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A new sound absorbing material made from 100% softwood fibres by means of a foam-forming technique is introduced. In foam forming, a wet foam is created by mechanically mixing water, fibres and a surfactant. The air bubbles keep the wet fibres separate, and a highly porous fibre network is formed during drying. The sound absorption of foam-formed structures was measured by means of an impedance tube. The results showed that foam-formed softwood materials possessed a competitive sound absorption coefficient compared to different types of commercial sound absorber materials. The material is based on 100% softwood fibres without added binders and is semi-rigid and does not completely recover from compression. Improvement in the strength properties of softwood material can be obtained by using starch or cellulose microfibrils. The material could be used in indoor applications, for example, in replacing mineral wool acoustic ceiling panels or polyester non-woven materials in office acoustics products.

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A microperforated panel (MPP) is generally defined as a perforated plate, in which the impedance of below one millimetre perforations is dominated by viscous losses. Using MPPs in duct and silencer applications, target is to maximize transmission loss (TL) by choosing proper surface impedance parameters. Additive manufacturing (AM) has recently reduced conventional design limitations and enabled fast prototyping of complex shaped structures. MPP-based model scale silencers can be printed within reasonable time, price, and accuracy. In this paper, design and validation of AM silencers with MPPs are studied. First, the theoretical background of MPP acoustics is summarized. Second, feasible parameters for a MPP absorber for a certain tuning frequency are sought numerically using acoustic finite element method (FEM). Third, several test MPPs are prototyped and their acoustic properties are measured. Finally, MPP silencers are simulated using different approaches and the results are compared against experiments.