Development of Supersonic Intensity in Reverberant Environments (SIRE) with applications in underwater acoustics

Open Access
Author:
Barnard, Andrew Robert
Graduate Program:
Acoustics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 24, 2010
Committee Members:
  • Stephen A Hambric, Dissertation Advisor
  • Stephen A Hambric, Committee Chair
  • Dean Capone, Committee Member
  • Anthony A Atchley, Committee Member
  • Julian Decatur Maynard Jr., Committee Member
  • Stephen Clarke Conlon, Committee Member
Keywords:
  • Nearfield Acoustic Holography
  • NAH
  • Acoustics
  • Sound Power Measurement
  • Directivity
  • Supersonic Intensity
  • Reverberant tank
  • Reverberation Room
  • signal separation
  • underwater intensity
  • acoustic vector sensors
Abstract:
A new measurement technique, Supersonic Intensity in Reverberant Environments (SIRE), has been developed analytically, and validated numerically and experimentally. The SIRE technique permits the measurement of narrowband radiated sound power and directivity in an environment with unknown field conditions. This type of measurement has previously been limited to environments with exact field conditions, such as the free field. Due to long acoustic wavelengths, underwater anechoic tanks are not cost-effective for low frequency measurements, nor are at-sea measurements time- or cost-effective. Unlike SIRE, techniques like nearfield acoustic holography (NAH) rely on knowledge of exact field conditions, which are usually unknown in a realistic measurement environment. SIRE is a cost effective, repeatable laboratory technique for narrowband evaluation of complex structural acoustic sources submerged in water. The technique leverages underwater acoustic intensity vector sensors in the near field of a source and allows the outgoing acoustic waves to be separated from unwanted incoming acoustic waves. Supersonic wavenumber filtering rejects the evanescent potions of the acoustic pressure and particle velocity from the separated, outward-propagating sound pressure and particle velocity. The SIRE technique was applied to a monopole source, dipole source, and point-driven, thin-walled cylinder with massive end caps. All sources were placed in an underwater reverberant tank and measured using custom underwater vector sensors specifically designed and built to reduce electromagnetic interference (EMI). The results are compared with theory, the ANSI S12.51 standard one-third-octave reverberation room method, and free field NAH. SIRE is shown to accurately measure radiated sound power to within the limits of ANSI S12.51. SIRE is also shown to accurately measure the directivity indices of simple sources to within ±3 dB. Finally, a coupled finite element/boundary element (FE-BE) model of a point-driven, thin-walled cylinder in a reverberant water tank is developed. The results of the FE-BE model show that the measurement standoff distance for the SIRE technique should be less than the reciprocal of the largest wavenumber in the frequency band of interest. Furthermore, the maximum measurement grid spacing is shown to be limited to less than twice the standoff distance.