Patent ID: 7604467

Claim:
A positive displacement supercharger comprising: a housing including a rotor cavity having a surrounding cavity wall; a pair of positive displacement rotors oppositely rotatable in the rotor cavity and having interleaved helical lobes forming rotor chambers operative to carry air axially from an inlet at an inlet end of the cavity to an outlet near an outlet end of the cavity; the outlet defined by an opening through the cavity wall and communicating the rotor chambers with an outlet plenum partially defined by an outer side of the cavity wall and subject to pulsations in the air discharged to the outlet plenum; the cavity wall being stiffened within the plenum by a stiff portion defining a plurality of lightening recesses opening into the plenum, the stiff portion limiting distortion of the wall by pulsations in the plenum; and restriction means partially covering the openings into the plenum of at least some of the recesses and forming tuning chambers, the restriction means including at least one tuning opening between each tuning chamber and the plenum and forming tuning volumes effective to attenuate selected frequencies of pulsations in the plenum and thereby reduce undesired noise emanating from the discharge air of the supercharger; wherein the restriction means includes a cover over at least part of the stiff portion and blocking air flow to the tuning chambers under the cover except through the tuning openings, and the tuning openings extend through the cover.