Conventionally, converging-diverging ducts are used to restrict the flow rate of gases. This process makes it possible to create a flow rate which is independent of the conditions prevailing downstream of the point of pressure drop and can be directly calculated by applying the standard formulae of compressible flows. Ultrasonic machining consists of making a tool vibrate at a very small distance from the work piece. The combination is placed in a coupling fluid containing abrasive grains. The tools are generally located at the extremity of a rod having an exponential profile which serves as an amplifier of displacement. This type of machining can produce various special shapes with a very good precision and the machined surfaces are remarkably smooth. Some manufacturers of dish-washers are thinking of incorporating these techniques in their product in order to reduce contamination by detergents as well as to reduce consumption of water and energy.