there are 3 ways to deal with sound.
1) Deflect it
This helps the sound go somewhere else but not actually go away
2) Absorb it
This works easily for high frequency sounds that have little wave energy but not so easily for low frequency sound which can have a large amount of energy that can travel long distances and through denser materials.
3) Cancel it out using Reverse phase sound wave cancellation
The sound you hear is the sum of the waves being created
1) Deflect it
Last year we took 2 4x8 sheets of sound board to help keep our quiet peace in the midst of our neighbors generators.
1 sheet we wired to a fence post as a short wall between us and our neighbors across the street who were kind enough to place their very loud generator on our side of their motor home camp, and the second sheet was given to our back door neighbors who had a newer quiet one. just by placing the sheet on the camp side of the newer generator the sound was almost totally dealt with.
The sound from the older one across the street was cut significantly , but was still reflected to us by the motor home wall behind it, but it was still much better , ,, from our side.
2) Absorb it
Sound board is a dense fiber board basically like the bulletin board from school days, it's about $7-18 a sheet and about 20-30lb per sheet, if you have room take an extra sheet for the ditz who sets up next to ya, it was well worth the cost ( we sent one sheet home with the back neighbors so he could make a screen with it.
Sound board deals with higher frequencies quite well, but sound board by it's self will only help cut down , but not stop low frequencies.
The best way to deal with low cycles is a concrete wall or a double 2x6 wall or a 2x 12 wall with split 2 x12s faced with rubber coated 3/4" plywood and filled with road base gravel mix, with your high frequency absorbers on the inner face of the wall.
This should only weigh a couple tons.
Particle board is very dense and heavy like the gravel and rubber, so it is used to contain the sound in speakers, but will reflect , not absorb the soundwaves.
3) Reverse phase sound wave cancellation
If you have 2 waves in sync you have twice the power , if you take 1 wave and phase it so it perfectly Mirrors the original, they cancel each other out and sum of the 2 is "0". this is how directional mics work.
I'm not writing a book , but they have worked on using this technique and if you have the desire and money it is possible to cut down the ambient noise with more noise.
This is actually how some mufflers work using baffles to break up and reflect the waves to cancel each other. they are often "tuned" to the motor they are designed for.
A box tapered like a popcorn box made from a layer of sound board inside dense particle board, preferably spaced so the layers are not touching, with a layer of those foam hospital mattress cushions glued to the inside of the sound board will do the best to absorb the high frequencies while deflecting the lower frequencies out to space ,, or up away from people.
The particle board is about 80lb a sheet so for a 4 x 4 box you are still talking 200 -300 lbs.
Be careful about igniting the foam or fiber with the exhaust.
If you add a muffler to your generator be sure you don't cut the air flow by sizing it too small or you could cause heat build up on the exhaust valves and have a mild melt down. Larger is better , not smaller and constricting.
Oh , our generator?
We had a 2x3' 80w solar voltaic panel that charged our batteries during the day and kept quiet at night.
no gas , no noise , and probably costs about as much as the generator, if you aren't planning on lighting the city by yourself.
Rouse, the long winded Mouse