Article Acoustic Properties of Larch Bark Panels Eugenia Mariana Tudor 1,2 , Lubos Kristak 3, * , Marius Catalin Barbu 1,2 , Tomáš Gergel’ 4 , Miroslav N ˇ emec 3 , Günther Kain 1 and Roman Réh 3   Citation: Tudor, E.M.; Kristak, L.; Barbu, M.C.; Gergel’, T.; Nˇ emec, M.; Kain, G.; Réh, R. Acoustic Properties of Larch Bark Panels. Forests 2021, 12, 887. https://doi.org/10.3390/ f12070887 Academic Editors: Michele Brunetti, Michela Nocetti and Alexander Petutschnigg Received: 10 June 2021 Accepted: 6 July 2021 Published: 7 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Forest Products Technology and Timber Construction Department, Salzburg University of Applied Sciences, Markt 136a, 5431 Kuchl, Austria; Eugenia.tudor@fh-salzburg.ac.at (E.M.T.); marius.barbu@fh-salzburg.ac.at (M.C.B.); gkain.lba@fh-salzburg.ac.at (G.K.) 2 Faculty of Furniture Design and Wood Engineering, Transilvania University of Brasov, B-Dul. Eroilor Nr. 29, 500036 Brasov, Romania 3 Faculty of Wood Sciences and Technology, Technical University inZvolen, 96001 Zvolen, Slovakia; nemec@tuzvo.sk (M.N.); reh@tuzvo.sk (R.R.) 4 National Forest Centre, Forest Research Institute, 96001 Zvolen, Slovakia; tomas.gergel@nlcsk.org * Correspondence: kristak@tuzvo.sk Abstract: The potential of tree bark, a by-product of the woodworking industry, has been studied for more than seven decades. Bark, as a sustainable raw material, can replace wood or other resources in numerous applications in construction. In this study, the acoustic properties of bark-based panels were analyzed. The roles of the particle size (4–11 mm and 10–30 mm), particle orientation (parallel and perpendicular) and density (350–700 kg/m 3 ) of samples with 30 mm and 60 mm thicknesses were studied at frequencies ranging from 50 to 6400 Hz. Bark-based boards with fine-grained particles have been shown to be better in terms of sound absorption coefficient values compared with coarse-grained particles. Bark composites mixed with popcorn bonded with UF did not return the expected results, and it is not possible to recommend this solution. The best density of bark boards to obtain the best sound absorption coefficients is about 350 kg/m 3 . These lightweight panels achieved better sound-absorbing properties (especially at lower frequencies) at higher thicknesses. The noise reduction coefficient of 0.5 obtained a sample with fine particles with a parallel orientation and a density of around 360 kg/m 3 . Keywords: bark-based panels; acoustic performance; sound absorption coefficient; sustainable materials; construction materials 1. Introduction The negative effects of chronic exposure to noise are, nowadays, an important issue [1]. The outdoor noise levels directly influence housing market prices, which can decrease by 0.3 to 3% in areas with noise pollution [2]. The noise level in a building is influenced by a variety of factors such as location [3], city planning [4], building design [5], vegetation [6], façade elements [7], construction features and material selection [8–10]. A proper selection of materials improves the noise control in buildings to a great extent. It facilitates minimizing many costly techniques of noise control in buildings. The use of natural insulation materials with minimal production processing is an emerging trend in construction. The use of sustainable and recyclable materials is an impor- tant aspect to warrant a healthy environment. Many studies scrutinized the environmental and technical benefits of using sustainable materials (natural or recycled) as basic elements to produce commonly used as well as new porous materials (granular or fibrous) [11–18] such as rice husk, cotton stalk, jute fiber, straws of wheat, hemp, flax, coconut fibers, stalks of maize and sheep’s wool [19–25]. These lignocellulosic materials, when used appropri- ately, can provide thermal and acoustic insulation performance comparable with the most used insulation materials, but with excellent environmental properties [26]. Forests 2021, 12, 887. https://doi.org/10.3390/f12070887 https://www.mdpi.com/journal/forests