A study on the noise optimization through the analysis of electric ve- hicle noise paths Yoonsang Yang 1 Daehan Solution Co., LTD. 67, Namdongdae-ro 369 Beon-Gil, Namdong-Gu, Incheon, South Korea Seung Lee 2 Daehan Solution Co., LTD. 67, Namdongdae-ro 369 Beon-Gil, Namdong-Gu, Incheon, South Korea

ABSTRACT Due to the absence of an internal combustion engine, the electric vehicle's noise was much lower than that of the existing internal combustion engine. However, road noise and wind noise masked by internal combustion engines emerged, and in particular, electric vehicle-specific current conver- sion noise and motor high frequency noise were generated, providing an unpleasant sound envi- ronment for drivers and passengers. Therefore, in this study, the noise path of the electric vehicle was confirmed through Power Based Noise Reduction (PBNR) and Acoustic Transfer Function (ATF) tests. Based on the results of the analysis of the noise path, it was confirmed that indoor noise was improved when reinforcing the weak path.

1. INTRODUCTION

In line with paradigm changes such as carbon neutrality, global car makers are rapidly shifting to eco-friendly cars such as Hybrid Cars (HEVs), Electric Vehicles (EVs), and Hydrogen Fuel Cell Vehicles (FCEVs) as they competitively declare the end to Internal Combustion Vehicles (ICVs). The trend of change has created a butterfly effect, affecting a number of industries beyond the au- tomobile industry; There have been significant changes in materials and components related to noise vibration (NVH) in vehicles. In terms of noise sources, engine explosions and absorption and exhaust noise, which were the main sources of noise for internal combustion engines, have been largely eliminated in electric vehicles, while road noise and wind noise masked by engines have been added. For accurate noise countermeasures for electric vehicles, a clear analysis of the noise source and the noise transfer path of the vehicle should be performed first. In this study, acoustic force and q(m^3/s^2) were used to measure the acoustic transfer function between several sources of electric vehicles and response points. [1-3].

1 ysyang@dhsc.co.kr

2 slee@dhsc.co.kr

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2. EXPERIMENTAL THEORY

2.1. Acoustic Transfer Function

The Acoustic Transfer Function (ATF) is widely used to determine the relationship between the noise source and the transfer path necessary to improve the interior noise of the vehicle. The acous- tic transfer function of air-borne noise can be calculated by measuring the sound pressure (p) for each acoustic force Q at the location of the motor room, tire, door, etc., which is the main path of indoor noise, and calculating as follows.

2 𝑁 𝑖=1 (1)

ATF r, motor = 1

𝑃 𝑖 𝑟 𝑚𝑜𝑡𝑜𝑟

𝑁

𝑄 ′ 𝑖 𝑟

2.2. Power Based Noise Reduction

The Power Based Noise Reduction (PBNR) value is defined as the ratio of the acoustic power to the mean squared sound pressure and is expressed as a function of the one-third octave frequency band. The acoustic power of the point sound source is expressed as a function of volume accelera- tion in the free-field state.[3]

𝜌𝑄 𝑎 ×𝑄 𝑎 ∗

4𝜋𝑐 (2)

=

∗ : auto power, ρ : density, c : sound speed) Meanwhile, the PBNR value is expressed by the following equation.

( 𝑄 𝑎 : volume acceleration, 𝑄 𝑎 × 𝑄 𝑎

Π 𝑟𝑒𝑓 𝑝 𝑟𝑒𝑓 2 (3)

Π 𝑝∙𝑝 ∗ −10 log

PBNR = 10 log

Π 𝑟𝑒𝑓 𝑝 𝑟𝑒𝑓 2 = 1/400 , p ∙𝑝 ∗ : measured pressure auto power)

(

Substituting Equation (2) into Equation (3), the PBNR value can be obtained as shown in Equation (4).

∗

𝑄 𝑎

𝑄 𝑎

100𝜌

𝑝 𝑄 𝑎 −9.5 (4) [dB]

PBNR = 10 log

𝑝 ×

𝑝

+ 10 log

𝜋𝑐 = 20 log

( p/𝑄 𝑎 is acoustic transfer function) 3. Experimental Setup

The Acoustic Transfer Function (ATF) was measured in a semi-anechoic chamber with a back- ground noise of 20 dB(A) or less. Through this, the area where is vulnerable to noise transfer to the interior of the vehicle was identified. Therefore, the possibility for improving the interior noise of the vehicle was identified. The Q source, which is a sound source, was installed at the ear position of the driver's seat and the second row VIP seat, and the microphone was installed in the motor room, tire, and floor to measure the acoustic transfer function.

