Rajagopal Srinath, Robinson Stephen P, Ablitt Justin, Miloro Piero, Wang Lian, Zeqiri Bajram, Hurrell Andrew
IEEE Trans Ultrason Ferroelectr Freq Control. 2023 Feb;70(2):120-127. doi: 10.1109/TUFFC.2022.3205851. Epub 2023 Feb 6.
Hydrophones are generally calibrated in acoustic fields with temporally localized (short pulse) or long duration (tone burst) signals. Free-field conditions are achieved by time gating any reflections from the hydrophone body, mounting structures, and surrounding water tank boundaries arriving at the active sensing element. Consequently, the sensitivity response of the hydrophone is a result of direct waves incident on its active element, free from any contaminating effects of reflections. However, when using tone bursts below 400 kHz to calibrate hydrophones, it may not be possible to isolate the direct wave from reflection artifacts. This means that the sensitivity responses derived at these frequencies using short pulse and tone burst signals might not be comparable as they can be characteristic of the acoustic field interaction with either/both the hydrophone active element alone or the hydrophone active element and body. Therefore, there is a need to consider an appropriate calibration method for a given hydrophone type, depending on whether the eventual application employs short pulse or tone burst acoustic fields. This article presents the findings from a short study comprising four needle-type hydrophones of active element diameters in the range of 1-4 mm. These hydrophones were calibrated from 30 kHz to 1.6 MHz using established calibration methodologies within the underwater acoustics (UWA) and ultrasound (US) areas employed at the National Physical Laboratory (NPL), Teddington, U.K. In UWA tone, burst acoustic fields are used, while in US, it is short pulses. The 2- and 4-mm-diameter needle hydrophones showed the largest variation at the overlapping frequencies, in which the maximum disagreement of UWA calibration was 30% relative to US calibration. For the 4-mm hydrophone, UWA calibration exhibited resonant sensitivity structure between 100 and 450 kHz, but which was absent in US calibration. This observed behavior was further investigated theoretically by using a validated acoustic wave solver to confirm the resonant sensitivity structure seen in the case of UWA calibration. The work contained within illustrates the need to ensure that the method of calibration is carefully considered in the context of the duration of the acoustic signals for which the hydrophone is intended.
水听器通常在具有时间局部化（短脉冲）或长时间持续（短纯音脉冲串）信号的声场中进行校准。通过对来自水听器主体、安装结构以及周围水箱边界并到达有源传感元件的任何反射进行时间选通来实现自由场条件。因此，水听器的灵敏度响应是直接波入射到其有源元件上的结果，不受任何反射污染效应的影响。然而，当使用低于400kHz的短纯音脉冲串来校准水听器时，可能无法将直接波与反射伪像分离。这意味着在这些频率下使用短脉冲和短纯音脉冲串信号得出的灵敏度响应可能不可比，因为它们可能是声场与单独的水听器有源元件或水听器有源元件和主体相互作用的特征。因此，需要根据最终应用是采用短脉冲还是短纯音脉冲串声场，为给定类型的水听器考虑一种合适的校准方法。本文介绍了一项简短研究的结果，该研究包括四个有源元件直径在1 - 4mm范围内的针型水听器。这些水听器使用英国特丁顿国家物理实验室（NPL）采用的水下声学（UWA）和超声（US）领域既定的校准方法，在30kHz至1.6MHz范围内进行校准。在UWA中使用短纯音脉冲串声场，而在US中使用短脉冲。直径为2mm和4mm的针型水听器在重叠频率处显示出最大差异，其中UWA校准相对于US校准的最大差异为30%。对于4mm的水听器，UWA校准在100至450kHz之间呈现出共振灵敏度结构，但在US校准中不存在。通过使用经过验证的声波求解器从理论上进一步研究了这种观察到的行为，以确认在UWA校准情况下看到的共振灵敏度结构。本文所包含的工作表明，需要确保在校准方法的选择上，要根据水听器预期使用的声学信号的持续时间仔细考虑。
