Experimental evaluation and modelling of the sound absorption properties of plants for indoor acoustic applications

• We measured the sound absorption of green walls to be used indoor.
• The investigated plants were Boston Ferns and Baby Tears.
• Non acoustical properties were deduced using an inverted model.
• A correlation between flow resistivity and leaf area density for Ferns was found.
• The acoustic correction of a restaurant using green walls was designed.

In this paper the sound absorption properties of two types of plants for indoor applications are analyzed. The aim of this research is to investigate the feasibility of using Nephrolepis Exaltata (Boston Fern) and Helxine soleirolii (Baby Tears) in green walls for indoor applications, in order to improve the acoustic quality of indoor environments. The normal incidence sound absorption coefficient of ten specimens of Fern and three of Baby Tears was measured in presence and in absence of a substrate. The chosen substrate is made of coconut and perlite soil, commonly used in hydroponics; its composition allows to obtain high porosity values. The sound absorption coefficient spectra were measured in the frequency range of 50–1600 Hz using a vertically mounted impedance tube with a diameter of 100 mm; measurements were carried out in accordance with ISO 10534-2 method. After measuring the normal incidence sound absorption coefficient with impedance tube, the equivalent fluid model for sound propagation proposed by Miki was used to deduce non-acoustical properties of the samples. The inverted Miki's model allowed to find the theoretical tortuosity (α∞) and the flow resistivity (σ). These non-acoustic properties were then related to morphological characteristics of specimens. It is shown that the leaf area density is closely related to flow resistivity, at least for Boston Fern; the data allowed to identify a semi-empirical relation between them. Finally an application of the developed green system for the acoustic treatment of a real case study is reported.