Developing an Underwater Low-frequency Projector Using Musical Instrument Schema

Open Access
Author:
Acquaviva, Andrew Arthur
Graduate Program:
Acoustics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 04, 2014
Committee Members:
  • Stephen Thompson, Thesis Advisor
  • Daniel Allen Russell, Thesis Advisor
  • Victor Ward Sparrow, Thesis Advisor
Keywords:
  • Acoustics
  • Electroacoustic Projector
  • Clarinet
  • Underwater
Abstract:
Designing an underwater source of sound that operates at very low frequencies, such that the wavelength is many times the longest source dimension, is often challenging. Issues with impractical physical dimensions, highly reactive radiation impedance causing low acoustic output power, and failure limitations from dynamic strain all inhibit low-cost, efficient products. However, a woodwind musical instrument, specifically a clarinet, effectively produces low frequencies despite a highly reactive input impedance. By examining the physical principles of a clarinet, it is feasible to construct an underwater low frequency projector (LFP) that operates on those principles. Specifically, understanding the thermoviscous losses and radiation characteristics of the clarinet bore, the functions of tone holes, and how the standing wave in the bore couples to the reed oscillation in a nonlinear positive-feedback loop allows the designer to create an underwater source that operates on the same positive-feedback coupling between the bore and reed. This thesis explains the relevant clarinet physics, followed by a description of the prototype construction and experimental method for measuring the signal produced by the prototype. A compendium of data are presented which show that the device produces a tonal sound of roughly 250 Hz, corresponding to ka < .001, at a sound pressure level averaging 140 dB re:$ 1uPa$. This tonal source is associated with a secondary broadband source corresponding to a cloud of cavitation bubbles which are present only when the system is oscillating. Spectrogram data show that, despite the complications from cavitation, the tone is stable in frequency and amplitude over periods of a minute or more. From this data, it is concluded that building an underwater sound source which can operate at very low ka is feasible and that further research is justified in improving the power output, removing the issues of cavitation, and improving the sophistication of the design.