M.P. Cooling

A nonlinear propagation model-based phase calibration technique for membrane hydrophones

A technique for the phase calibration of membrane hydrophones in the frequency range up to 80 MHz is described. This is achieved by comparing measurements and numerical simulation of a nonlinearly distorted test field. The field prediction is obtained using a finite-difference model that solves the nonlinear Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation in the frequency domain. The measurements are made in the far field of a 3.5 MHz focusing circular transducer in which it is demonstrated that, for the high drive level used, spatial averaging effects due to the hydrophones finite-receive area are negligible. The method provides a phase calibration of the hydrophone under test without the need for a device serving as a phase response reference, but it requires prior knowledge of the amplitude sensitivity at the fundamental frequency. The technique is demonstrated using a 50-mum thick bilaminar membrane hydrophone, for which the results obtained show functional agreement with predictions of a hydrophone response model. Further validation of the results is obtained by application of the response to the measurement of the high amplitude waveforms generated by a modern biomedical ultrasonic imaging system. It is demonstrated that full deconvolution of the calculated complex frequency response of a nonideal hydrophone results in physically realistic measurements of the transmitted waveforms.

Hydrophone area-averaging correction factors in nonlinearly generated ultrasonic beams

The nonlinear propagation of an ultrasonic wave can be used to produce a wavefield rich in higher frequency components that is ideally suited to the calibration, or inter-calibration, of hydrophones. These techniques usually use a tone-burst signal, limiting the measurements to harmonics of the fundamental calibration frequency. Alternatively, using a short pulse enables calibration at a continuous spectrum of frequencies. Such a technique is used at PTB in conjunction with an optical measurement technique to calibrate devices. Experimental findings indicate that the area-averaging correction factor for a hydrophone in such a field demonstrates a complex behaviour, most notably varying periodically between frequencies that are harmonics of the centre frequency of the original pulse and frequencies that lie midway between these harmonics. The beam characteristics of such nonlinearly generated fields have been investigated using a finite difference solution to the nonlinear Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation for a focused field. The simulation results are used to calculate the hydrophone area-averaging correction factors for 0.2 mm and 0.5 mm devices. The results clearly demonstrate a number of significant features observed in the experimental investigations, including the variation with frequency, drive level and hydrophone element size. An explanation for these effects is also proposed.

A comparison of hydrophone near-field scans and optical techniques for characterising high frequency sonar transducers

Two potential methods of fully characterising the response of high frequency sonar transducers and arrays operating in the frequency range 100 kHz to 500 kHz are compared. In the first approach two-dimensional planar scans, with a spatial resolution of better than half a wavelength, are performed in the acoustic near-field using a small probe hydrophone. The measured two-dimensional data are propagated numerically using a Fourier Transform method to predict the far-field response. Alternatively the data can be back-propagated to re-construct the pressure distribution at the source, a powerful diagnostic technique which can identify defects in transducers and array elements. The second approach uses a scanning laser vibrometer to measure the velocity of the transducer surface; with the resulting velocity data also being used to predict the far-field response by numerical propagation. The two approaches are compared for a number of devices. Comparison of the propagated hydrophone near-field scan data with direct measurements at these ranges shows very good agreement, indicating the usefulness of the method for deriving far-field transducer responses from near-field measurements in laboratory tanks. The potential limitations introduced to the optical approach by the acousto-optic effect are discussed.

Measurement of the phase response of a membrane hydrophone and its application to ultrasonic field characterisation

The accurate measurement of acoustic waveforms containing multiple frequencies is complicated by the need to know the frequency dependent phase response of the measurement system. This is particularly relevant for high amplitude ultrasound propagation resulting in non-linear distortion. However, if the phase response of the system is known then the true acoustic waveform can be recovered. This work describes a means of obtaining the required phase response of a hydrophone and receiver system over a wide frequency range, its application, and impact on measurements.

Transducer characterization by laser Doppler vibrometry.

A laser Doppler vibrometer (LDV) can be set up such that its laser beam passes through the glass wall of a water tank to be incident upon the radiating surface of an immersed ultrasonic transducer. Scanning the laser beam across a transducer in this way has the potential to be a fast, noninvasive method for source characterization and, in turn, field prediction. Such measurements are, however, significantly complicated by the acousto‐optic interaction—that is, the effect on the measurements of the acoustic field through which the laser beam passes. To increase understanding of the resultant acousto‐optic artifact, a detailed simulation of the LDV measurement of a circular, plane‐piston transducer emitting a tone‐burst has been created. The use of a transient field is important for simulation and experiment, such that measurements can be made over a time window which ends before any acoustic signal reaches the water tank boundaries. The simulation results show a significant acousto‐optic artifact contribut...