Possibilities of design and construction are presented by which the noise radiated by the surface of an engine can be reduced.
Vibration isolation, stiffening, or sound-reducing shells, can be applied within comparatively short time; however, their effect on reducing the total noise is restricted to about 5 dBA.
Total enclosure of the engine is highly effective. In this particular design, the space requirement is comparatively small, the weight increase is not too excessive, sound-absorbing material in the clearance is not required, and the noise reduction achieved is as high as 15-20 dBA.
New design concepts, taking into account acoustical principles right from the start, will be the most economical approach for the future. Knowledge of the structure-borne sound within the engine is essential in this respect; particularly, further knowledge gained through tests on the nonrunning engine, using a “banger rig” for simulating combustion excitation.