Design and Experimental Study of Blade-Type Extremely Low Frequency Underwater Sound Source System

To address the inherent limitations of traditional resonant underwater sound sources—specifically their excessive size and the sharp decline in sound pressure level (SPL) at frequencies below resonance—a blade-type extremely low frequency (<30 Hz) underwater sound source system was developed. The structure of the system and the principles of its coupled rotational and oscillatory motions are elucidated. The kinematic equations of the blade oscillation driven by a combined crank-connecting rod-slider-connecting rod-rocker mechanism are established, and the effects of the impeller rotational speed and the blade oscillation speed on the SPL are analytically evaluated. The prototype was tested in both air and underwater environments. The experimental results show that the blade-type sound source can emit acoustic waves of extremely low frequency ranging from 2 Hz to 7 Hz, with the fundamental frequency of the acoustic waves perfectly aligned with the blade oscillation frequency. Increasing rotational speed more strongly affects the SPL of harmonics, while changing the oscillation frequency has a greater effect on the fundamental. Notably, a steady increase in the SPL in this paper is maintained even when the oscillation-to-rotation frequency ratio exceeds 2, while the existing blade sound source gradually decreases when the ratio exceeds 1/2. With blade dimensions as small as 13 cm in length and 9 cm in width, the SPL at 10 Hz reaches 131.77 dB (re 1 μPa), highlighting its potential for high-performance acoustic radiation applications at an extremely low frequency.