Perforated Panel Absorbers with Viscous Energy Dissipation Enhanced by Orifice Design
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Currently, there is a great interest in panel absorber design where porous components are excluded due to environmental and cleaning considerations. For such absorbers, the challenge is to increase the natural, viscous losses to attain an acceptable absorption bandwidth. This dissertation presents two new perforated panel absorber concepts, where the viscous energy dissipation has been enhanced by the use of non-traditional design of the perforations. The first concept is a perforated panel where the perforations has been shaped as small horns. The inner part of the horns have dimensions comparable to microperforated panels. The purpose of the design is to increase the surface area of the opening, increase the flow velocity in the inner part of the horn, and offer a better impedance match to the incoming wave. The concept has been investigated primarily by calculations using the Finite Difference Method. The results indicate that a relatively large absorption bandwidth can be obtained for a horn with wide outer radius and small inner radius. The second concept is a double perforated panel, consisting of two parallel, perforated plates separated by a small distance, typically 0.1 – 0.3 mm. The main part of the energy dissipation takes place in the small gap between the plates. Both perforated and slitted variants have been investigated by simulations and experiments. For the slitted palne case, absorption bandwidths equivalent to microperforated panels has been observed. The slitted variant can also be designed to be adjustable, allowing tha lateral distance between the slites in the two plates to be varied. This offers two special features: The maximum absorption coefficient can be adjusted from unity to almost zero, and the resonance frequency can be shifted. A frequency shift of one actave at normal sound incidence has been obtained.