The operation of a geothermal steamfield or power station requires the intermittent discharge of process fluids to atmosphere to maintain stable operational control or to start-up or shut-down. The energy in the discharge is partially converted to sound which can impact on the receiving environment. Typically the design for two-phase or saturated water flows is a silencer consisting of a inlet jet pipe flowing into a horizontal duct to a vertical barrel. It is relatively simple, cost effective and the sound level design predictions have been approximated on previously installed units of similar size and duty. With increasing environmental awareness there needs to be more certainty in the sound level prediction so that new plant is not operationally constrained or require modification after commissioning. A design prediction method is proposed that was determined from test data of operational silencers, process industry jet sound power calculations and an adapted model for viscous sound attenuation within the inlet duct. Limitations in the method, designer guidance and future investigation areas are also discussed.