US Patent 7811070 Pneumatic vane motor with by-pass means

A pneumatic vane motor with a stator cylinder having an air inlet opening and an air outlet opening, a rotor eccentrically journalled in the cylinder and carrying a number of sliding vanes which divide the cylinder into moving cells, wherein the cylinder comprises a by-pass passage located between the air inlet opening and the air outlet opening and arranged to leak air from a pressurized working cell into a preceding cell before the working cell reaches the outlet opening, thereby using a preceding cell as an expansion volume for sound attenuation.