Experimental characterization of acoustic beaming from an elastic plate by coupled symmetric leaky Lamb modes

Designer mode dispersions with properties such as negative or zero group velocities have applications in tailored wave propagation and energy redistribution. The modes of elastic plates can readily exhibit such phenomena by virtue of their bulk elastic properties and thickness. Here, we investigate the symmetric leaky Lamb modes within a thin flat aluminum-alloy plate submerged in water using pulsed broadband ultrasound, and experimentally characterize acoustic beaming from the plate caused by resonant transmission, which is enhanced via intrinsic negative dispersion of energy within the plate. The beaming of acoustic power occurs within a narrow frequency band in transmission over the negative group velocity region of the first-order symmetric leaky Lamb mode. Experimental characterization utilizes Fourier analysis and measurements of transmitted fields in time and space to obtain the leaky Lamb modes dispersion, which agree well with the predictions of a numerical model. This model is then used to visualize the pressure field and confirm the nature of the energy flow inside the plate. The properties of the acoustic focusing depend on the plate thickness and elastic material parameters, and may be readily controlled for a range of applications. Such phenomena may be exploited in other areas, such as in the microwave domain where designer mode dispersions can be developed using metamaterial concepts.