Echo attenuation ratio

Packer and Rees [3] extended the expression derived by Murday and Cotts [7] to include the effects of a droplet size distribution, assuming a log-normal distribution. By curve fitting they were able to determine the principal parameters of such a distribution from the experimental R-values. In the presence of a distribution of sizes, the observed echo attenuation ratio R is expressed in terms of the calculated attenuation of individual droplets, R ... 

Zibrowius et al. (77) used an NMR spin-echo attenuation technique to estimate the ratio of the diffusion tensor element related to motion along the y-axis (the straight channel) and the average of the other two elements. For a methane loading of 8 molecules per unit cell, the ratio was estimated to be less than 5 at room temperature ... 

In the situation where V2DA is of the same order or larger than the distance between any diffusional barriers in the system, so-called restricted diffusion is observed. In a W/O emulsion, for example, the water molecules are restricted in the extent of their diffusion by the presence of the boundaries of the water droplets. The extent of the restriction of the diffusion of the water molecules is reflected in the ratio R = E /E. An expression for the echo attenuation R-factor as a function of droplet diameter has been derived by Murday and Cotts for uniform spherical droplet sizes [7] ... 

A corresponding composite probe with the same frequency and crystal size, however, detects the test flaw much better the echo has a 12 dB higher amplitude (see Fig. 4) and in addition, the noise level is much lower, resulting in an improved signal to noise ratio. This effect is especially observed at high sound attenuation. However, in materials with low attenuation or in case of shorter sound paths the standard probe yields a comparable good signal to noise ratio. 

With a test sample on the optically flat top surface of the bar, the pulse echo train is reduced in amplitude. This attenuation is owing to the refraction of part of the ultrasonic wave into the test sample at the frequency used. The ratio of successive peak amplitudes may be measured on the oscilloscope and expressed in decibels loss per echo. From this, the loss per echo with no sample on the bar can be substracted to give a value Adb which is related to the mechanical shear impedance of the sample. Rapid changes can be conveniently monitored by a recorder which follows the peak signal of a selected echo. 

FIGURE 60.4. Ultrasound pulse-echo pattern obtained at 10 MHz in a polystyrene disk 3 mm thick. The interval between successive reflections indicates the velocity of the longitudinal wave, and the ratio of intensity of any two successive reflections the attenuation. The horizontal scale is 2.00 xs/division. in this material the (longitudinal) speed of sound is 2.14 km/s, the acoustic impedance is 2.25 MRayls (units of 10 kg/(cm -s) and the attenuation coefficient is ca. 12 db/cm. See text below for the calculation [57]. 

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