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.

The flextensional class V ‘cymbal’ transducer has been widely adopted for low power ultrasonics applications, exhibiting high output displacement for low input energy, compared to a single ceramic, when used as an actuator. Despite its performance benefits, the original designs of cymbal transducers have inherent drawbacks for high power ultrasonics applications that require much higher output displacements. Asymmetries introduced during the fabrication process reduce the efficiency of the transducer, and degradation of the bonding layer between the end-caps and the electroactive material can alter the vibration response and ultimately lead to failure. A new design of the cymbal transducer is therefore proposed that delivers high output displacements. A comparison is presented between a cymbal based on the original design configuration and a new cymbal, to demonstrate the effects of input voltage levels on the dynamic characteristics and vibration response of the two different transducers. For the first cymbal the end-caps are directly bonded to the piezoceramic disc using a commercial non-conductive epoxy. The second cymbal incorporates a metal ring bonded to the outer edge of the piezoceramic disc to improve the mechanical coupling with the end-caps, thereby enhancing the operational capability of the device at higher voltages, allowing for excitation of higher output displacements by removing the problems associated with failure in the epoxy layer. This design is demonstrated to be particularly suitable for power ultrasonics applications such as miniature surgical devices, for example as drilling and cutting devices for orthopaedics procedures

An ultrasonic orthopaedic surgical device is presented, where the ultrasonic actuation relies on a modification of the classical cymbal transducer. All current devices consist of a Langevin ultrasonic transducer with a tuned cutting blade attached, where resonance is required to provide sufficient vibrational amplitude to cut bone. However, this requirement restricts the geometry and offers little opportunity to propose miniaturised devices or complex blades. The class V flextensional cymbal transducer is proposed here as the basis for a new design, where the cymbal delivers the required vibrational amplitude, and the design of the attached cutting insert can be tailored for the required cut. Consequently, the device can be optimised to deliver an accurate and precise cutting capability. A prototype device is presented, based on the cymbal configuration and designed to operate at 25.5kHz with a displacement amplitude of 30μm at 300V. Measurements of vibrational and impedance responses elucidate the mechanical and electrical characteristics of the device. Subsequent cutting tests on rat femur demonstrate device performance consistent with a commercial Langevin-based ultrasonic device and show that cutting is achieved using less electrical power and a lower piezoceramic volume. Histological analysis exhibits a higher proportion of live cells in the region around the cut site for the cymbal device than for a powered sagittal or a manual saw, demonstrating the potential for the ultrasonic device to result in faster healing.

A prototype CubeSat module to deploy a gossamer aerobrake, using strain stored in tape-springs, at end-of-life is described. A novel hub geometry to reduce bending shock at end-of-deployment while simultaneously permitting radial, as opposed to tangential, deployment is proposed. The rpm of the hub is measured under various deployment conditions to verify that the system offers highly-repeatable performance, while high-speed photography is used to characterise the behaviour of the tape-spring during unspooling and contrast it to the behaviour of a traditional tangential-deployment system. Secondly the folding pattern of the membrane, which takes advantage of the symmetrical deployment offered by the petal hub, is developed and the unfolding mechanism is verified by numerical and experimental analysis. Finally, the release of the stored strain is considered and a novel burn-though device is designed and prototyped to meet this requirement

Flexural ultrasonic transducers are robust and low cost sensors that are typically used in industry for distance ranging, proximity sensing and flow measurement. The operating frequencies of currently available commercial flexural ultrasonic transducers are usually below 50 kHz. Higher operating frequencies would be particularly beneficial for measurement accuracy and detection sensitivity. In this paper, design principles of High Frequency Flexural Ultrasonic Transducers (HiFFUTs), guided by the classical plate theory and finite element analysis, are reported. The results show that the diameter of the piezoelectric disc element attached to the flexing plate of the HiFFUT has a significant influence on the transducer’s resonant frequency, and that an optimal diameter for a HiFFUT transmitter alone is different from that for a pitch-catch ultrasonic system consisting of both a HiFFUT transmitter and a receiver. By adopting an optimal piezoelectric diameter, the HiFFUT pitch-catch system can produce an ultrasonic signal amplitude greater than that of a non-optimised system by an order of magnitude. The performance of a prototype HiFFUT is characterised through electrical impedance analysis, laser Doppler vibrometry, and pressure-field microphone measurement, before the performance of two new HiFFUTs in a pitch-catch configuration is compared with that of commercial transducers. The prototype HiFFUT can operate efficiently at a frequency of 102.1 kHz as either a transmitter or a receiver, with comparable output amplitude, wider bandwidth, and higher directivity than commercially available transducers of similar construction.

Flexural ultrasonic transducers are a widely available type of ultrasonic sensor used for flow measurement, proximity, and industrial metrology applications. The flexural ultrasonic transducer is commonly operated in one of the axisymmetric modes of vibration in the low-kilohertz range, under 50 kHz, but there is an increasing demand for higher frequency operation, towards 300 kHz. At present, there are no reports of the measurement of high-frequency vibrations using flexural ultrasonic transducers. This research reports on the measurement of high-frequency vibration in flexural ultrasonic transducers, utilizing electrical impedance and phase measurement, laser Doppler vibrometry, and response spectrum analysis through the adoption of two flexural ultrasonic transducers in a transmit-receive configuration. The outcomes of this research demonstrate the ability of flexural ultrasonic transducers to measure high-frequency ultrasound in air, vital for industrial metrology.