Technical note Vehicle interior noise source contribution and transfer path analysis.

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Int. J. Vehicle Design, Vol. 52, Nos. 1/2/3/4, 2010

Technical note: Vehicle interior noise source contribution and transfer path analysis F.O. Tando?an*
Temsa R&D and Technology A.S., Tubitak MAM Technology Free Zone, 41470, Gebze-Kocaeli, Turkey E-mail: okan.tandogan@temsa-argetek.com.tr *Corresponding author

A. Güney
Department of Mechanical Engineering, Automotive Division, Istanbul Technical University, Laboratory of Engines and Vehicles, 34390 Maslak-Istanbul, Turkey E-mail: guney@itu.edu.tr
Abstract: In this study, vehicle air-borne and structure-borne noise sources and noise transfer paths are identified. By applying the interior noise source contribution and transfer path analysis method, the contribution of each noise source in the vehicle is calculated by subsystem level measurements. Moreover, the actual vehicle level interior noise has been measured and compared with the calculated levels for correlation. When good correlation is achieved, a critical peak level, which may be a potential NVH problem, has been investigated with the measured and calculated data. Keywords: vehicle NVH; acoustics; noise; vibration; transfer path analysis; noise source contribution; FRF. Reference to this paper should be made as follows: Tando?an, F.O. and Güney, A. (2010) ‘Technical note: Vehicle interior noise source contribution and transfer path analysis’, Int. J. Vehicle Design, Vol. 52, Nos. 1/2/3/4, pp.252–267. Biographical notes: F. Okan Tandogan is a Mechanical Engineer and received his MSc in Automotive Engineering from the Istanbul Technical University. He worked for Ford Motor Company in Turkey in 2002–2006 as a Vehicle NVH Development Engineer. Since December 2007, he has been working for TEMSA R&D and Technology Corp. as a Vehicle NVH Development Engineer. A. Güney is a Professor at the Department of Automotive Engineering of Istanbul Technical University, Mechanical Engineering Faculty. He received his PhD from the same university in 1986. His research interest is vehicle NVH. He has been the President of the Turkish Acoustical Society since 2003.

Copyright ? 2010 Inderscience Enterprises Ltd.

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1

Introduction

Competition in the automotive market forces manufacturers not only to release their products in low costs, but due to the increasing quality and comfort demand of the customers; they also need to meet certain criteria. Because there are many competitors in the market and global manufacturing centres have been established, where production costs can relatively be minimised, products can be released with lower costs in the market. However, a product can hold its place in the market only if it meets certain quality parameters. Providing good-quality in the manufacturing lines is the final goal to be reached before the product goes to the customer; however, that is not sufficient unless the product is well-developed in vehicle attributes such as performance feel, vehicle dynamics, craftsmanship, NVH, etc. To be able to meet certain development targets, manufacturers establish well-defined strategies and development processes as well as they spend high amount of money to build up development centres. All those mentioned rely on the fact that the winner of the competition in the market will the one that succeeded in presenting satisfactory units involving customer demands. Automotive industry is one of the most important areas, where advanced engineering techniques have to be carried on together with effective marketing strategies. When the fact considered that the manufacturers are working with limited development budgets and timing, it becomes obvious that they are under strictly restricted circumstances. Although the initial impressions, when a customer goes to a car gallery or to an exhibition, are static parameters such as the external and internal design, craftsmanship and material quality, dynamic characteristics and comfort of the vehicle will become more dominant for the customer after a test drive. Therefore, to remain competitive in the market, it is essential for car manufacturers to optimise their design with regards to safety and comfort attributes such as crash safety, durability, vehicle dynamics, NVH, ergonomics, performance feel, fuel economy and etc. while keeping the product prices reasonable. It is already discussed that there is no unlimited time and budget for vehicle development. Therefore, it is of great importance for OEMs to use systematic and result-oriented product development methodologies. Vehicle NVH development is also one of the most important comfort parameters for vehicles and special care has to be taken to establish an effective and fast development methodology within the total restricted timing plan. To be able to reduce the investment costs, NVH development should be considered even in the very early phases of the vehicle program. It is of great advantage to start the initial investigations in virtual environments and to give as many inputs to the program design teams as possible before the prototyping phase. With the usage of CAE tools, most of the potential structural NVH problems will be addressed and even developed up to a certain level before the first vehicle prototype. When the first prototype is built, CAE tools can still be used to carry out further design iterations; however, as a real unit is available, it will be much faster and responsive to use test methodologies. One of the most important inputs to the program is to be aware of how much development will be done, which targets are required in vehicle level, how they can be cascaded to system, subsystem and even to component levels and even more importantly how to develop a certain structural problem or reduce noise in a specific operating condition. The answers to those questions ‘How much of the noise is coming from the exhaust orifice?’, ‘Which transfer path is more dominant in that specific noise problem?’ are very valuable and involve the main goal to be achieved during vehicle NVH development. In this study, ‘vehicle interior noise source contribution and transfer

