Abstract
Fabry-Pérot ultrasonic metamaterials have been additively manufactured using laser powder bed fusion to contain subwavelength holes with a high aspect-ratio of width to depth. Such metamaterials require the acoustic impedance mismatch between the structure and the immersion medium to be large. It is shown for the first time that metallic structures fulfil this criterion for applications in water over the 200–800 kHz frequency range. It is also demonstrated that laser powder bed fusion is a flexible fabrication method for the ceration of structures with different thicknesses, hole geometry and tapered openings, allowing the acoustic properties to be modified. It was confirmed via both finite element simulation and practical measurements that these structures supported Fabry-Pérot resonances, needed for metamaterial operation, at ultrasonic frequencies in water. It was also demonstrated the the additively-manufactured structures detected the presence of a sub-wavelength slit aperture in water.
Highlights
Acoustic metamaterials are those that have properties that arise from their inner structure and which are not ordinarily observed
Based on Eqn (1), and considering the effect of end correction, the fundamental FP resonance peak is expected to occur at a frequency (f1) of 82.9 kHz, which is outside the −6 dB bandwidth of the transmitter
This paper has demonstrated for the first time that metallic ultrasonic metamaterials can be fabricated to operate in water at the 200–800 kHz frequency range, important for many applications such as non-destructive evaluation and medical imaging
Summary
Acoustic metamaterials are those that have properties that arise from their inner structure and which are not ordinarily observed. Extraordinary effects can occur when acoustic signals are transmitted through multiple holes in a solid plate. This can occur when the diameter of the holes is less than the acoustic wavelength and when the hole exhibits an acoustic resonance [1]. If an array of such holes are placed side-by-side, an acoustic metamaterial is created with some novel properties such as subwavelength acoustic imaging [3]. This is made possible by coupling the evanescent waves which exist within the fluid close to the sample and the FP resonance within each hole
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