It was already discovered that a pair of spur gears generates the tooth mesh frequency sound due to a fluctuating air flow. This sound is generated independently from the sound corresponding to mechanical vibration. In this study, to clarify the detailed generation process of the sound including the Level Trough which is one of specific feature of sound emission, experimental investigations were performed. It has been found that there is two sorts of sound; one is placed at the middle in the mesh end region, and is point source which appears when the air flow velocity reaches a critical speed. The other one is placed at both face end surfaces, whose magnitude decreases by either shortening center distance or shallowing tooth root spaces even though the air flow velocity increases. By assuming that the air flow comes from the pumping through the mesh process, a general explanation for the sound generation is presented.