Sound Pressure and Bandwidth Enhanced PMUT with Volume Controllable Helmhotz Resonator for Respiratory Monitoring

This paper presents a piezoelectric micromachined ultrasonic transducer (PMUT) with volume-controllable Helmholtz resonator. The PMUT is fabricated with a bottom up technology. The active piezoelectric film of PMUT array is constructed on a titanium foil. The SU8 plate with a photolithography-defined square opening and the 3D stereolithography printed hollow base form the Helmholtz resonator and determine the boundary conditions of the PMUT. The novelties of this PMUT include (1) the thin SU8 plate functions as the neck portion of Helmholtz resonator; (2) the 3D printed base integrated with a threaded insert nut and a precision set screw forms the volume-controllable cavity of Helmholtz resonator. Experimental results exhibit that the sound pressure level (SPL) and operation bandwidth could be effectively tuned. The enhancement of 200% SPL increase and 2-fold bandwidth are achieved while setting the cavity length at 0.75mm compared with the open-cavity setting. The innovative wearable module integrated with the developed PMUT and MEMS microphone is also demonstrated. The wearable module successfully verifies the functionality of monitoring human respiratory patterns.