The relationship between the attenuation of sound at frequencies between 500 and 3500 Hz and the concentration of flowing suspensions of various powdered solids has been explored. The suspensions were conveyed in a pipeline of 53 mm diameter and the particle concentration was varied up to a maximum of 2.8 kg/kg. Three materials were tested: flour (mean radius, 27 mm), olivine sand (mean radius, 278 mm) and barytes. The feasibility of measuring the acoustic attenuation of particulate suspensions at audio-frequencies and relating the measured attenuation to concentration has been demonstrated. Although the audio-frequency attenuation is smaller at a given concentration than at ultrasonic frequencies, audio-frequency measurements are less susceptible to inhomogeneity of flow and to in-pipe turbulence than ultrasonic measurements.

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A novel method which uses reverberant decay rates to measure the concentration of solid particles suspended in a gas has been investigated. The impulse response, measured across a section of cylindrical tube containing the suspension of particles with mean diameter 6 m, was processed to determine the reverberant decay rates of selected frequency bands. The decay rates, observed with and without suspended particles, were used to give a value of excess decay rate in several frequency bands. The excess decay rates are found to vary linearly with the particle concentration. The frequency range used was 1.5-20 kHz, and the particle volume concentration was varied between 0-12 parts per million ͑ppm͒.

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A novel method to measure the particle concentration in a gaseous suspension has been investigated. The measured impulse response across a section of cylindrical tube containing the suspension has been processed to determine the decay rates of radial modes in the tube. These decay rates are sensitive to the attenuation coefficient of the suspension, and hence to the particle concentration. The decay rates observed with and without suspended particles have been used to establish an empirical relationship between the particle concentration and the model decay rates. The frequency range used was 1.5–15 kHz, and the particle volume concentration has been varied between 0.7–10 ppm. The mean diameter of the particles was 6 μm. [Work supported by the S.E.R.C.]