An affordable open-source hydrophone for low-frequency underwater acoustic measurements for educational and small-laboratory applications.

Infrasonic underwater acoustic signals (below 20 Hz) are essential for environmental monitoring, marine ecosystem studies, and evaluation of underwater robotic systems. Nevertheless, existing commercial hydrophones are primarily designed for broadband and high-frequency performance, increasing complexity and cost while limiting ultra-low-frequency stability. Additionally, rapid-prototyping hydrophone designs depend on available modules and lack standardized design guidelines, limiting further modifications. As a result, this research presents a low-cost, open-source infrasonic hydrophone (LUH) for underwater measurements, compact and portable with approximately 500 g, with a modular configuration fabricated by acrylonitrile butadiene styrene (ABS) using fused deposition modelling (FDM)- based 3D printing for rapid prototyping. Waterproofing, noise suppression, and signal transmission are achieved through multiple layers of elastic dip, copper foil, and silicone. From an electrical perspective, the LUH integrates a piezoelectric ceramic disk, high-impedance amplification, band-limited filtering, and a modular ± 12 VDC power supply within enclosures. The complete device can be fabricated within 100 h at a total cost of below 90 USD. The experimental validation demonstrates the stable detection of a 20 Hz sinusoidal signal under laboratory conditions, the 12 Hz accurate measurement using institutional-grade equipment, and the detection of a 5 Hz emission from a biomimetic fish robot at 3 m underwater. From there, these results have confirmed the reliable infrasonic sensing ability of the device in various situations.