The relationship between FFTs and gear quality as well as gear noise is investigated with some surprising conclusions, which leads to the discovery of a new interpretation of higher harmonics and the unknown influence of the residual error. An excursion into the psychoacoustic science in this phase of the research delivered many useful answers. Finally, a guideline was developed which proposes the combination of coordinate measurement, single flank test, structure borne noise, and additional analysis.

A rather exciting conclusion from the psychoacoustic research is the proposal of a gear transmission graph, which is a hybrid that connects different mathematical functions within the one pitch long contact area and outside of this area. A first surface development of a hypoid gearset has been realized by applying a UMC center section in connection with toe and heel sections, which are second order parabolas. The first results are very promising, which seem to confirm that the hybrid transmission function dramatically changes the way bevel and hypoid gearsets will be optimized in the future for silent operation.