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On the importance of consistent insonation conditions during hydrophone calibration and use.

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 artefacts. This means 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 1 mm to 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). In UWA tone burst acoustic fields are used while in US it is short pulses. The 2 mm 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 kHz to 450 kHz, but which was absent in US calibration. This observed behaviour 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 the method of calibration is carefully considered in the context of the duration of the acoustic signals for which the hydrophone is intended.