Underwater particle motion (acceleration, velocity and displacement) from recreational swimmers, divers, surfers and kayakers

Abstract

When humans take to the water, they generate sound. This is helpful for the detection, classification, localisation and tracking of certain activities for purposes of border security, health and safety of offshore industrial development, environmental management, etc. The most commonly measured acoustic quantity is pressure. Vector quantities related to particle motion, such as particle velocity and acceleration, can equally identify the activity and they carry directional information. Acoustic pressure and particle motion were measured from 10 water sports activities within an Olympic-sized pool: swimming backstroke, breaststroke, butterfly and freestyle; snorkelling with fins; kicking a boogie board with fins; paddling with alternating or simultaneous arms while lying on a surfboard; scuba-diving; kayaking and jumping into the pool. Activities that occurred at the surface and that involved repeatedly piercing the surface were the strongest sound generators. Surface activities that produced fewer bubbles and scuba-diving at depth generated less broadband power. The vector fields around water sports activities can be expected somewhat different in the open ocean from within a pool, and more research is needed to understand how marine fauna might perceive these vector quantities.