Acoustic properties of low growing plants

Abstract

The plane wave normal incidence acoustic absorption coefficient of five types of low growing plants is measured in the presence and absence of soil. These plants are generally used in green living walls and flower beds. Two types of soil are considered in this work: a light-density, man-made soil and a heavy-density natural clay base soil. The absorption coefficient data are obtained in the frequency range of 50-1600 Hz using a standard impedance tube of diameter 100 mm. The equivalent fluid model for sound propagation in rigid frame porous media proposed by Miki [J. Acoust. Soc. Jpn. (E) 11, 25-28 (1990)] is used to predict the experimentally observed behavior of the absorption coefficient spectra of soils, plants, and their combinations. Optimization analysis is employed to deduce the effective flow resistivity and tortuosity of plants which are assumed to behave acoustically as an equivalent fluid in a rigid frame porous medium. It is shown that the leaf area density and dominant angle of leaf orientation are two key morphological characteristics which can be used to predict accurately the effective flow resistivity and tortuosity of plants.

An application of normal mode decomposition to measure the acoustical properties of low growing plants in a broad frequency range

“…These properties are needed to calculate Plant that the reflection coefficient of the plant is angular-dependent is because the effective flow resistivity of the foliage is very low and its porosity is close to 1 (e.g. [6]). In this case it is common to use the extended reaction model in 120 which we consider that the refraction angle at which the sound wave propagates in the foliage depends on the incident angle.…”

“…It was suggested in [6] that the acoustical properties of a low growing plant in a low frequency regime at normal incidence can be represented by an equivalent fluid model, e.g. the empirical model developed by Miki[10].…”

“…They concluded that vertical greenery positively affects the absorption of sound, but more experiments needed to be done on actual building facades for a better understanding of green acoustic insulations. A more recent laboratory work [6] showed that the acoustical properties of low growing plants can be predicted by an equiv-20 alent fluid model which is typically used to describe the acoustic behaviour of porous media at low frequencies. In this model the effective flow resistivity was related directly to the leaf area density whereas the tortuosity was related to the dominant angle of leaf orientation [6].…”

“…below 1-2 kHz) where the acoustic wavelength is still much larger that the characteristic leaf dimension (e.g. 15-2 250 mm for typical plants [7]) the viscous dissipation is the prime absorption 30 mechanism [6,4]. In the higher frequency range (e.g.…”

“…Finally, published laboratory work on plants (e.g. by Horoshenkov et al [6]) obtained through a controlled experiment in a standard impedance tube presents data for the 50 normal incidence plane wave absorption coefficient determined for a relative small sample area and in a rather limited frequency range.…”