Calculating suspended scatterer velocity

Acoustic signals are transmitted into fluid medium 109 containing scatterers. Scatterer velocity is calculated based on a calculated rate of change of phase associated with a plurality of backscatter signals, each resulting from a corresponding transmitted signal. Two subsets of transmission signals have different frequencies or frequency profiles. Scatterers may be particles, droplets or bubbles. Backscatter signals may be quadrature sampled voltage signals. The velocity may be a rising or settling velocity, the signals being measured vertically. Transmission signals may have the same frequency and/or phase profile. A transmission period of a transmission rate may be varied. In an aspect, an acoustic backscatter instrument, perhaps having multiple transducers 101, transmits the acoustic signals and measures the backscatter signals. In aspects, a data-processing system and software perform the calculations.