Acoustic characterization of natural fibers for sound absorption applications * Umberto Berardi a, * , Gino Iannace b a Department of Architectural Science, Ryerson University, 350 Victoria Street, Toronto, Ontario M5B 2K3, Canada b Dipartimento di Architettura e Disegno Industriale, Seconda Universita’ di Napoli, Italy article info Article history: Received 1 April 2015 Received in revised form 23 May 2015 Accepted 26 May 2015 Available online xxx Keywords: Sustainable materials Natural materials Fibers Airflow resistance Sound absorption abstract Natural materials are becoming a valid alternative to traditional synthetic ones for sound absorption treatments. In particular, in recent years, natural fibers have been considered valid raw materials for producing sound absorbing panels at a reduced cost. Moreover, these fibers often have good thermal insulation properties, have no harmful effects on health, and are available in large quantities often as a waste product of other production cycles. Following a literature review of previous studies about the acoustic properties of some natural materials, this paper reports the acoustical characterization of the following natural fibers: kenaf, wood, hemp, coconut, cork, cane, cardboard, and sheep wool. The ab- sorption coefficient and the flow resistance for samples of different thickness have been measured. By using existing theoretical models, this study also compares the measured behavior with the theoretically predicted behavior. This comparison shows the limits of theoretical models originally defined for porous materials with homogeneous fibers, when they are applied to natural materials. Finally, some sugges- tions for use of these natural fibers for sound absorption applications in buildings are reported. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Sound absorption panels for room acoustic applications are generally composed of porous synthetic materials, such as rock wool, glass wool, polyurethane or polyester, which are expensive to pro- duce and are generally based on petrochemicals. The growing awareness towards the environmental implications and health issues associated with these materials has increased the attention towards natural materials [1e3]. These are generally defined according to natural and renewable sources of their constituent materials, the low level of environmental pollution emitted during their production or to their low embodied energy [4,5]. However, there is still little knowledge about the sound absorption behavior of natural materials. In order to absorb sound, materials should have high porosity to allow the sound to enter in their matrix, and for dissipation. Pores isolated from other adjacent pores, also called “closed” pores, allow some level of sound absorption, but only “open” pores, which guar- antee a continuous channel of communication with the external surface of the material, allow higher sound absorption properties [6]. Based on their microscopic configurations, porous absorbing mate- rials have also been classified as cellular, fibrous, and granular [6,7]. Granular materials consist of relatively rigid, macroscopic bodies whose dimensions exceed those of the internal voids by many orders of magnitude, and in this they differentiate from cellular materials. Conversely, fibrous materials consist of a series of tunnel-like open- ings that are formed by interstices in material fibers; these fibers may be continuous filaments or discrete elongated pieces. Fibers are often classified as natural or artificial. Natural fibers can be vegetable (kenaf hemp, wood), animal (wool, fur felt) or mineral (asbestos); whereas synthetic fibers can be mineral (fiberglass, mineral wool, glass wool) or polymer (polyester). Vegetable fibers are comprised mainly of cellulose [7] and can be categorized into:  stalk or wood fiber (e.g. straw of wheat, rice, softwood or hardwood);  bast fiber or skin fiber (e.g. flax, jute, kenaf, industrial hemp, ramie, rattan, and soybean);  leaf fiber (e.g. sisal, palm, and agave);  seed fiber (e.g. cotton and kapok);  fruit fiber (e.g. coconut). * An early version of this paper was presented at the International Congress on Sound and Vibration, ICSV22, Florence, Italy, 12e16 July 2015. * Corresponding author. E-mail address: uberardi@ryerson.ca (U. Berardi). Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv http://dx.doi.org/10.1016/j.buildenv.2015.05.029 0360-1323/© 2015 Elsevier Ltd. All rights reserved. Building and Environment xxx (2015) 1e13 Please cite this article in press as: U. Berardi, G. Iannace, Acoustic characterization of natural fibers for sound absorption applications, Building and Environment (2015), http://dx.doi.org/10.1016/j.buildenv.2015.05.029