The extinction of radiation in suspensions is traditionally described by the Bouger-Lambert-Beer law (BLBL). Based on a quasicontinuum approach, the BLBL does not account for the discrete nature of particles or their spatial extension and arrangement. If an extinction measurement is made with a high spatial and temporal resolution, the transmitted intensity signal shows signi®cant uctuations. The strength of¯uctuation is related to the physical properties of the suspension and the process of spatial and temporal averaging. Exploiting this connection, it is possible to calculate the particle size distribution and the particle concentration from transmission measurements. This part of the series of papers provides a method for the temporal decomposition of the transmission's power spectrum, which permits the information on the particle size and concentration to be extracted from the seemingly irregular¯uctuation of the transmission signal.

The measurement of particle size distributions and particle concentrations is an important task in laboratories and in technical processes that deal with suspensions. Especially the instruments designed for on-line applications in production processes mostly evaluate the interactions of light with the suspended particles, viz. extinction, scattenng and diffraction. While the interaction of particles with ultrasonics is essentially the same as with light, the application of ultrasonics offers special technical advantages, which are due to the extremely broad range of wavelengths and the coherent detection of ultrasom·c waves. Ultrasonic spectrometry makes possible the on-hne measurement of the particle size distribution and particle concentration in suspensions at concentrations of up to 30% by volume without diluting the suspension. This paper is intended to introduce the reader to the fundamentals of ultrasonic particle sizing and to explain present efforts in this field. In the last section, some successful realizations of ultrasonic particle sizing instruments are presented.

The Bouguer‐Lambert‐Beer law (BLBL), commonly used to describe the extinction of radiation in disperse systems, is known to be valid in the limit of low concentrations only. In Part [1] it was shown that steric particle‐particle interactions lead to important effects in the range of high particle concentration and two versions of a more general extinction equation including steric interactions were derived for the regimes of fraunhofer diffraction and ray propagation in this part experimental evidence for the effects of steric interactions on the extinction of radiation in suspensions of monodisperse spherical particles is given. Various experiments on light extinction and on ultrasonic extinction are combined to cover a broad range of particle size parameters, mechanisms of particle‐wave interaction and receiver apertures.

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