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The surface density or mass per unit area of a membrane is an important material property often used in acoustics and ultrasonics. In this paper, a new measurement and analysis technique for estimating the surface density as a function of frequency for a nonporous membrane or foil is introduced. This new, broadband technique is derived from the Thompson-Gray measurement model which can be simplified to the fluid layer transfer function commonly used in acoustics. The fluid layer transfer function can be further simplified to the limp-wall mass law for acoustically 'thin' membranes whose thickness is much less than a wavelength. The transfer function of the membrane can be efficiently measured with commercially available air-coupled ultrasonic transducers from which the surface density can be computed. Surface density estimates are presented for four membrane-like materials: aluminum foil, brass shim, polyester and polyethylene sheets. [ABSTRACT FROM AUTHOR]

Fibrous ensembles are, typically, multi-scale flexible assemblies with unique physical and rheological properties, unlike continuum materials. Macroscopic behaviors of these materials are greatly the result of non-linear interactions at the micro levels. These micro-scale interactions can be assessed by capturing the material behavior under low mechanical stress conditions. While ultrasonic based non-destructive testing was suitably implemented for continuum materials, their application to fibrous structures was limited primarily due to the inherent structural arrangements of these unique assemblies. Discontinuities, non-uniform orientations and multi-phase components make these ensembles difficult to study using the existing scan-based methods. This work presents a novel rapid and advanced analysis tool for complex fibrous systems using a noncontact air-coupled ultrasonic system. Five characteristic features of ultrasound signals transmitted through fibrous structures were studied, i.e., dampness in signal flight, signal velocity, power spectral density, signal power and rate of amplitude attenuation. Analysis of these features under two different acoustic frequencies, 500 kHz and 1 MHz, allowed us to study the componentized behavior of these materials for three of the key mechanical properties including bending rigidity, shear rigidity and low stress tensile stress. A material response index (MRI) was also derived using the signal features. [ABSTRACT FROM AUTHOR]

Ultrasonic transducer beam models are developed to predict the average received pressure reflecting from the front surface of finite-sized shapes with simple curvatures. A general surface integral model is derived that combines the Rayleigh-Sommerfeld integral with the method of stationary phase and the Kirchhoff approximation. Numerical evaluation of the 2-D surface integral is simplified by employing the edge element technique and ray tracing that obeys Snell’s Law. Algorithms are developed that determine the Snell’s Law ray path for a ray reflected from spherically curved interface between any given point source and field point. On-axis scans of the average pressure received at the transducer of the front surface reflections from spheres of various sizes are calculated and compared with scans of planar interfaces and point scatterers. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The ultrasonic attenuation coefficient of a fluid or solid material is an acoustic parameter routinely estimated in nondestructive evaluation (NDE) and biological tissue characterization. In this paper, a new measurement and analysis technique for estimating the attenuation coefficient as a function of frequency for a fluid or solid is described. This broadband technique combines two established concepts in attenuation coefficient estimation: (1) frequency spectrum amplitude ratios of front surface, first back surface, and second back surface reflections from interfaces of materials with plate-like geometries, and (2) equivalent diffraction points within the transducer wave field. The new approach yields estimates of the attenuation coefficient, reflection coefficient, and material density without the need to make diffraction corrections. This simplifies the overall estimation process by eliminating the transducer characterization step, that is, by eliminating experimental characterization of the effective radius and focal length of the transducer which are required when careful calculated diffraction corrections are applied. In this paper, attenuation coefficient and reflection coefficient estimates are presented for water and three solids with estimates based on measurements made with two different transducers. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

In same material joining processes, remnants of the original interface may be acoustically weak and difficult to detect ultrasonically in the presence of grain noise. For the nearly perfect (but potentially inadequate) bond or weld the remaining interface may be defined only by small voids, cracks, or by weak echo surfaces (grain boundaries) approximately aligned in the original interface plane. In this study, we investigate the utility of correlation-based techniques for revealing the presence of interface signals in the presence of grain noise as the signal-to-noise ratio is pushed below 1.0. These techniques are attractive since they are independent of absolute scale and since the grain noise near the bond/weld time window may be useful in establishing baseline results for a region without an interface (grain noise only). Experimental results from two different same material joining processes are included in the study along with supporting results from a simulation driven by experimental data. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]