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

Abstract

This paper presents a new application of the normal mode decomposition to measure the reflection and absorption coefficients of a low growing living plant in a large 300×300mm impedance tube. In this way the higher frequency limit can be extended by a factor of 3 in comparison to that suggested by the standard ISO 10534-2 method for this type of an impedance tube. The adopted method (Prisutova et al., 2014) is based on minimising the difference between the spatial Fourier transform of the measured sound pressure at a range of closely spaced positions along the impedance tube and the predicted transform arising from the normal mode decomposition method. The angular and frequency dependent complex reflection coefficients for the first 5 normal modes are recovered. The acoustical properties of three plants specimen, Pelargonium hortorum, Begonia benariensis and Hedera helix, are measured with the adopted method. These properties are related to the plant morphology through an equivalent fluid model. It is shown that in some cases the predicted and measured data are in close agreement. However, there are cases when the agreement between these data is poor. The possible reasons for this discrepancy are proposed and discussed. This work paves the way for a better understanding of the relations between the plant morphology and its acoustical properties.