Introduction and Background
As the technologies developed in automotive industry for decades, sound quality of a vehicle has been improved significantly. At the same time, the expectation from customers for a quieter car when driving keeps increasing as well. According to a survey done by an automotive company, the sound quality still plays an important role when customers make a decision on which cars they are going to purchase. As a result, many automotive companies continue to put great efforts to improve the sound quality as a approach to gain a better reputation and obtain bigger market shares.

Seat squeak and rattle analysis
Squeak and Rattle of a seat assembly is a critical factor which affects much to NVH performance of a vehicle. A seat assembly usually consists of three mains parts: rail, seat back and head rest. When a car is running on the road, the whole seat assembly is exposed to a low-frequency vibration acoustic environment which is possibly caused by road noise, tyre resonance, engine vibration even the speakers in the cabin. If the frequency of these noises or vibrations is close to the natural frequency of any part of the seat, resonance is expected to occur. If the vibration response of this part exceeds the clearance between other part and itself, then this can result in a non-negligible impact between components. The magnitude of the impact force between the components depends on the vibration response, clearance and impedances at the impact location. Each impact force causes additional excitation that then results in vibration and radiation. If the radiated sound is perceived by an occupant, it is then considered as a rattle which brings a negative effect to NVH performance.

Possible solutions
The first solution is to increase the stiffness of the structure of the seat assembly. If the forces which cause components slide or impact against each other can be minimised, then the squeak and rattle should be under control. Because of the high stiffness, the natural frequencies of the seat assembly can be shift to a higher range, which allows the assembly to be insensitive to the low-frequency vibration acoustic environment. As a result, small deformation of the assembly can be expected, where rattle is less likely to occur.

The second solution is to increase the clearances between the surfaces of components which have the potential to impact against each other when vibration is happening. According to the rattle analysis done previously, rattle comes due to insufficient clearances between vibrating parts. Therefore, eliminating rattle between components can be achieved if, in designing phase, adequate clearances are set properly for the two vibrating parts.

The third possible solution is to increase preloads on contacting surfaces. Rattle usually occurs not only between the two separating components, where vibration forces the two parts to impact, but also between the two contacting components. That is because vibration forces the two parts to separate where the forces of vibration exceed the preloads set on the components. Once the forces decline, smaller than the preloads, the two parts resume to their original positions potentially impacting each other. The way of increasing preloads is trying to prevent