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path analysis’ method is discussed, which can be used as a very effective test-based NVH development tool (Harrison, 2004). Many car manufacturers’ product development departments and independent engineering companies use noise contribution and transfer path analysis method in vehicle NVH development studies (Plunt, 2005). There are several aspects in this methodology (Van der Auweraer et al., 2007). Some approaches are more practical and fast to apply and still yield accurate results, however, do not take into account the coupling effects, which can sometimes mislead the development engineers in certain aspects. Other approaches yield more accurate results; however, are not easy to apply and require more time especially in vehicle instrumentation (Sottek et al., 2006). The main difference between these techniques is the intention to eliminate the coupling effects during measurements (Knapen, 1999). As an example, the engine and transmission mounting system usually has three or four attachment points to the body. If the engine is not taken out of the vehicle, forces acting through one point on the body will also have a contribution to the other attachment points and will create so called ‘coupling effect’. This effect will be produced in all similar systems in the vehicle. In this study, the sources such as engine and transmission are not taken out the vehicle; however, still accurate results are obtained by measuring the operational forces and the point inertance of each single point that are considered as an input source. There are still ongoing studies in the literature to be able to establish the most efficient transfer path analysis methodology, which can both be applied in a user-friendly way with less time requirements and also yield high-accuracy results (Kim and Lee, 2009). Further information can be searched through released technical papers. More detailed discussions for all steps of this study can be viewed in Tandogan (2006).

2

Human hearing and psychoacoustics

As it is clearly known, the field acoustics has a variety of areas of application. In general, sound waves having a wavelength less than 20 Hz are classified as infrasound; 20 Hz to 20 kHz are audible sound and greater than 20 kHz are classified as ultrasound (M?ser, 2004). In vehicle noise development studies, it is important to know how humans perceive sounds, which frequencies are critical for human ears and which are less important to work on. Because of the fact that human ear does not equally act to the sound waves in different frequencies, the basics of NVH development are based on human hearing and perception of sound.

2.1 Acoustic sensitivity of human ears
For engineers who are dealing with NVH development, it is important to be aware of how ears perceive sound. Therefore, before getting into details of the methodology, some basics about human hearing will be discussed in brief. Sound waves approaching the ear are picked up by the outer ear and directed to the middle ear through the auditory canal. Because of the structure of the auditory canal, all frequencies are not treated the same way before they reach the hearing centre. Due to the fact that the auditory canal resonance is around 4 kHz, human ears are more sensitive to the frequencies around 4 kHz (Zwicker and Fastl, 1999).

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Depending on the frequency and sound pressure information involved in the sound wave, the response of the ear changes as shown in Figure 1 (M?ser, 2004). As it is seen, the sensitivity of the ear is not the same in low and high frequencies compared to middle frequencies. This introduces the usage of acoustic weighting filters in NVH signal processing. Main usage reason of these filters is to investigate different designs’ effect on overall vehicle NVH performance in a more effective way, closer to the human hearing. A weighting filter is widely used in vehicle acoustic studies. For further details, books on acoustic fundamentals can be viewed.
Figure 1 Sensitivity of human ear

3

Fundamentals of vehicle NVH

During its operation, a vehicle is under the effect of various environmental and operational excitations. Therefore, in vehicle NVH development, engineers need to take into account of all the critical excitation sources as well as the critical transfer paths as systems and subsystems. Sound wave propagation from the noise source to our ears occurs in two different ways such as structure-borne and air-borne. In vehicle acoustic and vibration development studies, it is essential to determine the contribution of both structure and air-borne noise to be able to have the knowledge of which noise source is a strong contributor, or which region of the body/chassis is weaker, or if is it worth to improve sound radiation from the vehicle panels, or addressing the absorption of the noise in the interior.

