The low noise and linear sound level characteristics of passenger vehicles are receiving increased scrutiny from automotive journalists. A linear noise level rise with increasing engine rpm is the first basic aspect of insuring an acceptable vehicle interior engine noise sound quality. In a typical case of structural response to engine vibration input, interior noise begins to rise with rpm, remains constant or even drops as the engine continues to accelerate, and then exhibits a noise period corresponding to the structure's natural frequency. Frequently this nonlinearity is bothersome to the customer.
During the development process, Chrysler's Dodge and Plymouth Neon exhibited just such a nonlinear rise in noise level, heard within the passenger compartment, when the vehicle was accelerated through 4200 rpm. By generating operational mode shapes of the body front end structure under the conditions where microphone response data indicated the peak occurred, it was determined that the noise period was associated with a combination fore-aft and torsional bending of the front lower radiator cross-member responding to front engine mount input. Structural treatments to reinforce the crossmember such as cross-braces, an internal bulkhead, a bulkhead with C-channels, as well as a variety of typical mass dampers were investigated with varying degrees of success. Reducing the engine mount bushing rate was not an option. Most of the potential solutions simply changed the mode shape or moved the problem engine speed. What was desired was a simple solution, which simultaneously acted upon the vertical and torsional crossmember motion, that could be rapidly implemented. By devising a high polar moment of inertia mass damper, applied at the anti-node of both the torsional and bending modes, a significant reduction of resonant energy was achieved thus reducing the noise at 4200 rpm to an acceptable level.
This paper will outline the steps that were employed to methodically determine the source of the noise as well as develop an effective treatment. The development of the 2.3 Kg multi-degree of freedom mass damper as the best noise level reduction and production feasible solution, led to its implementation on all production Neons.