Abstract: This study addresses the challenges in calibrating hydrophone sensitivity at low frequencies, which arise from limitations in water tank dimensions, unavoidable reflections in water pools, and superposition of reflections in the low-frequency range. Building upon the free-field comparison method, we propose a technique that fits the amplitude, frequency, and phase of residual periodic signals to lower the frequency limit for sensitivity calibration. Experimental results demonstrate that this method enables sensitivity calibration in the 2.5-20 kHz frequency range, with errors within 0.6 dB. It effectively eliminates calibration errors caused by reflected sound fields, validating its efficacy in low-frequency calibration. However, the residual period method has some practical limitations. Factors such as environmental noise, attenuation of reflected sound fields, and nonlinear effects can impact the method's accuracy. Despite these challenges, the proposed approach shows promise in expanding the frequency domain for hydrophone calibration, particularly in low-frequency ranges where traditional methods face significant obstacles.