Abstract: A centrifugal fan used as a ventilating fan is provided which can increase a range of frequencies of noises in which noises can be suppressed and can adjust frequencies of noises at which noises can be suppressed. The centrifugal fan includes, in an outer casing, a motor that couples an impeller, a casing that surrounds a periphery of the impeller and has a bell-mouthed suction port on one side, and a bell-mouthed orifice that has an opening that is concentric with the suction port and has a diameter equal to or smaller than that of the suction port, wherein a member closing a clearance portion between an end of the orifice and the casing is a noise absorbing structural material, and a space formed by the orifice is a rear air layer, thereby forming a resonance type noise absorbing structure.

Abstract: A centrifugal fan used as a ventilating fan is provided which can increase a range of frequencies of noises in which noises can be suppressed and can adjust frequencies of noises at which noises can be suppressed. The centrifugal fan includes, in an outer casing, a motor that couples an impeller, a casing that surrounds a periphery of the impeller and has a bell-mouthed suction port on one side, and a bell-mouthed orifice that has an opening that is concentric with the suction port and has a diameter equal to or smaller than that of the suction port, wherein a member closing a clearance portion between an end of the orifice and the casing is a noise absorbing structural material, and a space formed by the orifice is a rear air layer, thereby forming a resonance type noise absorbing structure.