High Frequency Noise and Vibration of Automobile Transmissions Systems

Abstract

Investigations are discussed which were aimed at establishing the characteristics of rear axle gear noise which was considered to be a typical example of a power transmission noise. A series of preliminary investigations gave quantitative information on which the ensuing research could be based, It was hypothesised from this information that the amplification of the vibrations by the resonant response of the propeller shaft was the cause of the audible noise. Investigation of the influence of the propeller shaft showed that its resonant behaviour could not be the cause of the audible noise and that it was sufficiently well isolated to be disregarded in this respect. Further appraisal of the results indicated the possibility of transmission of low level vibrations to the vehicle body. A study of the rear suspension system showed that rear axle vibrations were transmitted to the body. It was concluded that the vibrations became audible noise due to the coupling of the acoustics of the passenger compartment with the vibrations of the enclosing panels, Redesign of the rear suspension system with particular regard to the dynamic properties of the rubber bushes is recommended as a course of action likely to lead to a reduction in the noise, Theoretical studies into the prediction of the natural frequencies and mode shapes of the power transmission system by numerical methods were pursued, The transfer matrix method of evaluation was found to be an excellent technique for this work and if larger computor facilities had been available extension of the analysis could have yielded useful results. It is shown that random process techniques must be used to obtain reliable results from road test data, and information about the analog data analysis systems developed for the analysis is presented.