This disclosure describes a sensor for detecting vibrations, particularly low-frequency vibrations. The sensor is of a cascaded gapped cantilever construction and may include a single sensing beam extending from a base to a proof mass across the plurality of gaps, a mechanical beam opposite the piezoelectric beam. The sensor may be included in a system including a housing, a support, and electronic components for converting and measuring signal. The sensor may be mounted to an item of furniture, such as a bed or a sofa, and measure physiological data including ballistocardiogram and respiratory data of a subject positioned on the furniture.