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Origin of Low Frequency Noise in Motor Cars
Technical Paper
725008
Sector:
Language:
English
Abstract
The paper describes the basic causes of the predominant low frequency noise inside motor cars of integral construction. It is shown that maximum levels of noise occur in the subaudible frequency range around 10 Hz and is caused by engine bodily vibration on its mountings resulting from road excitation. Noise in the audible frequency range is produced by the bending and torsional modes of the whole car structure, and in particular by the large number of "ring modes" of the body shell, excited by vibration forces from the engine. Investigations on a number of cars show that the characteristics of the noise spectra and the vibration response of the car bodies are very similar which has led to empirical equations for the prediction of average levels of noise inside the car and magnitude of the vibratory exciting forces. Radiation of noise inside the car has been found to be proportional to vibration velocity of the body shell up to 200 Hz and above this frequency to vibration acceleration. The effect of acoustic resonances in the car cavity are also considered.