Acoustic metamaterials are large-scale materials with small-scale structures. These structures allow for unusual interaction with propagating sound and endow the large-scale material with exceptional acoustic properties not found in normal materials. However, their multi-scale nature means that the manufacture of these materials is not trivial, often requiring micron-scale resolution over centimetre length scales. In this review, we bring together a variety of acoustic metamaterial designs and separately discuss ways to create them using the latest trends in additive manufacturing. We highlight the advantages and disadvantages of different techniques that act as barriers towards the development of realisable acoustic metamaterials for practical audio and ultrasonic applications and speculate on potential future developments.
Quarter wavelength acoustic impedance matching layers (IMLs) are often attached to an ultrasonic transducer (UT) to enhance energy transmission into a load medium. Natural materials are rarely suitable as an IML but even composites with a suitable impedance can be excessively attenuating, or unsuitable for UT mounting. It would be beneficial then, if a non-absorptive IML with the specified impedance could be manufactured using common fabrication tools. Recent publications have shown that phononic crystal (PC) structures, made using modern printing and curing technology, can operate as IMLs and may satisfy these criteria. It is important to continue this research because it has the potential to improve UT performance, and simplify or surpass existing solutions. Here, finite element analysis (FEA) is used to validate a hydrogel-matrix / steel-inclusion PC IML that separates PZT and water domains. The PZT boundary is excited over a frequency range and the resultant in water pressure is averaged. Results are compared with an ideal, bulk IML and common, in practice materials. Comparison of spectra demonstrate that the PC increases in water pressure at the expected frequency, is an effective medium in the long wavelength regime, and is comparable to the ideal IML.
The impedance matching layer has a critical effect on ultrasonic transducer performance, but it is difficult to source materials that have the appropriate acoustical properties. A method that utilises effective property relations of composites and finite element analysis is used to design a hydrogel-steel based phononic crystal, quarter wavelength impedance matching layer that can match bespoke configurations. Phononic crystal band structures are calculated to determine an appropriate lattice scale length, and frequency domain studies are carried out to compare this novel type of matching layer with an ideal bulk layer. Transmitted pressure curves are as expected and suggest that this design type will be suited for fabrication and testing.
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