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3.1 Structure-borne and air-borne noise in vehicles
An important portion of noise in the vehicle interior comes from the excitation forces transmitted from the vibrating sources to the vehicle body through the attachment points, which is classified as structure-borne noise. Main structure-borne noise sources in vehicles are powertrain and transmission system, exhaust and intake systems. Air-borne noise radiation from the sources is transferred in the air through the panels and openness of the vehicle body to the vehicle interior and finally to ears. Engine, exhaust and intake orifice can be classified as the most dominant air-borne noise sources in the vehicle powertrain.

4

Vehicle noise sources and transfer paths

Vehicle overall interior noise is the sum of the contributions from all the noise sources/transfer paths. Engine is one of the strongest noise sources. Very important engine noise contribution comes from the structure-borne excitations, dominantly through the engine and transmission mounts. Vibrations are isolated up to a certain level by the rubber or the hydraulic engine mounts; however, the remaining portion of the force is transferred to the body and turned into sound pressure in the vehicle interior by the excitations of the vehicle panels and driveline components. Air-borne engine noise is generated by the combustion and inertia forces and mechanisms on the engine block and transferred through the air to the vehicle interior. Another excitation source is the exhaust system, which is attached to the vehicle body in several points. Forces generated by the exhaust gas outlet and coming from the engine are transferred to the body from the exhaust attachment points. Moreover, exhaust orifice is also a strong air-borne noise source due to the outlet out the burnt exhaust gases with high speeds. As the last noise source to be discussed in this study, intake orifice can also be classified as a strong noise source. Gas inlet into the intake box generates usually a low frequency air-borne noise. The noise sources and transfer paths of the vehicle used in this study are given in Table 1. In the system level investigations, point inertance, accelerations at these points and the noise transfer functions (NTFs) from these points to the vehicle interior are measured. By these measurements, frequency-based forces transmitted from the noise source to the body from the attachment points and the noise response of the vehicle to a unit input from the attachment points are determined. Since the actual force levels on the attachment points in the operating condition and the noise response of the vehicle body to unit force from these points will be known, the actual noise level from a single transfer path can be calculated. Similarly, for the air-borne noise contribution, the radiated noise of the source is measured. Additionally, the noise reduction of the vehicle body at that local area is measured based on frequency, yielding the input noise and the body transparency. The difference of the noise reduction from the radiated noise will give the air-borne noise contribution. These steps are followed for each noise source and transfer path and summed up acoustically to determine the overall vehicle interior noise level.

Vehicle interior noise source contribution and transfer path analysis
Table 1 Interior noise contribution and transfer path of each source

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Vehicle overall interior noise Structure-borne contribution Engine Power plant mount forces (in x, y, z directions) NTFs from power plant mounts (in x, y, z directions) Suspension attachments (as transfer paths) Exhaust system suspension attachment point forces (in x, y, z directions) Suspension attachment point NTFs (in x, y, z directions) Exhaust attachment point forces (only in z direction) Exhaust attachment point NTFs (only in z direction) Air-borne contribution Engine Engine radiated noise Engine noise reduction Intake system Intake orifice radiated noise Intake orifice noise reduction Exhaust system Exhaust orifice radiated noise Exhaust orifice noise reduction

5

Measurement and calculation methodology

The approach in this study can be discussed in two steps. The structure-borne contribution is calculated through measurement-driven results for each source and transfer path. The main idea in determining a single contribution is to obtain a force spectrum and a transfer function from that point to the response location. To determine the operational force acting on a point, the accelerations are measured during the specified operating condition (complete RPM sweep in full load). Furthermore, the point inertance and the NTF from each point to the response location are determined by impact measurements. The multiplication of the acceleration spectrum with the inverse point inertance or ‘accelerance’ (Ewins, 2000) yields the actual force acting on the body. When this force is multiplied with the NTF, which is the response of the body to a unit input, the actual sound pressure level in the response location from a single excitation point is gathered. The summation of the contributions of all excitation points yields the total structure-borne sound pressure level. The air-borne noise contribution is determined with a similar approach but with different calculations. At this point, the required parameters can be named as the noise radiation from each single source, and the noise reduction spectrum of the vehicle at the local region of the vehicle where the noise source is located. The subtraction of the local noise reduction spectrum of the vehicle from the actual noise radiated from the source (i.e., exhaust orifice) gives out the air-borne contribution of a single exciter. To be able to conduct these calculations, a high frequency sound source is used for exhaust and intake orifices to determine the local noise reduction of the vehicle. The actual noises radiated

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from the sources are measured by applying the killing technique to eliminate the effect of other noise sources during vehicle operation.

