SONARtrac flow meters represent a new class of clamp-on flow monitoring technology well-suited for paper and pulp applications. Separate and distinct from ultrasonic technology, SONARtrac devices utilize clamp-on, strain- based sensors combined with sonar processing techniques to simultaneously measure the percent of entrained air present in a paper or pulp slurry as well as measure the volumetric flow rate of that slurry on a real time basis. To measure entrained air, the meter utilizes the sensor array to determine the speed at which naturally occurring, process-generated sound propagates through the process fluid. The speed of sound measurement is then used to determine entrained air levels accurate to within 0.01% for typical paper and pulp applications. In addition to entrained air measurement, sonar-based meters can also provide volumetric flow measurement by tracking the speed at which naturally occurring turbulent structures convect with the flow past the same sensor array. By utilizing these two, first-principles based measurements to track the naturally occurring pressure field within process piping, sonar- based flow monitoring systems provide new insight to multi-phase flow for the pulp and paper industry.