The combination of vibro-acoustic transfer path analysis and airborne source quantification makes it possible to quickly quantify structural and airborne contributors in order to identify the root causes of a noise problem at Nissan Motor Co. The problem here involves an intake system but the same methods can be applied to nearly any component that generates interior noise. All of the measurements are combined onto a single graph that shows that structural noise is negligible throughout the frequency range of interest while airborne shell noise dominates at lower frequencies and airborne nozzle noise was important at higher frequencies. Being able to pinpoint the source of the problem made it possible quickly identify modifications that solved the problem in less time and at less cost than would be required using the conventional trial-and-error approach.The problem with the conventional approach.
The traditional approach to addressing interior noise problems uses operational deflection shape analysis and decoupling tests that attempt to pinpoint noise sources. For example, a tube may be placed in the air intake in order to remove the nozzle noise from interior noise measurements. The intake manifold might be shielded to eliminate shell noise radiated from its housing, etc. The problem with these types of tests is that they provide only approximate indications of the source of the problem. Without an in-depth analysis and understanding of the cause of the problem, design engineers typically face a long and expensive trial-and-error process that usually requires making costly modifications whose impact is far from guaranteed.
Comprehensive approach identifies root causes
In this project, LMS consultants used transfer path analysis to accurately measure the contribution of noise transmitted from the intake manifold through the structure of the vehicle. Structure-borne transfer path analysis was performed by combining a force identification procedure with Frequency Response Function (FRF) measurements. In order to increase efficiency, the FRF’s were measured reciprocally by exciting the car cavity with a calibrated volume velocity source and measuring the response at intake mounting positions. The consultants then used airborne source quantification to accurately measure the impact of nozzle and shell noise. They quantified the nozzle source strength with an inverse procedure, using acoustic measurements in the vicinity of the nozzle. They estimated shell noise by dividing the manifold surface into patches and measuring the acceleration of each patch with the engine running. Combining these sources with measured FRF’s enables made it possible to quantify the impact of the different sources on interior noise.
Finding the root cause and solving the problem
When these measurements were combined into a single chart, it was easy to visualize the precise causes of the noise problem.
Having a clear picture of the mechanisms responsible for generating interior noise at each of the problem frequencies would substantially reduce the time and cost required to solve the problem.
For example, acoustic modes in the duct causing high nozzle noise and forced vibration of the intake manifold housing can be minimized
by changing the geometry of the piping and chambers and adding resonators.
Experimental and numerical modal analysis can also be used to optimize the geometry of components in the frequencies of concern.
