Abstract: An electronic noise attenuation system for attenuation of non-steady noise occurring in a propagation passage such a duct line by conducting adaptive controls by use of a computer system including a digital filter therein. Sensing microphones M.sub.1, M.sub.2 are located in the propagation passage with a cancellation sound source S therebetween at positions where the transfer functions Hr, Ht of a sound wave generated from the cancellation sound source are equivalent to each other. The output signal of the sensing microphone M.sub.1 and the phase-inverted version of the output signal of the sensing microphone M.sub.2 are input via an add circuit to the control part of the system. The system can prevent the occurrence of an acoustic feedback from the cancellation sound source to the sensing microphone M.sub.1 so as to realize a stable and highly accurate noise attenuation.

Abstract: A clean room wherein clean air obtained through filters from the upper portion of the clean room is blown toward the floor, through the openings in the floor, and with the clean air being discharged again through the filters from the upper portion of the clean room. The air flow rate of clean air in the aisle areas is greater than the air flow rate in the wafer handling areas, and the opening rate of the floor is smaller in the portion near to an air return under the floor than in the portion remote from the air return thereby greatly reducing the diffusion of dust to the wafer handling areas.