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Fig. 1 Location of sound source and microphone 4. RESULTS AND DISCUSSION

Figure 2 shows the result of calculating PBNR in the motor room, tire, and floor. The location of (a) is the driver's seat, and the location of (b) is the VIP seat respectively. As a result, it was confirmed that the front motor room is more vulnerable to noise than other areas due to the wire penetration of the panel, steering system, etc.

80

80

70

70

Poewr Based Noise Reduction[dB]

Poewr Based Noise Reduction[dB]

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60

60

50

50

40

40

30

30

20

20

FRT Motor room RR Motor room TIRE Floor

FRT Motor room RR Motor room TIRE Floor

10

10

1000 2000 3000 4000 5000 6000 7000 8000 0

1000 2000 3000 4000 5000 6000 7000 8000 0

Frequency[Hz]

Frequency[Hz]

(a) the driver seat (b) the rear seat Fig. 2 Result of PBNR measurement by receiving point Fig. 3 is regarding the noise countermeasures transmitted through the front motor room. The result is the measured PBNR when the bulkhead is implemented in the front motor room and the exposed area of the panel to which the dash inner insulator is applied. The location of the micro- phone is the driver's seat, and the location of the sound source is the front motor room. As a result of the measurement, it was confirmed that when the partition wall was implemented, the trans- mission noise decreased by 3 to 5 dB, and when the absorption noise area of the dash inner insu- lator increased, the low frequency band improved.

80

70

Poewr Based Noise Reduction[dB]

60

50

40

30

20

Base Add a bulkhead Increase the area of Dash Inner Insulator

10

1000 2000 3000 4000 5000 6000 7000 8000 0

Frequency[Hz]

Figure 4 Results of PBNR on front motor room Fig. 4 is the result of measuring the PBNR from the rear motor room to the room. In order to reduce the noise transmitted from the rear motor room to the room, the PBNR was measured by adding sound absorption materials to the flow carpet, luggage board, luggage side trim, and c- pillar. As a result of the measurement, it was confirmed that noise was reduced the most when the luggage side trim unit was improved. When additional sound absorption materials are applied to other trims, it is confirmed that the improvement effect can be confirmed in the frequency range of 3,000 Hz or higher, but there is no effect in the frequency band below it.

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Figure 4 Results of PBNR on rear motor room 5. CONCLUSION

The noise path of electric vehicle was confirmed through the PBNR test, and the performance was improved by applying sound absorption material to the noise path. 1) Based on the results, it is verified that the noise transmitted from the front motor room to the in- terior of the vehicle body was weak due to the influence of the dash panel has opened areas due to parts such as the wiring harness and steering system.

2) It was confirmed that the noise transmitted to the interior of the car was reduced by 3 to 5dB by implementing a partition wall as a countermeasure for noise path from the front motor room to the interior. 3) As a countermeasure for the noise transmission path of the Rear motor room, it was confirmed that the sound absorption material was additionally applied to the luggage side trim to reduce the transfer noise by more than 3dB. 6. REFERENCES

1. Y.S. Yang, S. Lee, A Study on the Effects of Compression of the Felt on Flow Resistance and

Acoustic Characteristics, Proceedings of INTER-NOISE 2021 , 2292. Washinton DC, US, Au- gust, 2021. 2. Jason Zhu, David Hammelef, Michelle Wood, Power-Based Noise Reduction Concept and

Measurement Techniques, SAE 2005 Noise and Vibration Conference and Exhibition, US, May, 2005. 3. Lee, Jonghyun , Cho, Sehyun, Yi, Juwan, Lee, Chulhyun, Yang, Jungmin, A study on the char-

acteristics of high frequency road noise transmission at the rear seat of a hatch back compact car using PBNR (Power Based Noise Reduction) method, The Journal of the Acoustical Society of Korea, Volume 37 Issue 4, PP.248-255, 2018

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