The automotive industry’s shift towards electric vehicles (EVs) has brought with it a new set of challenges, particularly in the realm of noise, vibration, and harshness (NVH). Among these, road noise booming—especially in the low-frequency range below 50 Hz—has emerged as a critical issue affecting passenger comfort. A recent study published in the journal Noise and Vibration Control delves deep into the mechanisms behind a specific 31 Hz booming noise in an electric SUV, offering innovative solutions that could reshape how automakers approach NVH development.
The research, led by Huang Yinglai, Zhao Mingbin, and Shan Xile from Geely Automobile Research Institute (Ningbo) Co., Ltd., focuses on a two-box hatchback electric SUV that exhibited significant low-frequency booming in the front row during steady-speed driving. What sets this study apart is its systematic investigation into road-induced booming, a topic that has received less attention compared to engine-related booming in traditional vehicles.
The Problem: A Persistent 31 Hz Booming
During testing, the team observed that when the vehicle traveled at a constant speed of 60 km/h on rough roads, the front row experienced a pronounced low-frequency noise with a peak at 31 Hz, reaching 45 dB(A). Notably, the rear row remained relatively unaffected, a detail that would later prove crucial in pinpointing the root cause.
Initial assessments ruled out tire-related issues. Despite the fact that EV tires are designed to be harder, wider, and flatter to meet low rolling resistance and load requirements—factors that typically exacerbate road noise—the tire modal analysis showed no correlation between tire structural characteristics and the 31 Hz peak. Tests on tire vibration transfer functions from the tread to the wheel center revealed that the X and Z directions did not exhibit matching peaks at 31 Hz, eliminating the tire as the primary source.
Tracing the Vibration Path: Suspension Force Transmissibility
The investigation then turned to the vehicle’s suspension system, a key component in the transfer of road-induced vibrations to the cabin. The researchers introduced the concept of suspension force transmissibility as a critical metric to evaluate how effectively vibrations are transmitted through the suspension.
By establishing a detailed model of the rear suspension system, the team analyzed the force transfer from the wheel to the vehicle body through bushings. The model considered the vibration velocities on both the suspension and body sides of the bushings, as well as the forces acting on them. Through this analysis, they identified a significant peak in the suspension force transmissibility at 31 Hz, aligning perfectly with the observed booming frequency.
Further modal testing of the rear suspension uncovered a rigid body mode at 30 Hz in the RY (rotation around the Y-axis) degree of freedom. This modal frequency was found to be the primary driver behind the elevated force transmissibility at 31 Hz. The team’s analysis revealed that the stiffness of the motor’s rear mount and the rear bushings of the subframe had a substantial impact on this RY modal frequency.
The Role of Body Structure and Vibro-Acoustic Coupling
While the suspension system played a pivotal role in transmitting vibrations, the vehicle body’s response to these vibrations was equally critical. The researchers conducted body noise transfer function tests, which involved exciting various suspension mounting points and measuring the resulting noise in the cabin.
The results were striking: the rear subframe mounting points exhibited a high noise transfer function, particularly when excited in the Z-direction, with amplitudes reaching 60 dB in the 30-40 Hz range. This indicated that the vehicle body was overly sensitive to Z-direction vibrations at the rear subframe mounts, amplifying the noise in the cabin.
A deeper dive into the tailgate’s vibration and noise transfer functions revealed a clear peak around 30 Hz, suggesting that the tailgate’s modal characteristics were closely linked to the booming issue. This led the team to explore the complex interaction between the tailgate and the vehicle’s interior acoustic cavity—a phenomenon known as vibro-acoustic coupling.
Unraveling the Vibro-Acoustic Coupling Mechanism
The researchers developed a theoretical model to analyze the coupling between the tailgate and the interior acoustic cavity. Treating the tailgate as a rigid body vibrating in the x-direction and other boundaries as rigid, they derived equations governing the sound pressure distribution and particle velocity within the cavity.
Their findings revealed a critical insight: the 31 Hz booming was the result of impedance matching between the tailgate’s first-order modal frequency and the acoustic cavity. When the tailgate’s vibration impedance matches the acoustic impedance of the cavity, resonance occurs, creating a new eigenfrequency that would not exist if the tailgate were a rigid boundary. This explains why two-box vehicles, with their distinct tailgate design, are particularly prone to such booming.
The model also helped explain the front-rear disparity in noise perception. At 31 Hz, the standing wave pattern within the cabin creates a pressure antinode near the front row headrests and a node near the rear row, resulting in the pronounced front-row booming.
Innovative Solutions: From Theory to Practice
Armed with a comprehensive understanding of the booming mechanism, the research team proposed a multi-faceted optimization strategy:
- Suspension Rigid Body Mode Adjustment: By increasing the stiffness of the motor’s rear mount and the rear bushings of the subframe, the team successfully shifted the RY modal frequency from 30 Hz to above 35 Hz. This adjustment significantly reduced the suspension force transmissibility in the 28-35 Hz range, cutting off the primary vibration transmission path.
- Body Modal Control: Targeted modifications to the vehicle body structure aimed at reducing its sensitivity to Z-direction vibrations at the rear subframe mounts. This involved strategic reinforcement to alter the body’s natural frequencies, avoiding resonance with the suspension-induced vibrations.
- Tailgate Constraint Optimization: The team installed specialized hoses within the tailgate’s left and right sealing strips and increased the interference of the tailgate buffer blocks. These measures effectively reduced the tailgate’s amplitude, minimizing its role in exciting the acoustic cavity.
The results of these optimizations were dramatic. The 31 Hz booming noise was reduced from 45 dB(A) to 35 dB(A), and the overall road noise sound pressure level decreased from 65.5 dB(A) to 63.7 dB(A). Subjective evaluations confirmed the elimination of the annoying booming sensation, marking a significant improvement in passenger comfort.
Broader Implications for EV NVH Development
This study’s findings carry far-reaching implications for the automotive industry, particularly as EV adoption continues to rise. Unlike traditional internal combustion engine vehicles, EVs lack the masking effect of engine noise, making NVH issues—especially low-frequency noises—more perceptible to passengers.
The research highlights the importance of a holistic approach to NVH engineering, considering not just individual components but their interactions. The introduction of suspension force transmissibility as a key metric provides automakers with a new tool to evaluate and optimize suspension systems. Similarly, the detailed analysis of tailgate-acoustic cavity coupling offers valuable insights for the design of two-box vehicles, which are increasingly popular in the EV market.
Moreover, the study’s emphasis on road-induced booming fills a critical gap in existing literature, which has traditionally focused on engine-related noise. As EVs become more prevalent, road noise will only grow in importance, making this research timely and relevant.
Conclusion
The successful mitigation of the 31 Hz booming noise in the electric SUV demonstrates the power of systematic engineering analysis and innovative problem-solving. By combining theoretical modeling with practical testing and optimization, the team at Geely Automobile Research Institute has not only resolved a specific NVH issue but also contributed valuable knowledge to the broader field of automotive engineering.
As the industry continues to push the boundaries of electric vehicle performance and comfort, studies like this will play a vital role in ensuring that EVs meet—and exceed—consumer expectations for a quiet, smooth ride.
Huang Yinglai, Zhao Mingbin, and Shan Xile are engineers at Geely Automobile Research Institute (Ningbo) Co., Ltd., located in Ningbo, Zhejiang, China. Their research was published in the Noise and Vibration Control, Volume 44, Issue 5, October 2024. The article’s DOI is 10.3969/j.issn.1006-1355.2024.05.046.