Quieting your generator

#1

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.

#2

Just a side note: The normally accepted delta for an “average” human to perceive is 3 dB. So 0.5 dB is very, very small, and well within error range on a sound level meter. In other words, you might not have materially changed it at all, or ambient noise was a higher factor than you might have realized. Also, for your future acoustic endeavors, the distance you might want to use is 1m, or about 3.3 ft. You can extrapolate from there any distance/intensity you want, but that’s the standard from which to start.

#3

Ah so. I was getting the 3dB threshold confused with the 10dB double volume threshold. I was pulling the #'s out of distant recall because I did this testing last Summer. Upon reflection, I think my 0.5dB change was actually 1.0dB. I’ll change it in the OP. Thanks for the correction.

You make a fair point the the #'s that I got were within the “noise” of the accuracy of the device I was using and my informal test methodology. I thought that there’d be a big difference and I thought that it would be cool to actually have #'s to document it, not having used the sound meter much since I bought it for a “make my damned truck quieter” project years ago. I was obviously over-optimistic re. the “big difference” thing .

#4

3db is double the acoustic energy, 10db is what your ear interprets as twice the volume (as mentioned)