02.11.2023
The determination of the effective size of hydrophones is important for their calibration and application. The respective standard, which was revised with PTB participation, contains several innovations that were implemented promptly at the existing measuring station. The characterisation can now be carried out very efficiently for frequencies from 1 MHz to 50 MHz [1] and thus supports the calibration services of WG 1.62 "Ultrasound" and its users.
The impact of the effective size of the receiving area of hydrophones on the measurement of ultrasonic fields emitted for instance by medical equipment has recently gained increased attention [2]. It is expected that future editions of the measurement and safety standards will consider this aspect more intensely for the purpose of harmonisation. In order to numerically compensate for spatial averaging in hydrophone measurements, the individual hydrophone size must be determined. The recently published standard IEC 62127-3 Ed.2 [3] provides new guidelines for measurement and data analysis. As a matter of principle, the directional characteristic is now always measured in the angular range from -35° to 35° and evaluated at most up to a drop of -6 dB of the maximum received signal by least square fitting of a model function. A measurement setup that has existed at PTB for many years was adapted, extended and tested accordingly. A series of hydrophones of different designs (membrane, needle and capsule hydrophones) was investigated by measuring the directivity characteristics, using broadband ultrasonic pulse excitation [1]. By exploiting non-linear sound propagation, the evaluation and derivation of the effective receiver size for frequencies from 1 MHz to 50 MHz can be performed with high frequency resolution from measurements with only one excitation signal. Where available, the results were compared with older results from the previous measuring station, with which the determination was possible up to a maximum of 20 MHz and only at a few frequency points, and was also considerably more time-consuming. The range above 20 MHz could so far only be determined by extrapolation (Figure). For membrane hydrophones, monotonically decreasing results are obtained, each approaching a limit value, and the deviations between older and current data remain below ±10 µm. For the other hydrophone types, less uniform progressions and partly somewhat larger differences occur. Due to the higher frequency resolution, the new characterisation now contains details that could not be detected before. Overall, the normative changes, especially the specification of the angular range of the directivity data to be used, should lead to a better harmonised hydrophone characterisation and thus support the improved compensation of spatial averaging in ultrasound output measurements on medical devices.
Figure: Measured directional response of a membrane hydrophone (PTB 42) (left) und effective receiving radius for three different membrane hydrophones in dependence on the ultrasonic frequency (right).
Literature:
[1] V. Wilkens and M. Weber, Determination of effective hydrophone sizes from 1 to 50 MHz according to IEC 62127-3:2022 using short pulse excitation, Proc. 2023 IEEE Ultrason. Symp., Montreal, Canada, (2023), Paper 1077. 
confcats-event-sessions.s3.amazonaws.com/ius23/posters/1077.pdf
[2] G. R. Harris S. M. Howard, A. M. Hurrell, P. A. Lewin, M. E. Schafer, K. A. Wear, V. Wilkens, and B. Zeqiri, Hydrophone measurements for biomedical ultrasound applications: A review, IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 70, no. 2, pp. 85–100, Feb. 2023. ieeexplore.ieee.org/document/9913943
[3] Ultrasonics—Hydrophones—Part 3: Properties of Hydrophones for Ultrasonic Fields, International Standard IEC 62127-3, Ed. 2, 2022.
Contact:
Volker Wilkens, FB 1.6, E-Mail: 
volker.wilkens(at)ptb.de