Acoustic and Electroacoustic Spectroscopy

Both acoustic and electroacoustic spectroscopy are related to a sound propagation through a heterogeneous system such as a suspension or an emulsion. An acoustic spectrometer measures only the changes in the properties of the sound wave, whereas an electroacoustic spectrometer deals with coupling between electrodynamic phenomena and the sound wave pressure field. Acoustic and electroacoustic spectroscopy are independent methods because the attenuation has little effect on the electroacoustic spectra and conversely electrokinetic phenomena have negligible effect on the attenuation spectra. Both techniques require a comprehensive theory to extract information about the particle size distribution or electrosurface parameters such as zeta potential from the measured spectra. The existing acoustic theory takes into account both compression and shear waves generated by oscillating particles. In contrast, the present theory for electroacoustics considers only shear waves. We will show that compression waves can also be important in electroacoustic phenomena. In fact, compression waves cause the dominant effect for conducting particles at the sufficiently low frequencies. As one would expect, each technique has advantages and disadvantages. Acoustic spectroscopy cannot give a complete characterization of the disperse system because it is able to characterize only particle size distribution. Electroacoustic spectroscopy, in certain cases, can provide a more complete characterization but requires assumptions about the sample which may not be valid, particularly in concentrated systems. A new approach is suggested here which combines acoustic and electroacoustic spectroscopy and eliminates the disadvantages of both.