When an alternating voltage is applied to a colloid, the particles move back and forth at a velocity that depends on their size and zeta potential and on the frequency of the applied held. As they move, the particles generate sound waves. In this paper we describe a new device for measuring this electroacoustic effect, From this measurement the frequency-dependent electrophoretic mobility (or "dynamic" mobility) of the particles can be determined in suspensions of arbitrary concentration, and from the dynamic mobility spectrum it is possible to obtain the size distribution and the zeta potential of the particles. The great advantage of the technique is that it is not limited to dilute suspensions. After describing the operating principles and calibration of the device, we present mobility spectra obtained on model silica suspensions. It is found that these spectra are in good agreement with O’Brien’s thin double layer formula, down to a kappa alpha value of 20. We then show how size distribution and zeta potential can be obtained from the mobility spectrum, and we compare results for a number of suspensions with measurements made using other techniques.