An adjustable acoustic metamaterial cell using a magnetic membrane for tuneable resonance

One of the most exciting challenges in acoustic metamaterial (AMM) research is to incorporate tuneability, for example where the resonance of the AMM can be actively adjusted, whilst also ensuring ease of manufacture. Many of the challenges in realising physical AMM prototypes, such as ensuring small feature sizes, introducing specific material characteristics, and enabling scale-up fabrication, remain significant barriers in the design of AMMs in general. As such, the dynamic control of device response is an ongoing obstacle in the field of AMMs. One option for practical AMM implementation in the future is to mitigate some of these barriers by enabling adaptable resonant properties to tailor device response. To this end, this research demonstrates the design, fabrication, and characterisation of an adjustable magnetic AMM cell using stereolithography with a bespoke superparamagnetic resin. To underpin the experimental approach, a mathematical model able to predict the behaviour of the adjustable AMM is developed and implemented, for which the mechanical properties of the AMM membrane were experimentally measured and used to define the model. This research demonstrates a 3D-printable magnetic AMM cell able to perform sub-100 Hz acoustic resonant tuning. The developed superparamagnetic formulation recorded a magnetisation of saturation of 2.4 emu/g, coercivity of 5 mT, and a Young’s modulus of 17.89 MPa. The device is thus ultra-subwavelength (λ/77.5) and displays magnetically-actuated resonant tuneability from 88.73 Hz – 86.63 Hz with a consistent 12 dB increase in resonance.