Hello PeterC,
The exhaust will not be the main source of noise from your ST3, or any highspeed aircooled diesel. A good car or truck muffler will take care of most
exhaust noise, but the whole engine radiates noise very effectively from its cooling fins, shrouds and other surfaces. This is unavoidable, and the alternative is to
screen the unit with sound reflective or absorbent surfaces while not restricting air flow. Silenced generator packs use metal panels clad inside with sound absorbent foam
and baffled air intake and outlet grilles. A stationary set can use brick, concrete, masonry panels to just above engine height to reflect sound upward. Again, air circulation
must be provided for. Lister handbooks detail the requirements plainly. Regards, Combustor.