Hydrophone Measurements for Biomedical Ultrasound Applications: A Review.

This article presents basic principles of hydrophone measurements, including mechanisms of action for various hydrophone designs, sensitivity and directivity calibration procedures, practical considerations for performing measurements, signal processing methods to correct for both frequency-dependent sensitivity and spatial averaging across the hydrophone sensitive element, uncertainty in hydrophone measurements, special considerations for high-intensity therapeutic ultrasound, and advice for choosing an appropriate hydrophone for a particular measurement task. Recommendations are made for information to be included in hydrophone measurement reporting.

The Practicalities of Obtaining and Using Hydrophone Calibration Data to Derive Pressure Waveforms.

This paper considers the means by which calibration data are used to convert hydrophone output voltage into pressure. Hydrophone frequency responses are complex-valued quantities, and only by correcting for the magnitude and phase variations, is it possible to accurately recover the original pressure waveform. The limitations of current hydrophone calibration techniques are discussed, and a new method of obtaining hydrophone phase data is presented. Magnitude and phase information is measured via both coarse increment (1 MHz) and fine increment (50 kHz) calibration techniques for three exemplar hydrophones (0.5 mm needle, 0.2 mm needle, and 0.4 mm membrane). Frequently hydrophone calibration data are available at frequency increments that do not match that required by the deconvolution process. Therefore, a variety of methods to interpolate the calibrated system response to obtain correctly spaced data are considered, and two spline interpolation methods are found to offer best performance. Data preconditioning and filtering to address artifacts above and below the 1 to 40 MHz bandwidth of the coarse frequency increment calibration are also investigated, and a simple procedure for selecting an appropriate low-pass filter is presented. The revised calibration data are used to deconvolve the hydrophone frequency response for experimentally derived waveforms. Standard ultrasonic output parameters (such as peak compressional and peak rarefactional pressures, pulse intensity integral, and temporal peak and pulse average acoustic intensities) are calculated from these waveforms. Although the three hydrophones used in this paper are of different types and have a range of active element sizes, all output parameters derived from the deconvolved waveforms have <5% variation from their respective population means (with the majority being within <2%).

A finite difference analysis of the field present behind an acoustically impenetrable two-layer barrier.

The interaction of an incident sound wave with an acoustically impenetrable two-layer barrier is considered. Of particular interest is the presence of several acoustic wave components in the shadow region of this barrier. A finite difference model capable of simulating this geometry is validated by comparison to the analytical solution for an idealized, hard-soft barrier. A panel comprising a high air-content closed cell foam backed with an elastic (metal) back plate is then examined. The insertion loss of this panel was found to exceed the dynamic range of the measurement system and was thus acoustically impenetrable. Experimental results from such a panel are shown to contain artifacts not present in the diffraction solution, when acoustic waves are incident upon the soft surface. A finite difference analysis of this experimental configuration replicates the presence of the additional field components. Furthermore, the simulated results allow the additional components to be identified as arising from the S(0) and A(0) Lamb modes traveling in the elastic plate. These Lamb mode artifacts are not found to be present in the shadow region when the acoustic waves are incident upon the elastic surface.