Applied PhD in underwater acoustics modelling:Particle Motion – the flip side of sound

BACKGROUND
Galway-Mayo Institute of Technology (GMIT), Ireland, have partnered with Irwin Carr Consulting (ICC), Northern Ireland (UK), to further our understanding of important aspects of our underwater environment. This project is focussed on the real-world application of the research and the impact it will have on our understanding of underwater sound propagation. With the technical expertise of our industrial partner ICC, we will be able to offer facilities, technical knowledge, sound sparring and guidance to an outstanding aspiring PhD student to carry out internationally important work. The successful applicant will be employed by ICC during the project, while remaining associated with GMIT, and receiving guidance from both organisations. See more about the format at the Irish Research Council.

INTRODUCTION
Sound has historically been characterised either in terms of how the local pressure oscillates or how much energy passes through a given area per unit time. These two approaches lead to the units we assign to sound to characterise it: Sound Pressure Level (dB_SPL re 1 µPa) and Sound Intensity Level (dB_I re 1pW/m2).
Sound must propagate through matter. And as matter is made up of particles, those particles move for the sound to travel. The movements are almost always oscillations around a mean position that particles will return to after being affected by a sound. Those movements are termed particle motion, and can be measured and described with respect to the peak particle acceleration, velocity and displacement, all ways to characterise the level of the associated sound.
Humans are, for the most part, sensing sound as pressure fluctuations between the two sides of the tympanic membrane. This works because air is compressible. Seawater however, is almost incompressible, with a bulk modulus of 2.34 GPa. This is 16,500-23,000 times the resistance to compression of air. A liquid-filled middle ear would therefore barely be able to detect fluctuations in pressure, as even high pressure fluctuations would lead to only very small compressions of the liquid-filled middle ear, with associated minimal displacements to detect for the inner ear (or other functional equivalent).
This high resistance to compression is what makes sound travel much faster in water than in air.

Marine mammals have air-filled ears, capable of detecting pressure analogously to us, but many fish and most invertebrates have no air-spaces associated with their ear, rendering them insensitive to pressure fluctuations.
In a large proportion of situations sound waves can be legitimately characterised as plane compression waves moving through a fluid. In these situations, it’s straightforward to calculate the particle motion elements.
Close to interfaces however, the particle motion aspect is poorly understood.

SCOPE OF PHD
1. Mapping particle motion
It’s envisioned that the graduate take on the task of measuring pressure and peak particle motion in a semi-controlled environment, such as a harbour enclosure or a dock. This should result in a detailed characterisation of a real sound field.
2. Modelling particle motion
The graduate will attempt to model the particle motion using a choice of modelling approach and the available literature. The aim is to develop a tool that can approximate the particle motion from source information such as source level and spectral distribution while incorporating environmental factors. We envision a piece of software made available for researchers and developers around the world.
3. Third scope to be described by applicant.
Ideas include:
- Boundary layer investigation
- Non-compressional waves (Rayleigh or sheer waves)