Conventional, Bayesian, and the modified least-squares Prony's plus curve-fitting (MLSP þ CF) methods were applied to data acquired using 1 MHz center frequency, broadband transducers on a single equine cancellous bone specimen that was systematically shortened from 11.8 mm down to 0.5 mm for a total of 24 sample thicknesses. Due to overlapping fast and slow waves, conventional analysis methods were restricted to data from sample thicknesses ranging from 11.8 mm to 6.0 mm. In contrast, Bayesian and MLSP þ CF methods successfully separated fast and slow waves and provided reliable estimates of the ultrasonic properties of fast and slow waves for sample thicknesses ranging from 11.8 mm down to 3.5 mm. Comparisons of the three methods were carried out for phase velocity at the center frequency and the slope of the attenuation coefficient for the fast and slow waves. Good agreement among the three methods was also observed for average signal loss at the center frequency. The Bayesian and MLSP þ CF approaches were able to separate the fast and slow waves and provide good estimates of the fast and slow wave properties even when the two wave modes overlapped in both time and frequency domains making conventional analysis methods unreliable.

Acoustic vortex waves have previously been shown to be able to overcome the diffraction limit, through the use of subdiffraction-limited pressure nulls that propagate well into the farfield. Acoustic vortex waves, therefore, present a novel means to achieve farfield superresolution imaging. However, generating acoustic vortex waves in an experimental setting typically requires a complicated phased array consisting of multiple active elements in a fixed geometrical configuration. In this work, we describe how an acoustic vortex wave can be generated using a synthetized vortex wave array applied during post-processing to in-water 2D plane measurements obtained with a single moving acoustic source and a fixed receiver. The geometric versatility of the synthetic vortex array enables different shaped acoustic vortex patterns to be achieved using the same in-water measurements. Experimental and theoretical results of the magnitude and phase of the nearfield and farfield pressure fields will be presented for a variety of geometric configurations and vortex integer wave modes of the synthesized vortex wave array. The data show excellent agreement with expected results and demonstrates shaped acoustic vortices with superresolved features in both the nearfield and farfield. [Work supported by the Office of Naval Research.]

Acoustic metasurfaces are ultrathin (subwavelength thickness) structures that can demonstrate extreme acoustic properties, such as negative or near-zero values, similar to those seen in acoustic metamaterials. The negligibly small thickness of acoustic metasurfaces make them a more practical alternative to metamaterials for certain applications. In this work, we experimentally demonstrate an aqueous acoustic metamaterial that employs subwavelength, flexural elements that acoustically act in parallel to achieve enhanced acoustic transmission and broadband negative effective density. The metasurface was constructed from a brass plate, which was machined to have a parallel arrangement of circular, flexural elements on the surface, and experimentally tested in water over the nominal range of 50–100 kHz. The reflected and transmitted acoustic data were compared between a brass plate with and without the circular flexural elements. It was observed that the aqueous acoustic metasurface made from a brass plate with circular flexural elements showed improved broadband transmission. A detailed description of the experimental testing and analysis will be discussed. [Work supported by the Office of Naval Research]

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The goal of this study was to compare the results from two proposed methods for separating fast and slow waves from mixed-mode signals, one using Bayesian probability theory and one using modified least-squares Prony’s (MLSP) [Wear, J. Acoust. Soc. Am. 133, 2490–2501 (2013)], on measurements of cancellous bone. Ultrasonic through-transmission data were acquired on a bovine femoral head specimen for thicknesses ranging from 15 mm to 6 mm. The thickness was reduced in 1 mm increments and measurements were acquired at each sample thickness [Nagatani et al., Ultrasonics 48, 607–612 (2008)]. A Bayesian parameter estimation analysis was performed on the experimentally acquired signals to isolate the fast and slow waves and to obtain estimates of the fast and slow wave’s ultrasonic properties. Results for the corresponding ultrasonic properties were estimated using the modified least squares Prony’s method plus curve fitting (MLSP + CF) on the same bovine sample by Wear et al. (J. Acoust. Soc. Am. 136, 2015–2024). The results show good agreement between the phase velocities estimated by Bayesian and MLSP + CF analysis methods for both the slow wave and the fast wave. Both analysis methods yielded fast and slow wave phase velocities that depended on sample thickness. This may imply that the propagation model, which is used in both methods, may be incomplete.