Bottom line. If you want a quiet generator, buy one. Don’t buy a loud generator with any expectation that you’ll be able to later make it quiet.
Details. I have (oxymoron alert) a nice Harbor Frieght (HF) invertor generator that is pretty darn quiet, and a crappy little $88 HF that is not quiet. The little one is darn handy tho, or at least it became so once I pulled it’s fuel tank off and figured out why it was leaking gas.
The little generator is enough to run my trailer’s fridge, a coffee pot, and to run the charger that keep’s the trailer 2 large 12V batteries charged up. It can run an electric blanket in the winter, a fan in the Summer and multiple slow cookers when we’re cooking chili for 40. It was $88 well spent and it’s a perfectly serviceable solution right up until I want to run the trailer’s AC unit. Then the little generator falls short.
The downside of the little generator is that it’s loud. So I got to thinking about how I might make it quieter.
I home-brewed an external muffler by taking a Home Depot pail and putting 4 layers of carpet padding in it. Then I cut some holes in it and ran a hose from the generator exhaust to the pail. The hose went thru the layers of padding into the center of the pail. I figured that the padding was porous enough and had some much surface area that the exhaust would not cause a high pressure zone inside. So the noise had no way to avoid the 4 layers of carpet padding. I figured that had to be enough to significantly reduce the noise volume.
I was wrong about that.
Using a sound meter at a distance of 10’ I measured the sound volume from the generator with and without my fabulous external noise muffler. I got a reduction of about 0.5dB 1.0dB. Call it a bit less than the human ear can notice
The lesson I learned is that if you want a quiet generator, buy one that was engineered to be quiet. Don’t try to quiet down a noisy one, because unless you put the entire thing into an enclosures, you’re not going to quiet it down enough to make the effort worth the trouble. I don’t really know anything about "what makes generator’s noisy? Is there something fundamentally more quiet about inverter generators, or are their quieter because they have smaller engines? Does a lot of the noise from cylinder detonations emanate from the block itself? Are 2 stroke engines fundamentally more noisy than 4 stroke? All I can say for sure is that quieting your cheap generator’s exhaust after the OEM muffler doesn’t seem to do much good.