Method and apparatus for reducing noise in a roots-type blower

A Roots-type blower with reduced noise emission comprising: a housing (22) defining a chamber (28) of said rotor chamber rotor an inlet (80) and an outlet (82) comprising; a first (24) and second (26) rotors rotatably mounted in said chamber (28), each rotor a plurality of lobes (70), cooperating adjacent lobes of each rotor with said housing to define one or more transport chambers gas (103), said rotors configured to move gas from said inlet (80) through said gas transport chamber (103) to said outlet (82); and at least one channel (90a, 90b) of gas flow outlet extending from said outlet along an inner surface of said housing at said rotor chamber in an opposite direction to a direction of rotation of said rotor, configured said flow channel of exhaust gas at least to allow gas to flow from said outlet chamber (103) gas transport as said lobes of said rotor rotate towards said outlet, characterized by: at least said channel (90a, 90b) gas flow outlet so configured that a pressure of said gas in said chamber (28) as said chamber moves towards said outlet changes at an approximately linear rate; or defining at least said channel (90a, 90b) gas flow outlet flow area increases generally non-linearly towards the direction of said outlet.