Just my two cents worth regarding how to quiet a gen-set.
Of course you know you are dealing with a generator that runs at 3600 RPMs all the time, compared to the quieter and more expensive inverter type that throttle the engine RPMs to match the load.
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Years ago at the old Buhl Science center there was an exhibit that consisted of a room similar to two U put together and a large multi-pane glass in the center. Imagine one U upside down and one U normal, and them over-lapping so there is a walk way on each side before you enter. The room was carpeted on ALL SURFACES (floor, walls, and ceiling) and so was the entrance path along the legs you had to walk through to get in or out. Kids would go into that room and while looking out at their friends through the multi-pane glass they would yell as loud as they could, and you literally could not hear any sound that they made. Outside walls of the Buel exhibit were not carpeted.
I have often thought that the design of that room would make a great generator sound-deadening structure, just add fans. The only drawback was the size of that structure. Those walk through legs with turns add a lot of area to what the entire structure takes up.
Anyhow, just keep in mind that any inside wall coated with carpet will absorb sound and can not have sound bounce off of them.
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BTW, speaking of fans, you want to have the fan(s) in a wall so that none of the air leaving the fan has any way to re-enter.
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I have been paying attention to how to quiet down gen-sets for several years now. I added a car muffler to my 5000 Watt / 6250 Watt Peak Coleman Powermate Generator with a single cylinder Tecumseh 10 HP HM-100 gasoline engine, as that engine with the stock muffler can wake the dead. Before I added the car muffler I spoke to a mechanic who works on them. He said that a car muffler helps some, but not as much as you might hope because of the other sources of noise from those engines. He also said that if you remove the muffler most engines have pipe thread at the exit hole and you can buy the correct pipe size and thread it into those exhausts, but you must include some means of having flex or things will end up breaking because of the engine moving when it runs. I used a flex pipe from McMaster-Carr.
Adding the car muffler did not reduce the noise enough to make any noticeable difference if you are within 20 feet of it. But once you are in a house near by, there is a world of difference, in fact it is almost not even noticeable in the house with the car muffler, and without it was very annoying in the house.
These engines have several sources of noise, not just the exhaust. The intake going into the air filter is also in pulses, and the internal moving parts also make quite a racket.
One of the best post I read on another forum was about someone whose brother in law dug a very deep hole, put a pipe for the exhaust down to the bottom of the hole and filled it with sand. The person who owned the generator said it was the quietest muffler he has ever heard. I though of doing this, and besides the obvious problem that you have to be plenty far enough from any structure that may be occupied, in areas where the ground freezes during the winter there is some question about how the top layer of soil being frozen will affect such a system. And it can't be too healthy for plants and earth worms.
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