Automotive transmissions. Contribution to noise and vibration inside vehicles

Vibro-acoustic phenomena associated with lateral transmissions can be classified into three categories, depending on whether the role of the transmission is direct, indirect or both.

• Direct role: acoustic or vibratory excitation occurs at the level of the transmission itself. This is the case for phenomena linked to the forces generated by constant velocity joints;

• Mixed role: excitations come from both the transmission and another source, such as the motor. It is the combination of these various excitations that creates an acoustic or vibratory disturbance;

• Indirect role: the excitation is no longer located in the transmission, but comes from another source. This is the case, for example, with the hum phenomenon, when a natural frequency of the wheel half-shaft amplifies a motor excitation.

Phenomena originating in the transmissions are dealt with in detail in the article [BM 2 596] . In this article, we deal with the "indirect role" category, more precisely, vibro-acoustic phenomena felt in the passenger compartment whose source is the engine and which pass mainly through the suspension buffers (high path) and lateral transmissions (low path).

In vehicles fitted with internal combustion engines, hum, rumble and engine presence generally account for most of the noise and vibration from the engine source.

• Engine hum: this is a low-frequency noise (20-200 Hz) similar to the sound of a bumblebee in flight. It comes directly from combustion in the cylinders and inertia forces linked to the movement of the moving linkage. Its frequency corresponds to the 2 harmonic of crankshaft rotation in a 4-cylinder engine, and to the 1.5 harmonic of crankshaft rotation in a 3-cylinder engine. In general, hum is the strongest component in terms of vibration level;

• Engine rumble and presence: these are medium-frequency noises or vibrations (200-800 Hz). More precisely, rumble and presence represent levels in the 250 Hz and 500 Hz octaves respectively, which correspond to levels of motor harmonics higher than those of hum.