Acoustical characterization and modelling of agricultural byproducts for building insulation

Abstract

In a general context of global warming, an increasing interest is observed on the use of biobased insulating materials. Indeed, this kind of materials ensures good thermal characteristics while drastically reducing fossil energy consumption and greenhouse gas emissions associated with their manufacture. In addition, thanks to their permeancy (ability to allow diffusion of water vapor within their porous network), these materials present an alternative option to isolate and rehabilitate buildings made with non-industrial materials (rammed earth, cob, adobe,...) and whose stability requires to maintain a water exchange with the outside. Bio-based materials from plants have promising thermal and acoustical characteristics which are the subject of several studies now. In this work, the acoustical properties of hemp particles (shiv), flax (shiv), sunflower (pith and bark) and rape (straw) are measured and compared. These "green" materials are generally considered as harvesting “residues” and reuse is not always considered. These particles are all produced in most European countries and are the main available agricultural by-products which may be used for building insulation. It should be underlined that those products can be mixed with a binder to manufacture particular insulators (e.g. hemp concrete or sunflower pith/chitosan). The aim here is to deal with raw materials and not a particular mix of particle/binder. The sound absorption and transmission coefficients of those particles are characterized using laboratory experiments. Results show that they can all be used for acoustical absorption and insulation. We apply models to estimate not only the inter- and intra-particles porosities and thus the total porosity of the samples but also the particle and skeletal densities. Then, we show that equivalent fluid models can predict the sound absorption and transmission accurately. Thus inversion of these models enables to estimate microstructural parameters such as the tortuosity, viscous and thermal characteristic lengths and resistivity.