Directivity optimization of MEMS ultrasonic transducers by implementing acoustic horns

The applications of microscopic ultrasonic transducers are often limited due to their non-optimal directivity pattern. The aim of this work is to design passive structures to adapt the directivity pattern to an intended application, e.g. range finder. The horns were designed and optimized using a Finite-Element-Method (FEM) model and manufactured by a conventional 3d-printing technique. The experimental validation was done using a novel MEMS-based ultrasonic transducer based on lateral actuation developed by Fraunhofer IPMS. This transducer type generates sound waves by displacing air inside a 3x3 mm² silicon chip using microscopic sized beams instead of using a diaphragm. This article presents several horn structures that exhibit a pronounced main lobe, inter alia an exponential horn and a folded horn with reduced overall dimensions, both optimized for an operating frequency of 40 kHz. We have shown numerically and experimentally that directivity properties of the transducer were significantly improved considering peak pressure, reduction of the side lobes and main lobe width by simple horn geometries. The implementation of the designed horns will enable additional applications where a specific directivity pattern is required. Furthermore, the given results imply that the presented design strategies can be used to create various directivity patterns.