Solid Micro Horn Array (SMIHA) for Acoustic Matching S. Sherrit, X. Bao, and Y. Bar-Cohen, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA ABSTRACT Transduction of electrical signals to mechanical signals and vice-versa in piezoelectric materials is controlled by the material coupling coefficient. In general in a loss-less material the ratio of energy conversion per cycle is proportional to the square of the coupling coefficient. In practical transduction however the impedance mismatch between the piezoelectric material and the electrical drive circuitry or the mechanical structure can have a significant impact on the power transfer. In this paper a novel method of matching the acoustic impedance of structures to the piezoelectric material are described and discussed in relation to the objective of increasing power transmission and efficiency. In typical methods the density and acoustic velocity of the matching layer is adjusted to give “ideal” matching between the transducer and the load. The approach discussed in this paper utilizes solid micro horn arrays in the matching layer which channel the stress and increase the strain in the layer. This approach is found to have potential applications in energy harvesting, medical ultrasound and in liquid and gas coupled transducers. KEYWORD: piezoelectric devices, acoustic wave, Acoustic matching, backing layers, impedance, power transmission. 1. INTRODUCTION Piezoelectric transducers are used for emission and reception of ultrasonic signals in medical diagnosis and treatment, nondestructive evaluation, energy harvesting, active structural damping, and many other applications. In this paper, a novel method of producing highly efficient ultrasound transducer using impedance matching layers that consist of an array of micro-horns sandwiched between two layers is presented. This design combines the use of horns that are applied in high power ultrasonics to enhance the displacement of a surface and micromachining to produce highly efficient ultrasound transducers that can be tailored to match a wide variety of acoustic loads. For example, in medical ultrasound the acoustic impedance of the transducer is approximately 20-25 times greater than the acoustic impedance of human tissue and therefore the pressure transmission coefficient is only about 8 percent. The use of a matching layer is one of the most critical elements in the proper design and manufacturing of ultrasonic transducers. Our approach allows for the design and manufacturing of matching layers that can be produced by variety of proven industrial processes using well defined materials with known mechanical properties. Prior techniques to increase the acoustic transmission include the use of ¼ wave matching layers [1], [2], graded layers [3] composite material matching layers [4], and piezoelectric/polymer composites [5], [6]. The micro-horn array for the impedance matching layer can have a variety of cross sections including tapered shape, such as stepped, linear, exponential, parabolic, and Fourier, and of the order of 10’s to 100’s of microns long. They can be laid out in a two dimensional array (regular or randomized) and are connected between two plates of the order of tens of microns thick. This sandwich structure is then bonded to a piezoelectric transducer. The theoretical models suggest that by tailoring the various features of the horn (e.g. cross sectional area profile, length, plate thickness and material choice for the horns, back plate and face plate) one can produce transducers with near perfect matching to the acoustic load and power transmission efficiencies of 90% or greater with very large bandwidths. Other features of these matching layers are that their design is amenable to MEMS fabrication techniques. These techniques can include silicon integration and micromachining, tape casting, laser micro-machining and multilayer co-fired fabrication techniques. They may also be integrated into 1D and 2D array transducers. By adjusting the horn distribution density one may also be able to pulse shape the acoustic signal (e.g. Gaussian rather than Piston) to produce a desired Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2008, edited by Masayoshi Tomizuka, Proc. of SPIE Vol. 6932, 69322X, (2008) 0277-786X/08/$18 · doi: 10.1117/12.776384 Proc. of SPIE Vol. 6932 69322X-1 2008 SPIE Digital Library -- Subscriber Archive Copy