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Magnitude and phase characterisation of a reference broadband membrane hydrophone up to 100 MHz using nonlinear field modelling.

Gélat, P N; Wright, L; Bickley, C J; Sadhoo, N (2007) Magnitude and phase characterisation of a reference broadband membrane hydrophone up to 100 MHz using nonlinear field modelling. NPL Report. AC 3

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Abstract

A feasibility study aimed at assessing and improving a hydrophone calibration method employing a combination of ultrasonic field measurement and nonlinear field prediction has been carried out. The magnitude and relative phase characteristics of a broadband membrane hydrophone were obtained between 5 MHz and 100 MHz by comparing its open-circuit output voltage with the ultrasonic pressure at the hydrophone location in a harmonically rich field, theoretically predicted using the KZK equation. The methodology employed differs from previous work (Cooling and Humphrey 2007, Bleeker and Lewin 2000) in that prior knowledge of the hydrophone sensitivity between 5 MHz and 40 MHz is assumed and that the predicted field is optimised against the measured field over this frequency range by varying the source and medium parameters. The predicted hydrophone magnitude response shows consistency within ±10% at transducer-hydrophone separations between 90 mm and 180 mm.
The radial distribution of the ultrasonic field has been mapped over the radius of the hydrophone active element at selected axial positions, hence providing further insight into effects of spatial averaging due to finite hydrophone element receiving area.
3.5 MHz fundamental frequency checksource waveforms were acquired using both the broadband membrane hydrophone characterised in this study and a laser interferometer. At an axial position where the frequency content of the acoustic pressure is within the bandwidth of the interferometer (51.5 mm), agreement for p+ and p- is within 3% and better than 1% for Ispta and td. At transducer-hydrophone separations where the frequency content of the acoustic pressure extends beyond the bandwidth of the interferometer (80 mm), p+ appears underestimated by 14% in the interferometer measurement. This highlights the need for better measurement capability of laser interferometry beyond 50 MHz.
This method shows promise in assisting the development of laser interferometry beyond 50 MHz, aiding mapping of transducers fields in the axial and radial directions and estimating the magnitude and phase characteristics of a test hydrophone by using the characterized hydrophone as the reference device.

Item Type: Report/Guide (NPL Report)
NPL Report No.: AC 3
Subjects: Acoustics
Acoustics > Underwater Acoustics
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/4025

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