The behaviour of tooth mesh frequency sound radiated from a tooth mesh point of a spur gear pair was investigated to see how it is generated. after examining the effect of bottom clearance, torque and material (steel or nylon) upon the radiation, the sound proved to be an aerodynamic sound; i.e., the sound is born from the pumping action of the clearance between meshing teeth, under high speed conditions. Especially towards the exit side of meshing, the radiation is prominent, with a "level trough" feature, which occurs due to the existence of two sorts of sounds having a phase difference of 180° near the meshing point. It has also become clear that this aerodynamic sound can not necessarily be reduced by increasing the clearance, after a consideration based on an analogy to the fluctuation of the flow rate of a gear pump.