Low-Frequency High-Directional Second-Order Electrochemical Vector Hydrophones

This paper presents two low-frequency high-directional electrochemical second-order hydrophones based on the velocity gradient pure partial derivative (VGP) and the velocity gradient mixed partial derivative (VGM) principles. Both consist of two independently vibrating first-order hydrophones, which are characterized by sensitivity of -180.12 dB (ref. 1 V/ μPa) within the 20- 400 Hz frequency range. The device is capable of simultaneously measuring first-order and second-order quantities through different signal processing methods. The first-order in-band sensitivity is -175.46 dB (ref. 1 V/ μPa), which is 4.66 dB higher than the sensitivity of a single first-order hydrophone. The sensitivity of the VGP hydrophone is -196.41 dB@400 Hz (ref. 1 V/ μPa), while the sensitivity of the VGM hydrophone is -201.83 dB@400 Hz (ref. 1 V/ μPa). The sensitivity of the second-order hydrophone shows an overall increasing trend of 6 dB per octave, with the VGP being 5.42 dB higher than the VGM. Compared to first-order hydrophones with 90° beamwidths and “8”-shaped directivities, the VGP hydrophone demonstrates a narrower “8”-shaped directivity with a beamwidth of 60°, while the VGM hydrophone features a “four-leaf” directivity with a beamwidth of 40°/30°. The results demonstrate that the directivity performances of second-order hydrophones are significantly superior to that of first-order hydrophones, implying higher positioning accuracy and greater significance for underwater target detection.