A properly dispersed population of small bubbles can mitigate cavitation damage to a spallation neutron source target. In order to measure such a bubble population, an acoustic device was developed and implemented in a mercury loop at ORNL. The instrument generated pulses of various frequencies and measured their acoustic propagation in the bubbly medium. It then deduced sound speed and attenuation at the various frequencies and used an inverse problem solver to provide near real-time measurements of bubble size distribution and void fraction. The measurements were then favorably compared with an optical method.