Attenuation Coefficient Estimation Using Equivalent Diffraction Points with Multiple Interface Reflections

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. INTRODUCTION The ultrasonic attenuation coefficient of a medium is an acoustic parameter routinely estimated in nondestructive evaluation (NDE) and biological tissue characterization. Knowledge of the ultrasonic attenuation of a given material is useful to the NDT field inspector searching for flaws in various structural materials, the material scientist characterizing the mechanical properties of the material, and the biologist investigating the acoustic properties of various types of biological tissue. One of the challenges associated with making accurate attenuation coefficient measurements is to separate the energy loss due to absorption and scattering within the medium from other possible sources of energy loss including those due to reflection and transmission at interfaces, diffraction of the transducer's wave field, measurement system inefficiencies, and misalignment of the transducer and specimen. In this paper, we will consider four attenuation coefficient estimation approaches (see Table 1): 1) a Classical Approach driven by the ratio of magnitude spectra from two interface reflections; 2) the Papadakis Approach which eliminates the need to make explicit corrections for reflection CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti © 2003 American Institute of Physics 0-7354-0117-9/03/$20.0