according to the invention , discharge noise from rotary vacuum machines having complementary profiles is attenuated by interposing between the discharge from the primary pump and the outlet to the atmosphere , a transfer device having independent cavities , e . g . cavities on a rotor rotated on a shaft , the cavities moving sequentially from the discharge to the outlet while simultaneously providing isolation between the discharge and the outlet . this achieves dynamic attenuation of the noise in a manner that is particularly simple and low cost , while also being very effective in suppressing discharge noise .