Attenuation measurements with ultrasonic diffraction grating show dependence upon particle size of slurry and viscosity of base liquid

How can using an ultrasonic diffraction grating lead to slurry characterization? The diffraction grating, which is formed by machining triangular grooves on the flat surface of an aluminum unit, has send and receive transducers fastened to the unit at an angle of 30°. The ultrasonic beam strikes the back of the grating, in contact with the slurry, and reflects a beam to the receive transducer; m = 0 and m = 1 beams are transmitted into the slurry. The angle of the m = 1 beam changes with frequency and, at the critical frequency fCR, it reaches 90°. When f < fCR, the m = 1 beam disappears, its energy is shared with all other beams, producing a peak in the receive transducer. The change in peak height with slurry concentration determines the attenuation; the frequency at the peak yields the velocity of sound. The attenuation has been measured for polystyrene spheres, ranging in size from 98 µm to 463 µm, and slurry concentrations up to 20 wt%. When the spheres are immersed in water, sugar water, or mineral oil, the attenuation measurements show the effect of particle diameter and the viscosity of the base fluid.