This paper presents the sound absorption properties of a composite micro-perforated panel absorber (MPPA) whose front layer is produced by additive manufacturing. The MPPA layers are printed using a polymer material, where the different hole spacing is used to create different perforation ratios. The sound absorption coefficient is measured by using an impedance tube method, to investigate the effects of the perforation ratio and the depth of an airgap behind the MPPA. Also a porous material layer is attached behind the MPPA layer in order to produce a multi-layer sound absorber. The measurement results are compared to theoreticaltheoretical results. The comparisons show that the measured sound absorption coefficients are in good agreement with the theoretical model. The user of a porous sound absorbing material behind the microperforated panel broadens the frequency bandwith of the MPPA. The frequency of maximum sound absorption can be varied by altering the perforation ratio of the perforated panel and/or the depth of the airgap behind the panel.
a b s t r a c tThis paper aims to investigate the acoustic properties of porous polycarbonate material (PPM) fabricated by additive manufacturing, and the feasibility to tailor artificial porous sound absorbing material is studied. Four PPM samples with different perforation angles were printed by using a 3D printer. Polycarbonate material was used, and the samples were printed with 25.4 micrometre layer resolution. Their sound absorption coefficient was experimentally measured using the two-microphone impedance tube method. It was found that with increased the perforation angle and constant porosity, the sound absorption was decreased. The results indicated that by adjusting the perforation angle and the airgap behind the sample, significant sound absorption can be achieved in the low frequencies where conventional porous materials may not be that effective. The results obtained in this paper provide a new approach for the fabrication of new porous sound absorbing materials.
An ideal engine mount should provide a dual behavior. It needs to be soft to reduce the transmitted force, and to be hard to limit the relative displacement. The constant parameter linear mounts are unable to provide a good isolation when the excitation frequency is variable. Hydraulic engine mounts were invented as smart isolators to passively produce a soft isolator at low amplitude and a hard isolator at high amplitude. Having a dual behavior puts the mounts in the domain of nonlinear systems which in turn causes many new phenomena which have never appeared in linear analysis. The dual behavior hydraulic engine mounts were introduced around 1980 and passed through many analytic and technical improvements. This article will review these improvements up to 2012 and discusses the technical problems and methods of remedy.
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