Measuring the effect of ground impedance on the vector field, both in air and underwater

Abstract

One direct advantage to characterizing the acoustic field with a vector sensor is the ability to measure both components of acoustic energy, potential (pressure) and kinetic (particle velocity). While it is well known that the kinetic energy exceeds potential in the near-field of a source, equivalent and opposite imbalances are prevalent in the far-field due to propagation effects. For example, at normal incidence the field exhibits a distinct frequency dependent relationship that depends explicitly on the impedance of the ground reflector, including any internal reflections from sediment layers. In this paper, we examine the measured vector field, both in-air and underwater, with a specific focus the energy balance when a source is directly overhead. While the construction of a neutrally buoyant volume for use in dense media (such as water) is fairly straightforward, the application to airborne acoustics requires extremely lightweight materials. Airborne measurements are provided by an Accelerometer-based Intensity Vector Sensor (AIVS) system, which is constructed from a lightweight MEMS accelerometer and microphone embedded in a spherical, expanded polystyrene (EPS) foam volume.