Investigation of engineering models for sound propagation in a near-shore environment

Abstract

This work presents an engineering approach to model the acoustic response of a sandy beach and its contribution to atmospheric sound attenuation of a near-shore acoustic source. Predictions of the sound pressure level at different ranges along the acoustic path are obtained with a parabolic equation solver that accounts for refraction and surface impedance variation along the propagation path. In this case study, sound propagates over flat water (126 m) and flat, smooth, sand (40 m). Experimental data are collected using a technique that uses chirp diversity to synchronize acoustic data with meteorological data. Sound pressure levels measured at the water’s edge and two range locations beyond the shoreline were compared to numerical results. Three models for acoustic behavior of sand were used to predict the sound pressure level at the beach. In particular, one of the models shows that transport properties of sand can be approximated by assuming sand grains are spherical particles. Generally strong agreement was found between predicted and measured sound pressure levels at the water’s edge. At 20 meters beyond the shoreline, modeling the beach with a reflective surface demonstrates an accuracy within 2 dB. When modeling the beach as a homogeneous absorber, none of the models proposed provided accurate results. However, when the sound absorption heterogeneity of the beach is accounted for using direct measurements of the effective flow resistivity, a better agreement (± 2 dB) was found at both 20 and 40 m beyond the shoreline. This work shows that good agreement between predictions and data can be also achieved by displacing the water–sand transition. Moving the transition to the location where the water saturation of the sand is below 10% and modeling the remaining beach with a semi-infinite layer of dry sand also produces agreement within 2 dB.