Measurement of Sound Attenuation

A classical problem in acoustics is the absorption of sound in solid suspensions. Sewell (1910) first conducted a theoretical study of the case of small rigid spherical particles suspended in fluids. The condition of immobility in this case is satisfied by water droplets in air thus, Sewell s treatment can be applied to sound propagation in fogs and clouds. 

Rayleigh (1894) laid out the foundation for the scattering theory of sound wave propagation in fluids that contain suspended solids. He discussed the plane-wave disturbance produced by small obstacles and observed that (a) the zero-order term in the partial wave expansion of the disturbed field is a manifestation of the compressibility difference between the particles and the suspending fluid and (b) the first-order term is determined from the density difference as well as from the relative motion of particles (viscous drag losses). 

Urick (1948) measured ultrasonic attenuation in aqueous kaolin as well as sound dispersion. The results were in good agreement with the losses predicted from viscous drag at the particle surface. Sound propagation in a suspension can also produce temperature gradients at the particle/suspending-fluid interface, and thus, results on attenuation via thermal diffusion. 

Another process that attenuates sound waves is wave scattering. A theory that describes wave propagation in solid suspension (Ishimaru, 1978) has been well established for a medium in which uniform particles are homogeneously suspended. The attenuation can be determined from the coefficients of the reflected compression wave, viz., 

FIGURE 5-25 Relative attenuation vs. slurry velocity for various coal concentrations. 

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