Identification of moan-noise generation mechanisms by an experimental method and verification of the mechanism by finite element analysis

Moan' noise in a vehicle is known to cause discomfort and anxiety to passengers. This abnormal noise is generated by a stick-slip phenomenon between the brake pad and the disc during breaking. The exciting force from the brake system is transmitted to a coupled torsion beam axle module through the brake pad, the caliper and an extra bracket. In this study, moan noise is reproduced during the vehicle's operating test, and the parts of the coupled torsion beam axle module generating moan noise are identified. Further, it is verified that the resonance of the coupled torsion beam axle module due to the exciting force of the brake system is the main cause of moan noise. Coherence analysis and transfer path analysis were conducted to identify how the vibrations from the coupled torsion beam axle module are related to moan noise in the passenger compartments of vehicles. In the coherence analysis, the vibration accelerations of the brake pad, the caliper, the spindle bracket, the trailing arm and the V-beam are set as the inputs, and the output is the noise at the driver's ear level. In terms of the frequency response, the results from a modal analysis of the coupled torsion beam axle module under the vehicle's test conditions agreed well with those from a finite element modal analysis in which the boundary conditions of the vehicle were taken into consideration. The dynamic characteristics of the response from the coupled torsion beam axle module were identified using the exciting-force transfer mechanism of the brake system. The locations producing a large strain were determined by calculating the modal strain energy, and an improvement scheme with design modification was proposed for moan noise reduction.