Measured Effects of Varying Moisture Content
Several studies of the influence of moisture content on the acoustical properties of sands and soils [58-61] show that the higher the water content, the higher the measured acoustic surface impedance. Since it influences water retention in soils, organic matter plays a role also . An interesting high- frequency resonance (~5 kHz) in the surface impedance spectrum has been found when applying water to a soil surface to form a thin surface film . When impedance or admittance spectra deduced from impedance tube measurements have been fitted with single- or two-parameter models, increasing water content leads to an increase in the best-fit effective flow resistivity , at least until the saturation reaches 50%. However, when using the single parameter model, different values of effective flow resistivity were needed to fit the real and imaginary parts of surface admittance . In contrast, a two-parameter model was found to enable reasonable fits to both real and imaginary parts. This supports the suggestion made elsewhere (, Chapter 5 and Section 6.6) that the single parameter empirical model is a poor choice for modelling the surface impedance of soils.
Independent acoustical influences of air and water contents have been investigated through impedance tube measurements on a sandy soil .
A larger air content than in the ‘dry’ state is found when there is 20% by weight of water. For a given water content, increase in air content results in higher absorption coefficients. On the other hand, for a fixed air content, increasing water content results in higher absorption also. This is attributed to increased particle cohesion associated with water content resulting in larger pores and a greater fraction of interconnected pores. Nevertheless, typically, an increase in water content is associated with a decrease in air content and hence with higher surface impedance and lower sound absorption [58,59,61].
The decrease in the noise shielding by a green roof due to increasing soil moisture in a vegetation (approximately 3-cm thick sedum plants)-covered substrate (7-cm thick, mineral fraction between 70% and 90%; organic content between 3% and 8%) has been investigated by measurements of the influence of natural rainfall and artificial wetting experiments . This decrease in diffraction-induced noise shielding is related to the measured decrease in ground effect attenuation associated with the increase in moisture content, which is discussed in Section 6.4.4.