Noise-free exhaust ventilation
Modern exhaust ventilation on the basis of centrifugal pump makes an equivalent noise of jet aircraft, so I suggest small speed exhaust ventilation for rooms where you need silence while the high concentration of workers or visitors (theaters, academic institutions, etc.).
The pump consists of: 1) body 2) restrictive ring 3) hinge 4) joint blade 5) rigid ring on the axle 6) axis in the center of rotation.