Noise reduction in an exhaust stack of a papermill

Porous or fibrous material are used in a great variety of applications to absorb acoustic energy at medium and high frequencies above approximately 200 Hz. There are, however, cases where their open and rough surface brings about certain disadvantages with respect to hygienic and cleaning requirements. For medium and low frequencies, which would also require a relatively large absorber thickness and weight, there is a need for an alternative absorber, the acoustically active components of which are formed exclusively by ideally even and smooth membranes. The sound-absorber presented herein is of the reactive or resonant type with several different modes of vibration excitable in a complex system of rather thin, though comparatively stiff metal or plastic membranes. This "case history" of a noisy papermill demonstrates the effectiveness of the new type of membrane absorbers under extremely harsh working conditions.