An acoustic filter based on layered structure

Abstract

Acoustic filters (AFs) are key components to control wave propagation in multi-frequency systems. We present a design which selectively achieves acoustic filtering with a stop band and passive amplification at the high-and low-frequencies, respectively. Measurement results from the prototypes closely match the design predictions. The AF suppresses the high frequency aliasing echo by 14.5 dB and amplifies the low frequency transmission by 8.0 dB, increasing an axial resolution from 416 to 86 lm in imaging. The AF design approach is proved to be effective in multifrequency systems. systems. Specifically in multi-frequency vibration systems, delicate wave propagation control is needed to make sure of the soundness and efficiency of the system. One approach towards the wave propagation control is using acoustic filters (AFs) made of polarization-patterned piezoelectric solids 6 or periodic structures, namely, phononic crystals.7-9 Alternative methods include acoustic diodes/switches, 10-12 which combine periodic structures and nonlinear medium. 13-16In multi-frequency acoustic systems, the wave sources and propagation are typically spatial-asymmetric. For instance, in dual frequency ultrasound transducers 4,17,18 that showed promising results for super-harmonic microscopy, 19 fundamental, and harmonic frequency ultrasound waves propagate in opposite directions. It is necessary that the low frequency (LF) wave propagates forward efficiently, while the harmonic high frequency (HF) wave does not propagate backward after reaching the high frequency active layer ( Fig. 1(a)). 20 However, few of the present wave control methods have been adapted to multi-frequency ultrasound transducers, due to their bulky size (multiple wavelengths in dimension) or their high cost resulting from very fine microstructures. Based on the space-frequency correlation, an AF can act as a unidirectional switch that allows acoustic penetration in one direction while blocking it in the reverse direction. In this approach, all elements in the system are highly coupled and the design must be undertaken at the system level.In this letter, we elucidate an AF design with an antimatching effect for a bandstop of HF ultrasound and a passive amplifier effect for transmission enhancement of LF ultrasound. Inspired by earlier work, 21-24 we applied microwave transmission line theory 25 to guide the AF design. The goal of the AF design is to prevent the HF wave from backward propagation while passing the forward LF wave efficiently ( Fig. 1(a)). The transmission line equivalent circuit of the system is schematically shown in Figs. 1(b) and 1(c).In a dual frequency system, acoustic structural design for HF acoustic waves is usually more sensitive than that for LF waves due to the wavelength difference. Therefore, our strategy is to first design the HF bandstop filter and subsequently design the LF passive amplifier.The HF bandstop filter could be designed based on wave reflection (type I) or wave absorption (type II) mechanisms. For the type I filter design,...

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