Abstract: A multi-phase fluid flow meter based on passive and non-intrusive acoustics is described for use in field applications on pipes. The present design uses the pipe's characteristic acoustic frequency and its amplitude variation in conjunction with a differential pressure measurement to obtain the total mass flow rates and mass flow rates of each phase. In addition to the mass flow rates the void/liquid fraction, fluid velocities and densities become viable estimates. Instrument construction is simple and very robust, allowing for use in extreme environments. This includes down hole as well as surface measurements.