Acoustic properties attenuation coefficient

For plane waves propigating in an isotropic homogeneous medium, three acoustic properties are important the speed of sound, the attenuation coefficient (to be discussed), and the characteristic impedance of the media. This impedance z is defined as the ratio of the acoustic pressure to the particle velocity associated with the wave motion in the material. For simple free-field plane waves, tliis is simply the product of the sound speed and density p. 

As the design matures, the direct measurement of the acoustic properties becomes necessary. These properties include the longitudinal wave speed, the coefficient of attenuation and the acoustic impedance, which can be obtained from measurements of the reflection and transmission of sound by the material. Two acoustic techniques are available for these measurements, the impedance tube and the panel test. 

Like the ultrasonic velocity and attenuation coefficient, the acoustic impedance is a fundamental physical characteristic which depends on the composition and microstructure of the material concerned. Measurements of acoustic impedance can therefore be used to obtain valuable information about the properties of materials. 

Before we discuss the applications of US-based detection techniques, we should emphasize the difference between the measured data and the desired output parameters. In ultrasound spectrometry, the measured data can be the attenuation coefficient, the sound speed and the acoustic impedance. However, the researcher is rarely interested in these measured properties, but rather on elastic moduli of solid samples particle size distributions or rheological properties of heterogeneous samples and concentrations, rheology, stability and chemical reactivity of liquids (particularly emulsions). 

Convection of heat via blood depends primarily on the local blood flow in the tissue and the vascular morphology of the tissue. Thermal diffusion is determined by thermal conductivity in the steady state, and thermal diffusivity in the unsteady state. In addition to these transport parameters, we need to know the volumes and geometry of normal tissues and tumor. In general, tumor volume changes as a function of time more rapidly than normal tissue volume. In special applications, such as hyperthermia induced by electromagnetic waves or radiofrequency currents, we need electromagnetic properties of tissues—the electrical conductivity and the relative dielectric constant. In the case of ultrasonic heating, we need to specify the acoustic properties of the tissue—velocity of sound and attenuation (or absorption) coefficient. 

As the acoustic properties of water-saturated sediments are strongly controlled by the amount and distribution of pore space, cross plots of P-wave velocity and attenuation coefficient versus porosity clearly indicate the different bulk and elastic properties of terrigenous and biogenic sediments and can thus be used for an acoustic classification of the lithology. Additional S-wave velocities (and attenuation coefficients) and elastic moduli estimated by least-square inversion specify the amount of bulk and shear moduli which contribute to the P-wave velocity (Breitzke 2000). 

The local variations of mineral content were determined from synchrotron radiation micro-computed tomography (SR-pCT). By means of a monochromatic x-ray beam, SR-pCT directly provides accurate 3-D maps of Ae linear attenuation coefficient within the sample. The absorption depends on the amount of mineral content which can be related to the differences in gray levels in reconstracted images. As acoustic microscopy and SAXS provide information about the surface properties, the corresponding surface is extracted from the 3D reconstraction of the bone mineral density. 

Acoustic properties (i.e., reflection coefficient, attenuation, and velocity of acoustic wave), and surface condition (i.e., surface roughness and discontinuities) of the specimen are factors in forming acoustic images. For a nanoscaled thin film system, (1) deference in the velocity of the surface acoustic wave propagating through the portion of the system and (2) increase of the amplitude of the acoustic wave caused by returning of the acoustic wave from the discontinuity located within the system are important for contrast factor. 

Applications of ultrasonic techniques to solid-gas systems rely on the fact that velocity and attenuation of US-waves in porous materials is closely related to pore size, porosity, tortuosity, permeability and flux resistivity. Thus, the flux resistivity of acoustic absorbents oan be related to US attenuation [118,119], while the velocity of slow longitudinal US is related to pore tortuosity and diffusion, and transport properties, of other porous materials [120]. Ultrasound attenuation is very sensitive to the presence of an external agent suoh as moisture in the pore space [121] and has been used to monitor wetting and drying prooesses [122] on the other hand, US velocity has been used to measure the elastic coefficients of different types of paper and correlate them with properties such as tensile breaking strength, compressive strength, etc. [123]. 

For studies of dilute polymer solutions, measurements of high precision are necessary to obtain the small differences between the properties of solutions and solvent. Apparatus for pulse propagation in such solutions at 20 MHz has been described by Miyahara, Wada, and Hassler," and for variable path interferometry by Cerf, and for standing waves by Miyahara. jjj jjjg latter method, the frequency can be varied continuously from 1 to 20 MHz. At lower frequencies (10 to 700 kHz), the free decay of waves in a spherical vessel can be measured. In such measurements, the data are ordinarily left in terms of M (or simply velocity and attenuation, or the acoustic absorption coefficient identified in Chapter 18) with no attempt to convert them to K. ... 

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