6

Results

The correlation between the calculated and the measured interior noise levels is illustrated in Figure 2. Due to the fact that good correlation between calculated and measured is observed, it is concluded that vehicle interior noise source contribution and transfer path analysis method can be used as an efficient NVH development tool in vehicle NVH development.
Figure 2 Measured and calculated noise levels (see online version for colours)

Steps of this method are discussed by investigating one of the critical peaks that can be a potential vehicle NVH issue. In Figure 3, the change of the interior sound pressure level versus engine speed is given. In this data, it is not possible to figure out the situation in frequency domain, therefore, another parameter should be introduced and data should be represented in a waterfall diagram. In Figure 4, the data is analysed such that the change of sound pressure level is given as a map where x axis is engine speed and y axis is frequency. As another display, the contributions of the most critical engine harmonics are given in a 2D diagram in Figure 5 by order analysis. In Figures 4 and 5, it is observed that the most critical excitation, in the RPM range where problem exists, comes from the fourth engine order. The problem is now cascaded one level below and the following step will be to figure out whether the issue comes from a structure-borne or air-borne excitation. To be able to do this, the overall structure-borne and air-borne contributions of sources and transfer paths will be compared against the overall level. In Figure 6, overall, structure and air-borne sound pressure levels are given. As it can be observed in Figure 6, it will be correct to judge that the problem is nearly completely dominated by structure-borne excitation. Therefore, the problem is again one level cascaded and in the following steps, only the fourth order structure-borne

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contributions of the relevant sources and transfer paths will be investigated. This will enable elimination of the time consumed for less important sources and focusing on the dominant ones.
Figure 3 Potential noise problem (see online version for colours)

Sound Pressure Level [dB(A)]

Potential Noise Problem
Measured Calculated

Engine Speed [RPM]

Figure 4

Waterfall diagram (see online version for colours)

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Figure 5

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Contributions of orders on overall level (see online version for colours)

Figure 6

Contributions of overall structure-borne and air-borne on sound OA level (see online version for colours)

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In the next step, the aim is to determine from which source and/or transfer path, the fourth order structure-borne excitation dominantly comes from. To be able to do this, total fourth order contributions of the power plant mounts, suspension attachment points and exhaust attachment points will be compared to the overall fourth EO structure-borne level. From Figure 7, it is observed that the contribution of the total exhaust attachment points is not as severe as the contributions of the power plant mounts and the suspension attachment points. Therefore, in the following steps, it will be appropriate to investigate the subsystems of only the dominant sources and transfer paths.
Figure 7 Contributions of each sources’ total fourth EO structure-borne excitation (see online version for colours)

At this point, the investigations will be aiming to determine which power plant mounts and suspension attachments are the dominant sources. In Figures 8 and 9, the subsystems are compared individually with the total overall levels. From Figure 7, it is seen that the first and the second engine mounts have more dominant contribution on the overall power plant mount fourth EO structure-borne level. Therefore, the third engine mount will be neglected in further investigations.

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Figure 8

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Contributions of each power plant mount (see online version for colours)

Figure 9

Contributions of each suspension attachment point (see online version for colours)

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In addition to the subsystem analysis of the power plant mounts, the individual contributions of the suspension attachment point will also be investigated due to the fact that the total suspension attachment also has strong contribution on the problem. It is observed for the suspension attachment points that the most dominant contribution comes from the third and the fourth point. It is important to explain that the suspension system is not considered as a path for road excitations but it is investigated as part of the engine attachment system on the vehicle chassis. From the investigations made so far, it can be concluded that to be able to determine the root cause of the specific NVH problem, it will be appropriate to deal with the first and second engine mounts and also with the third and fourth suspension attachment points. At that point, the noise problem is cascaded from the vehicle level to the subsystem level. In the light of the information gathered so far, only the most critical sources and transfer paths will be investigated in details. From the comparison of the most critical sources with the overall fourth EO structure-borne level, it is concluded that the third suspension attachment point is the dominant transfer path that should be investigated in more details. At that point, since the problem is cascaded to a specific point, the next step will be to determine the reason why that point has a strong contribution on the problem. Sound pressure from a structure-borne source is the multiplication of the input force and the NTF in all directions. In Figure 10, it is aimed to figure out in which direction the overall sound pressure level is more dominant for that particular point. Therefore, the contributions of x, y and z directions have been compared with the overall level of that point.
Figure 10 Contributions of the most dominant sources and transfer paths (see online version for colours)

