Values Of Acoustic Attenuation Research Articles

The influence of bone model geometries on the determination of skull acoustic properties.

In this study, we investigated the impact of various simulated skull bone geometries on the determination of skull speed of sound and acoustic attenuation values via optimization using transmitted pressure amplitudes beyond the bone. Using the hybrid angular spectrum method (HAS), we simulated ultrasound transmission through four model sets of different geometries involving sandwiched layers of diploë and cortical bone in addition to three models generated from CT images of ex-vivo human skull-bones. We characterized cost-function solution spaces for each model and, using optimization, found that when a model possessed appreciable variations in resolvable layer thickness, the predefined attenuation coefficients could be found with low error (RMSE < 0.01 Np/cm). However, we identified a spatial frequency cutoff in the models' geometry beyond which the accuracy of the property determination begins to fail, depending on the frequency of the ultrasound source. There was a large increase in error of the attenuation coefficients determined by the optimization when the variations in layer thickness were above the identified spatial frequency cutoffs, or when the lateral variations across the model were relatively low in amplitude. For our limited sample of three CT-image derived bone models, the attenuation coefficients were determined successfully. The speed of sound values were determined with low error for all models (including the CT-image derived models) that were tested (RMSE < 0.4 m/s). These results illustrate that it is possible to determine the acoustic properties of two-component models when the internal bone structure is taken into account and the structure satisfies the spatial frequency constraints discussed.

Акустические и акустооптические свойства стекол Ge-=SUB=-a-=/SUB=-Se-=SUB=-b-=/SUB=-Te-=SUB=-c-=/SUB=- и Si-=SUB=-a-=/SUB=-Se-=SUB=-b-=/SUB=-Te-=SUB=-c-=/SUB=- в случае дифракции на продольных и сдвиговых акустических волнах

We investigated acoustic and acousto-optic properties of glasses based on germanium, selenium, silicon and tellurium. The results of measurement are presented for two group of glasses including germanium, selenium and tellurium (GeSeTe) and silicon and tellurium (SiTe). The major characteristics of glasses such as the velocity of longitudinal and shear acoustic waves as well as the values of figure of merit are presented. The values of figure of merit are obtained by diffraction on longitudinal and shear acoustic waves at the wavelength of optical radiation λ = 3.39 µm. We also present the results of measurements of acoustic attenuation values α for longitudinal and shear acoustic waves in tellurium glasses at ultrasound frequencies in the range f = 75-100 MHz.

PARTICLE IRREGULARITY AND AGGREGATION EFFECTS IN AIRBORNE SUSPENSIONS AT AUDIO- AND LOW ULTRASONIC FREQUENCIES

In the inertial regime of frequency-radius space, irregularity and aggregation of particles can result in values of acoustic attenuation that are significantly different from those predicted by assuming separated smooth spherical particles. Data obtained previously from suspensions of alumina particles and olivine sand in air at audio-frequencies, together with new data obtained at low ultrasonic-frequencies in suspensions of glass beads and silica flour, are compared with predictions. It is shown that neither a coupled phase theory modified to allow for non-spherical shapes nor effective radius theories are able to account for these data. Qian [21] has suggested that suspensions may be treated as fractal media and used the acoustic Reynolds number as the fractal dimension in modifying scattering theory. A new fractal modification of multiple-scattering theory for acoustic attenuation is derived. The theory uses ωτv(ω is the angular acoustic frequency, τvis the dynamic relaxation time of the particles) as a fractal scale. It requires an empirical determination of the difference between the fractal dimension of the measured suspension and that of a hypothetical suspension of spheres with the same particle size distribution. However, values obtained at a single frequency also enable fits with data at other frequencies. The new fractal modification of scattering theory is found to enable better agreement with measured attenuation as a function of concentration for irregular particles than an effective radius model. Also, the fractal modification is able to predict the observed frequency dependence at a given concentration rather better than effective radius approaches. Moreover, the fractal approach is found to enable discrimination between the effects of particle irregularity and aggregation.

New models and measurements concerning sound attenuation in concentrated airborne suspensions

In the inertial regime of frequency-radius space, irregularity and aggregation of particles can result in values of acoustic attenuation that are significantly different from those predicted by assuming separated smooth spherical particles. Data obtained previously from suspensions of alumina particles and olivine sand in air at audio frequencies, together with new data obtained at low ultrasonic frequencies in suspensions of glass beads and silica flour, are compared with predictions. It is shown that neither a coupled-phase theory modified to allow for nonspherical shapes nor effective radius theories are able to account for these data. Qian [Phys. Rev. E 53(3), 2304–2306 (1996)] has suggested that suspensions may be treated as fractal media and used the acoustic Reynolds number as the fractal dimension in modifying scattering theory. A new fractal modification of multiple scattering theory for acoustic attenuation is derived. The theory uses ωτv (ω is the angular acoustic frequency, τv is the dynamic relaxation time of the particles) as a fractal scale. Fitted values of fractal dimension obtained at a single frequency are found to enable fits with data at other frequencies. Moreover, the fractal approach is found to enable discrimination between the effects of particle irregularity and aggregation. [Work supported by EPSRC (UK).]

Near-bottom 4-kHz reflectivity observations in real time

A sea-going digital computer system has been utilized to process and display quantitative near-bottom 4-kHz sounding data in real time for an abyssal hill region of the sea floor 200 miles west of San Diego. Over a region approximately 50 miles square, variations in bottom reflectivity of as much as 10 dB were observed, with abrupt changes in sediment character and reflectivity (as much as 7 dB). Four precisely navigated cores were taken from the area for physical property analysis, one of which sampled a buried 3-cm-thick manganese layer. Quantitative 4-kHz reflectivity data from the above area as well as several others in the Pacific are being further analyzed in an attempt to infer acoustic attenuation values for sedimentary layers from variations in layer reflectivity with burial depth. Preliminary results are in good agreement with published values of directly measured attenuation in ocean sediments. A value of 0.12 dB/m at 4 kHz obtained for calcareous ooze of approximately 75% porosity from the Carnegie Ridge represents a relatively low value in comparison with published attenuation values, suggesting significant lithification of this high-porosity sediment.