A process for making particles is provided. The apparatus is a microprocessor based controller that provides an interface for the user to interactively locate and target areas of affected muscle tissue to specifically provide a beneficial effect which alleviates pain or provides other therapeutic results. The engine enclosure is constructed to include an ambient air screening wall around a muffler 5 so that the cooling ambient air taken from the ambient air inlet ports 24 and 38 may flow to a fan without contacting with the muffler 5.