Both gas and propane generators are internal combustion engines. They can be very noisy in operation, especially older units and those that run at higher revs per minute (RPMs). A resonating bare metal housing will amplify the noise pollution, but installing sound insulation must not cause the generator to overheat. Universal rules apply to creating an effective quiet box whether it is a cargo bay dedicated to the generator, a shroud permanently mounted on a platform welded to the recreational vehicle (RV) or a free-standing unit.
Fabricate your quiet box from sheet steel or plywood. Cut panels sized to balance safe usage with storage considerations. The corners may be joined with internal battens, but using hinges allows that a free-standing box be collapsed flat for travelling and storage. Adding a roof will allow the generator to be run in the rain without getting wet. Sleeve the outside of the quiet box in tar shingles for further insulation.
Line the quiet box using high hysteresis rubber sheeting. This self-adhesive material is manufactured to minimise noise transference through thin auto body panels such as door skins, and is available at auto parts stores. It is easily trimmed to the required size and shape. Reinforce the adhesive by spacing screws and penny washers around the edges if fabricating a wooden quiet box, or use rivets with broad heads if using sheet steel.
Use wood or metal hole saws and an electric drill to cut three appropriately sized openings in separate walls. Mount a vent fan in one, oriented so as to suck hot air out of the box, not to blow cooler air in. Using a fan that moves a considerable volume of air allows for a smaller opening, further reducing sound outflow.
Attach flexible exhaust hosing to the generator exhaust, and route it through the second hole. It is important to prevent the generator from rebreathing its own fumes, and venting exhaust gasses will also dramatically reduce heat build-up. Attach the external end of the hose to an aftermarket generator muffler, which must be sturdily secured to prevent vibration damage. Pointing the outlet downward prevents rain from entering, and pointing it away from your RV stops exhaust noise transferring to the vehicle as pulses which may resonate into loud booms.
Pass flexible exhaust hosing through the third hole and attach a sound-deadening filter to the air intake.
Radiated sound pressure is not a generator engine's only problem. Vibration can transfer as noise through a phenomenon called sympathetic vibration. A quiet box that can be easily removed allows the generator to be dismounted and moved away from the RV, stopping all vibration. Heat dispersion is aided by convection when the generator and quiet box are lifted off the ground on a plinth.
Heat removal is imperative if the generator has an integral fuel tank. Heating causes gasoline to expand which in turn causes gas tanks to swell and rupture. Warranties are usually invalidated by using a quiet box. Some manufacturers invalidate a warranty if they find a generator has been run inside a garage or shed.