LMS Engineering - Vehicle Interior Noise Reduced by 6 dB in 5 Weeks

LMS engineers helped an automotive car OEM in solving a mid-frequency road-induced booming noise that was discovered close to production. Solving the problem by trial and error and testing different hardware prototype modifications was no option. LMS applied advanced analysis practices to identify the different sources of the noise levels, and the noise transfer paths from these sources to “the problem point”. These insight lead to a targeted design modification on a lower suspension arm, that reduced the road-induced interior noise with 6dB


Trial and error was not an option

Only nine weeks before a new midsize sedan was scheduled to move into production, engineers working for a major automobile manufacturer had to address a mid-frequency road-induced booming noise reported by test track drivers. The traditional approach to fixing the problem would have begun with making deformation measurements of the body and suspension system on a dynamometer or road simulator. Engineers would then attach a large mass to areas where deformation was the highest in an effort to find a change that reduced the noise. Once they felt they had found the cause of the problem, engineers would try out various modifications, such as adding a beam in the area that was causing the problem. The problem with this approach is that a large number of modifications can be applied without really solving the problem. So it normally takes months to track down the one that is causing the noise using trial and error methods. Sometimes reducing deformation in one area will even make a noise problem worse. Making the wholesale changes that are usually required by this approach can also have a negative impact on other aspects of the design, such as the steering or the stability of the vehicle.

Transfer Path Analysis identified the root cause

The automobile manufacturer needed a way to quickly identify the problem so that modifications could be limited to the area that was actually causing the noise. LMS consultants applied Transfer Path Analysis (TPA), a test-based technique that traces the flow of vibro-acoustic energy from multiple sources to a receiver. TPA determines the phase as well as the amplitude of each path, helping engineers avoid the possibility of actually making the problem worse by reducing an excitation that is opposite in phase to the dominant path. In a first step, a coherence analysis brought LMS to the conclusion that one of the suspensions was clearly more critical than the other.

The focus of the project was thus limited to the rear suspension. The consultants first indirectly measured the forces using the complex stiffness method. They applied accelerometers to each side of the body mounts and ran the vehicle on a dynamometer with a rough road profile at the same speed that the boom had been reported. They used LMS software to calculate the operational forces at each mount by multiplying the stiffness of the body mounts times the difference between the displacements on the body and suspension sides of the mounts. They combined this approach with a so-called matrix inversion method, which enabled to improve the analysis results. The sum of the excitations contributed by all of the connections was shown to correspond very closely to the measured interior noise, confirming the accuracy of their measurements.

A quick and painless solution











This analysis, combined with engineering insight, enabled to understand the critical noise generation mechanisms. The TPA results pinpointed one particular connection as the source of nearly all of the excitation at the frequency of interest, so the consultants focused on the lower suspension arm that was connected to this mount. They added mass to the arm and verified that the noise was reduced with this modification. Then they proposed adding braces to increase the stiffness of the arm. During this process, LMS consultants worked closely with the OEM to determine which aspects of the design could be changed and which had already been optimized to solve a different problem. For example, if the resonant frequency of the lower arm couldn’t be changed, the consultants would probably have proposed adding dampers or tuning the body mounts instead.