A substitution calibration technique for piezoelectric ultrasonic hydrophones is presented that uses an optical multilayer hydrophone as the reference receiver. Broadband nonlinearly distorted focused pulses are first measured with the reference hydrophone and then with the hydrophone to be calibrated. By Fourier transformation of the time wave forms and division of the frequency spectra, the complex-valued frequency response of the hydrophone under test is obtained in a broad frequency range in a very fast and efficient way and with high frequency resolution. The results obtained for a membrane hydrophone and a needle-type hydrophone are compared with those obtained by independent calibration techniques such as primary calibration using optical interferometry and secondary calibration using time-delay spectrometry, and good agreement is found. The calibration data obtained are apt to improve the results of ultrasound exposure measurements using broadband voltage-to-pressure conversion. This is demonstrated for standard pulse parameter determination from exemplar exposure measurements on a commercial diagnostic ultrasound machine. For the membrane hydrophone, the evaluation method commonly used leads to an overestimation of the positive peak pressure by up to 50%, an underestimation of the rarefactional peak pressure by up to 11%, and an overestimation of the pulse intensity integral by up to 28%.

Amplitude and phase calibration of hydrophones up to 70 MHz using broadband pulse excitation and an optical reference hydrophone

A primary calibration method for ultrasonic hydrophones which uses a Michelson interferometer to determine the particle displacement in an ultrasonic field is discussed The acoustic pressure is derived from this measurement and used to determine the free-field sensitivity of a hydrophone in the frequency range 05-15 MHz The random uncertainty of the method is typically 1%, whereas the systematic uncertainty varies from 23 to 66% over the frequency range To obtain this accuracy, the performance of the system has been carefully examined and appropriate correction factors derived The greatest difficulty in the method lies in determining the frequency response of the optical detection system, and two different approaches have been used to measure this response Several acoustical effects have also been studied and the calibration procedure modified to take account of them The calibration results are in agreement with those of other methods and with the theoretically predicted frequency response of a hydrophone The method has been used to determine the temporal stability of a hydrophone over a period of two years >

Primary calibration of ultrasonic hydrophone using optical interferometry

A efficient time-domain algorithm, based on the spatial pulse response approach, is proposed for the determination of the acoustic fields radiated by means of acoustical sources. The computations are performed by the discrete representation array modelling (DREAM) procedure, specially adapted to study the planar and arbitrarily structured multielement transducer arrays. DREAM, based on the discrete representation computational concept, acts as the generator of the array velocity potential impulse response, and thus, does not require any analytical solutions prior to the computations. The computations are valid for all field regions and may be performed for any excitation form. Apart from the classic case of rigid baffle conditions, the free and soft planar baffle also can be considered. The use of the time-domain solution for causal Green's function for lossy media enables the wideband absorption effects to be modeled. The accuracy of computations depends on temporal and spatial discretization and can be obtained as required. The quantitative rules, which determine the required discretizations to be predicted, are proposed. The computational examples show that DREAM allows the different and various transducers to be modeled. Its possibilities are illustrated by computations for the multielement transducers, including the beam-steered, amplitude-weighted sonar array, the focusing annular transducer, and the diverging and converging cylindrical array.

A new approach to calculate the field radiated from arbitrarily structured transducer arrays

Calibration and measurement issues for therapeutic ultrasound.

A technique for the primary calibration of hydrophones based on an optical principle is presented. An interferometer determines the displacement of a pellicle mounted on the surface of the tank fluid perpendicular to the sound propagation axis; the known acoustic field is then measured using the hydrophone. The application range extends from 1 to 50 MHz and, with a lesser accuracy, to 70 MHz. The loaded and the open-circuit sensitivity are obtained pointwise, and the frequency and the number of the measurement points can be arbitrarily chosen. As an example, two membrane hydrophones with a spot diameter of 0.5 mm are calibrated, and the comparison with a theoretical model describing the acoustic and electrical properties of the hydrophone shows good agreement.

Primary calibration of hydrophones with extended frequency range 1 to 70 MHz using optical interferometry

The results of shock-wave measurements using a calibrated fiber-tip sensor based on a Michelson interferometer are presented. A transfer function, obtained by an independent experiment that describes the properties of the sensor system, was used to correct the measured shock-wave data in the Fourier frequency domain. The phase of the transfer function was determined from its amplitude by a fitting procedure using minimum-phase terms. As an example of application, the acoustic output field of an electromagnetic lithotriptor was investigated, and the shock-wave source was reliably characterized. The measured data provide a basis for estimating the hazard to which a patient is exposed during shock-wave treatment and for optimizing a lithotriptor system to produce a sharply localized and effective acoustic field.

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R. Reibold

Papers

Shock-wave measurement using a calibrated interferometric fiber-tip sensor

Reconstruction of ultrasonic fields by deconvolving the hydrophone aperture effects: I. Theory and simulation

Determination of the Klein-Cook parameter in ultrasound light diffraction

Uncertainty considerations of ultrasonic field mapping by light-diffraction tomography

Radiation of a rectangular strip-like focussing transducer