# Meshing Stiffness--A Parameter Affecting the Emission of Gearboxes
## 2 Materials and Methods
### Automobile Transmission Noise Sources  
The gearbox is an acoustically closed system from which noise is propagated mainly through vibrations of the housing surface or connected units, including the basic structure. The most important cause of noise is considered to be transmission error, which is related to the kinematic accuracy of the gear and gear stiffness.  
Vibrations from the gears transmitted to the gearbox housing are the most significant sources of noise [17]. From a physical point of view, the cause of vibrations is a dynamic force that can change its amplitude, direction, or field of action. In the case of involute gearing, the most significant is the amplitude change. This change can cause variable stiffness of the gearing and impact the entry of the gear mesh due to deformations, deviations of the pitch, and profile of the teeth from the theoretical ones.  
Vibrations in the meshing of the gears are influenced by many other factors, including vibrations transmitted to the gearing from the drive or driven unit, and oscillations of the shafts and bearings. All these phenomena are involved in increasing the amplitude in the gearing. The total energy of the radiated noise continues to increase.  
A specific source of noise is the occurrence of shocks due to the axial and lateral (tooth) clearance of helical gears. This occurs mainly with lightly loaded gears (for example, at the idle speed of an internal combustion engine) or, conversely, with very loaded gears at low speed. Irregular operation of the drive unit contributes to this and generates torsional vibration (change in angular acceleration during one revolution). This type of noise is referred to as rattling and knocking (ringing, rattling, etc.). The engines of modern cars therefore have a dual-mass flywheel and clutches with shock absorbers.  
Other phenomena causing noise during gear meshing include air pocketing (related to air pockets in the lubricant) and lubricant entrainment (due to small clearances, excess lubricant is not pushed out of the mesh and stresses the gearing by additional dynamic forces that cause an increase in vibrations).  
The gearbox contains components that can be a source of noise and vibration themselves, or they can excite or transmit and amplify vibrations. These include, for example, auxiliary elements such as steering elements, bearings, and shafts. The most significant of these elements are bearings, which are the second-most important source of noise after gearing. Vibrations are caused by the rolling elements of the bearing rolling along the inner and outer paths [18]. Their frequency is given by surface irregularities (pitting) or irregularities of functional surfaces, which are caused by wear or faulty production (deformation during clamping).  
However, the dominant share of noise in gearboxes is almost always due to the excitation of vibrations in the gear mesh. Figure 1 shows an example of the overall evaluation of the noise of an automobile transmission, where the share of the separated gear mesh noise (labeled as N and 3) is in the region of a maximum of 40% of the transmission noise, which contributes 53% to the total noise. The remaining 47% is background noise (labeled Bgr), which can mainly include bearing noise.