Dissemination of the Acoustic Pascal: The Role and Experiences of a National Metrology Institute

Abstract

Hydrophones are pivotal measurement devices ensuring medical ultrasound acoustic exposures comply with the relevant national and international safety criteria. These devices have enabled the spatial and temporal distribution of key safety parameters to be determined in an objective and standardized way. Generally based on piezoelectric principles of operation, to convert generated voltage waveforms to acoustic pressure, they require calibration in terms of receive sensitivity, expressed in units of V•Pa −1 . Reliable hydrophone calibration with associated uncertainties plays a key role in underpinning a measurement framework that ensures exposure measurements are comparable and traceable to internationally agreed units, irrespective of where they are carried out globally. For well over three decades, the United Kingdom National Physical Laboratory (NPL) has provided calibrations to the user community covering the frequency range 0.1 -60 MHz, traceable to a primary realization of the acoustic pascal through optical interferometry. Typical uncertainties for sensitivity are 6% to 22% (for a coverage factor k = 2), degrading with frequency. The article specifically focusses on dissemination of the acoustic pascal through NPL's calibration services which are based on comparison with secondary standard hydrophones previously calibrated using the NPL primary standard. The work demonstrates the stability of the employed dissemination protocols by presenting representative calibration histories on a selection of commercially available hydrophones. Results re-affirm the guidance provided within international standards for regular calibration of a hydrophone in order to underpin measurement confidence. The process by which internationally agreed realizations of the acoustic pascal are compared and validated through Key Comparisons, is also described.

Characterisation of hydrophone sensitivity with temperature using a broadband laser-generated ultrasound source

“…transmission of the pellicle, interferometer laser spot size, etc) to be known at each temperature the hydrophone is calibrated (IEC 62127-2:2007 Annex F [2]). For this reason, most primary hydrophone calibrations are performed in a temperature controlled environment close to 20 • C. Other secondary hydrophone calibration techniques based on the substitution or comparison principle [14] will comprise the difficulty of the temperature dependence of the source transducer sensitivity, and may provide only relative differences between the substituted hydrophones when applied to investigations of temperature dependency.…”