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Figure 11 Contributions of all directions on third suspension point total fourth EO structure-borne level (see online version for colours)

It is concluded that the contributions from x and y directions are more dominant compared to the z direction. In Figure 12, NTFs from all directions are given.
Figure 12 NTFs from all directions of third suspension attachment point (see online version for colours)

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As a final investigation, the overall and the fourth EO force levels of all directions will be investigated.
Figure 13 Third suspension attachment point overall and fourth EO force levels versus engine speed (see online version for colours)

From the force level and the NTF curves, it is seen that the noise problem is mostly dominated by the body weakness in x and y directions as observed in the NTFs and also the high force levels in that specific RPM band. Since the root cause of the noise problem is cascaded to a very specific point, the following steps as a corrective action would be to improve the design in the mentioned issues. After the design changes, it is proper to conduct verification measurements and confirm the effect of the change on total vehicle NVH performance. If the improvement achieved is not found sufficient, other dominant sources and transfer paths should also be improved in the same manner.

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7

Conclusions

In this study, a potential interior noise problem is investigated by the interior noise source contribution and transfer path analysis. This methodology enables cascading a certain NVH issue from vehicle level to system, subsystem and even component level and provides a time saving and cost-effective approach in root cause identification. As a next step in addition to this study, further investigations should be carried out to pinpoint and improve the main source of this NVH problem. Panel contribution analysis (PCA) should be one of the tools that can be used to determine the percentage of sound radiation from the panels of the vehicle. Focusing on this specific noise problem, improvement iterations on the dominant contributing panels will be an effective approach in the reduction in sound radiation. As a second step, the root cause of the high frequency response of the body should be investigated. Point mobility of the suspension connection point, vibration transfer function (VTF) and NTF improvements by using CAE tools will enable reduction of the responses. It is concluded that interior noise source contribution and transfer path analysis methodology is an effective tool in vehicle NVH development and can be used in target setting, target cascading and specific problem root cause identification.

Acknowledgements
The authors would like to thank ‘Ford Otomotiv Sanayi A.S.’ for the test systems and software support.

References
Ewins, D.J. (2000) Modal Testing, SRP, England. Harrison, M. (2004) Vehicle Refinement Controlling Noise and Vibration in Road Vehicles, Elsevier Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 30 Corporate Drive, Burlington, MA 01803, pp.151–159 Knapen, P.L. (1999) Transfer Path Analysis Related to Booming, Performed on a Car, WFW Report No. 99-018, NedCar Report No. 52233/99-0125, Netherlands Car B.V. Vehicle Dynamics Department Steenovenweg 1, 5708 HN Helmond, The Netherlands. Kim, S.J. and Lee, S.K. (2009 – in press) ‘Prediction of structure-borne noise caused by the powertrain on the basis of the hybrid transfer path’, Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, Professional Engineering Publishing. M?ser, M. (2004) Engineering Acoustics, Springer, Germany Plunt, J. (2005) Finding and Fixing Vehicle NVH Problems with Transfer Path Analysis, Acoustical Publications, Inc., November, ProQuest Information and Learning Company. Sottek, R., Genuit, K., Behler, G. and Vorl?nder, M. (2006) Description of Broadband Structure-Borne and Airborne Noise Transmission from the Powertrain, Fisita 2006, Yokohama/Japan. Tandogan, F.O. (2006) ‘Vehicle interior noise source contribution and transfer path analysis’, MSc thesis, Istanbul Technical University, Turkey.

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Van der Auweraer, H., Mas, P., Dom, S., Vecchio, A., Janssens, K. and Van de Ponseele, P. (2007) ‘Transfer path analysis in the critical path of vehicle refinement: the role of fast, hybrid and operational path analysis’, SAE 2007 Noise and Vibration Conference and Exhibition, May, St. Charles, IL, USA, Session: vehicle subsystem NVH: body structure/chassis (Part 1 of 3). Zwicker, E. and Fastl, H. (1999) Psychoacoustics, Springer, Germany.


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