Since the discovery of the unique auditory system of the Ormia ochracea fly, researchers have designed various directional acoustic sensors inspired by its principles. However, most of these sensors operate only within a single- or dual-frequency band and typically exhibit high eigenfrequencies, making them unsuitable for low-frequency applications. This paper proposes a low-frequency, multi-band piezoelectric MEMS acoustic sensor that incorporates an improved coupling structure within the inner diaphragm to enable low-frequency signal detection in a compact design. Additionally, an asymmetric wing and coupled structure are introduced in both the inner and outer diaphragms to achieve multi-band frequency response. Aluminum nitride (AlN), a material with low dielectric and acoustic losses, is selected as the piezoelectric material. The sensor operates in the d₃₃ mode and employs a branched comb-like interdigitated electrode design to enhance the signal-to-noise ratio (SNR). Simulation results demonstrate that the four eigenfrequencies of the sensor are evenly distributed below 2000 Hz, and at all eigenfrequencies, the sensor exhibits a consistent cosine response to variations in the incident elevation angle of the sound source.