| Popis: |
The purpose of this study was to verify and refine a phase calibration technique reported last year, based on the nonlinear acoustic wave propagation in water. The nonlinearity of the medium leads to the generation of harmonics and the relative phasing of the harmonics causes a distinct asymmetry between the positive and negative pressure levels affecting the rise and fall time of the examined waveforms. Knowledge of the relative phase shift measured in terms of radians versus frequency when referenced to the uniform simulated phase can help deconvolve the pressure-time signal, and hence produce its faithful reconstruction, including the rise times and peak amplitudes. The modified scheme discussed in this dissertation, uses an advanced semi-empirical computer model which predicts the near and the far field distributions using the hyperbolic propagation operator, in contrast to the parabolic approximations used elsewhere. Two PVDF membrane hydrophones were first calibrated in terms of their amplitude sensitivity in Volts/Pascals or dB re 1V/μPa. The sensitivities were needed to calculate the pressure levels generated by the HIFU (High Intensity Focused Ultrasound) sources. The sources operated at the frequencies of 5 MHz and 10 MHz to enable studies up to 100 MHz. The phase responses of these two hydrophones - a Marconi 50μm thick, 500μm diameter bilaminar PVDF film membrane hydrophone and a custom made Precision Acoustics, 9μm thick, 400μm diameter hydrophone - were determined with respect to the relative phase extracted from the complex frequency response of the nonlinear field simulated by the advanced semi-empirical hyperbolic operator model. The results indicate that the nonlinear technique is primarily suited for membrane hydrophones having a flat frequency response with variation in the range of ±10 %. Another PVDF hydrophone probe design, namely the needle one, does not exhibit uniform frequency response due to intrinsic (radial mode) resonances. These resonances introduce electrical distortions in the measured signals, which complicate the separation of the medium generated harmonics and those produced at the hydrophone output. Therefore, in order to calibrate the needle probes, a fiber optic hydrophone with a flat frequency response and zero phase-shift in the frequency range considered would be preferable. The main limitation of the nonlinear approach to determine phase response of membrane hydrophones is caused by the fact the technique can only provide phase information at discrete frequencies which are multiples of the fundamental of the acoustic source. Another limitation is associated with the use of preamplifier. No preamplifier and hence no electrical impedance matching was used to measure high pressure levels above 4 MPa (corresponding to the HIFU sources excitation levels of about 100Vpp), because of the observed saturation (clipping) of the electrical signal. This clipping reduced the number of harmonics which could be measured, and effectively limited the calibration bandwidth. The uncertainties of the measurements were analyzed and are presented at 95% confidence levels.In conclusion, we have demonstrated that the nonlinear acoustic wave propagation can be used as an effective tool to determine complex frequency response of the ultrasound PVDF membrane hydrophones in frequency range of 1-100 MHz. |
