronics Engineering

http://jdy.qks.cqut.edu.cn

E-mail:jdygcyw@126.com

7 September 2014;revised 21 October 2014;

11 December 2014

Guo-hong DENG，Professor.E-mail:dengguohong@cqut.edu.cn *Corresponding author:Wei LIU，Graduate student for master de

gree.E-mail:603779113@qq.com