Low frequency Piezoelectric Micromachined Ultrasonic Transducers optimized for concrete structures.

Embedded sensors operating within a reinforced concrete structure enable the timely detection of structural degradation causes, such as chloride ions present in the pore solution. The best communication protocol to use for inter‐device communication within the concrete structure would ideally be wireless such as an ultrasonic system. Concrete is a composite material which imposes constraints on ultrasonic transmission, especially when operating at frequencies above 100kHz where incident radiation is scattered by the aggregate. Furthermore, for an effective coupling mechanism, a liquid with high acoustic impedance is required to reduce energy reflection at the interface. This paper outlines the design, construction, and characterisation of a range of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) which are specifically designed to operate in liquids, at frequencies near or below 100kHz. The PMUT devices described in this paper have diaphragm diameters ranging between 550μm and 2,000μms which sizes are needed for PMUTs to resonate at the required low frequency. This paper outlines the device geometry calculations and the design and fabrication of the prototype devices. The resonant frequencies were determined using a laser vibrometer.