Search

Achieving sufficient energy transmission over a wide frequency range is a challenge which has restricted the application of many types of air-coupled ultrasonic transducers. Conventional transducer configurations such as the piezoelectric micromachined or flexural ultrasonic transducers can be considered as narrowband. This study reports a type of ultrasonic transducer, the wideband electromagnetic dynamic acoustic transducer (WEMDAT), which operates through a combination of electromagnetic induction and Lorentz force with dynamic behaviour of a micro-scale-thick conductive film. WEMDAT prototypes have been designed, fabricated, and tested, showing their compatibility with both low and high power inputs, operating efficiently as a wideband transmitter from 46.4 kHz to 144.6 kHz with a good directivity. The WEMDAT has also been shown to operate effectively as a wideband ultrasonic receiver through the measurement in a pitch-catch configuration. The WEMDAT prototypes possess an adjustable drive coil lift-off distance from the active membrane, providing flexibility for optimizing the sensitivity of the transducers for different input levels. The performance of the WEMDATs can be optimized, showing significant potential for air-coupled ultrasonic applications. [ABSTRACT FROM AUTHOR]

A tunable isotropic negative effective permeability is experimentally demonstrated in a three-dimensional (3D) dielectric composite consisting of dielectric ceramic cube arrays by temperature changing. It shows that a strong subwavelength magnetic resonance can be excited in dielectric cubes corresponding to the first Mie resonance mode and can be continuously and reversibly adjusted from 13.65 to 19.28 GHz with the temperature changing from -15 to 35 °C. Accordingly, negative permeability can be performed in the frequency range of about 6 GHz by adjusting the temperature. It provides a convenient route to design adaptive metamaterials and 3D invisible cloak. [ABSTRACT FROM AUTHOR]

An electrically tunable negative permeability metamaterial consisting of a periodic array of split ring resonators infiltrated with nematic liquid crystals is demonstrated. It shows that the transmitted resonance dip of the metamaterial can be continuously and reversibly adjusted by an applied electric field, and the maximum shift is about 210 MHz with respect to the resonance frequency around 11.08 GHz. Numerical simulation shows that the permeability is negative near the resonance frequency, and the frequency range with negative permeability can be dynamically adjusted and widened by about 200 MHz by the electric field. It provides a convenient means to design adaptive metamaterials. [ABSTRACT FROM AUTHOR]