In this Letter, a low-frequency acoustic sensor based on an extrinsic Fabry–Pérot (FP) interferometer with a silicon nitride (Si₃N₄) membrane is demonstrated. Using micromachining techniques, the 800 nm thick Si₃N₄ membrane is deposited on an 8 mm × 8 mm × 400 μm silicon (Si) substrate. All the assembly procedures of the sensor are focused on the substrate to avoid any damage to the membrane itself, compared to general membrane transfer methods. The frequency response of the proposed sensor is discussed theoretically and experimentally demonstrated. The sensor exhibits an excellent flat response to the tested acoustic frequency range of 1 Hz to 250 Hz. The phase sensitivity is around ?152 dB re 1 rad/μPa with sensitivity fluctuation less than 0.8 dB. The frequency response characteristic shows a promising potential of the sensor in low-frequency acoustic signal sensing applications.