11 HP Briggs & Stratton noise reduction

My S.O. complained that my 11 HP B&S / Craftsman was too loud so I am trying to quiet it down.
I worked a few years in the noise reduction industry so I may be doing this the difficult but self-educational way.
I put some pictures in an album at https://www.flickr.com/gp/181709411@N08/BWkd2C
My stock muffler was the rectangular one that bolted on with 2 flange bolts and a third around the corner by the spark plug.
It has several chambers with various size holes in a pattern like a 4 room house, and no sound absorbent material left in it if there ever was any.
The nearest parts guy said it would be $38 and take a few days to get.
I saw a YouTube where a guy welded a long pipe and resonator and muffler that sounded quiet.
But I don't weld like that and I want 'more'.
I read good stuff about SuperTrapp, a muffler-diffuser that quiets high-performance exhaust enough to pass noise regulations when they exist.
It uses a stack of disks with thin spaces between them and fewer disks make less noise with more back pressure.
The theory is, on my part, that the thin spaces keep big sounds from coming out (not really true).
Keeping it cheap, I started with a 1" iron pipe & elbow threaded into the B&S engine.
I made my own 'supertrap' with 2 flanges spaced by washers and a cap on the end of the straight pipe.
With spacers leaving open area equal to the straight pipe, it soounded like a straight pipe, maybe louder because the sound wasn't going in the direction of the pipe any more.
With thin washers leaving about half the area of the pipe, it got quieter but messed up the engine firing.
I have a noise meter and it was about 95 dB at the steering wheel.
That's a Concorde SST on take-off if I recall correctly.
I ordered a real SuperTrapp disk-only muffler, 2" ID inlet and 12 disks 3" OD in the assembly.
It was about $69 but I always wanted one so... I have one now.
I shaved down a 1" to 1-1/4" pipe coupling to get the 2" ID that the SuperTrapp slips over.
It was louder, like 98 dB, likely because of the greater open area.
12 disks is supposed to handle a 100 HP motor.
The Briggs is about 344cc, so equal to about half of an 800cc (50 CID) V-twin motorcycle.
I had one of those and it sounded good, not like a lawn tractor with no muffler.
With 3 disks it got quieter, 88 dB but the engine didn't sound as healthy as it should, and it was still in the 95 dB range.
I'm not sure the 1st & last disk are as effective as middle disks are so the exhaust might have been very restricted.
I got an 18" Cherry Bomb glass-pack muffler, close to being a straight 2" ID.
They're $23 today, I think mine was on sale.
The openings were actually 2-3/32 ID, still small enough to fit a shaved down pipe coupling.
With the cherry bomb and no supertrapp it was 95 dB at the steering wheel.
(They're all measured at the steering wheel unless I say otherwise).
Adding the SuperTrapp brought it up to 97 dB at the wheel.
The noise was then aimed sideways towards the noise meter instead of upwards to the sky.
I put 5 disks in (or took 7 out, however you say it), noise level was about the same.
Then I made a baffle with an 8" long by 1" iron pipe.
It has 7 holes 1/4" diameter along the length, 5 holes 1/8" diameter, and 3 hole .410" diameter, all adding up to about the same area as a straight 1" pipe.
The end cap is not drilled but it is shaved to allow 1" of iron pipe area pass by.
This pipe was screwed into an IPS bushing and then into the top of the muffler.
With the baffle and Super Trapp  installed I get 87 dB at the steering wheel.
The odd thing is I get 85 dB about a foot from the Supertrapp on the side away from the tractor.
The noise isn't the exhaust any more.
There is 100 dB adjacent to the engine.
So I think some sound comes from the carb or crankcase.
More as the time comes.

So someone complained a stock muffler on a mower was too noisy? Sounds like a busy body.

For sure some (possibly much) of the noise resonates from the engine itself and moving parts, rather than just the exhaust exit. Liquid cooled engines have some of the engine surrounded by water, which might help deaden the sound a bit. Also, things like snowmobiles and cars/trucks have sound absorbing foam under the hood and around the engine ect. Plastic shielding on the engines of modern cars, sometimes with foam, sometimes without are mostly for noise reduction I think. Anything tin that vibrates I think must resonate some sound. It's tough to cut it down a lot on a tractor once you get the exhaust quieted down.

And mighty, he's talking about his wife! lol.

I want to get my simplicity quieted down a bit too. Kind of annoying in some cases.

OH, My sincere apologizes then.  I did not know what S.O. was.  I was thinking it was some sort of home owners organization official.

Yes, the wife complained and I like her so I'm doing my best.  I have an extra air box for my motorcycle and I think I can run a big radiator hose from the carb to the box.  It has an oiled air filter built in and weighs less less than 5 lbs I'm sure.
By the way, the edits in the first post were to correct mistakes & nonsense words.  The spell correct thing drives me nuts.
S.O. was for Significant Other, but it also means the one woman homeowners association ;-) We work nights and sleep days so it would be great to be as quiet as a car.

Again, I apologize.  My mind was in a totally different timezone.

I think Mr.Mod may be on to something.  Maybe box in the engine to quiet it down more.  Biggest hurdle would be the cooling.  I definitely think the muffler is a great upgrade in the quieter department.  Your fabrication as always is pretty top notch.

I built my kid a mini bike, with Briggs 5 hp motor. Made the exhaust header, piping, and muffler for it too. The muffler was 2" X 4" X 6" long. Had 3 chambers in it. Well after finishing the mini bike and such, he started riding  it around the neighborhood. I heard him 2 blocks away. I was sure I nailed it on being quiet, so the cops wouldn't bust him and take the bike. Had to rethink what I did. When I built the muffler, I was thinking of a gun suppressor. That kind of thinking doesn't work for exhaust of engines. Got to thing differently, Got to slow the gasses down another way. If you slow down the gasses, the sound separates from the gasses and cancels itself out. What I came up with is I needed to expand the gasses in a big chamber. A certain ratio comes to mind but space for the chamber is usually limited. The ratio that pops in my head is a 10 to 1 size ratio chamber. The exit of the chamber can be the same as the inlet and doesn't have to be a bunch of little holes on the exit to equal the same as the inlet.

My theory is-when the exhaust exits the cylinder, it is moving with the sound waves, but the sound waves are bouncing around in the pipe and the exhaust gas is carrying it along through the pipe. If you separate the 2 in a large chamber, then most of the sound waves stay bouncing around in the large chamber canceling each other out(sound waves will keep moving fast), while the exhaust gasses slow down in the big chamber and separate from the sound waves then speed back up when exiting the chamber. The bigger the chamber, the better or quieter it will be. Don't need any baffling or packing either. The packing will help with a thin layer on the walls but usually blows out the tailpipe eventually. The exhaust gasses will find the least resistance going through the big chamber on it own with no baffling to direct it.

I put this theory to the test and built an addition to the muffler. I made a big chamber after it, a 4" X 4" X 8" long chamber. Helped alot. Now when he rides the mini bike, I here him going by only a couple houses away.

What I remember is that "limp mass" absorbs sound, and limp mass is heavy stuff with no stiffness. The cherry bomb muffler I have is 2" diameter straight through but the middle is perforated and there's a layer of fiberglass roving between the perf liner and the bulbous exterior that is most of the length.
My baffle made a lot of difference. The sound bounces back from the end cap on the baffle and the end cap on the Supertrapp. The wavelengths will cancel at some frequencies and amplify at others, except when they amplify they shake the glass liner more and turn more sound energy into heat.
The holes in the baffle pipe were started as equal area to the B&S muffler, 52 holes 1/8" diameter.
But drilling those holes was a pain so I finished one side with 7 1/4" dia holes and did the other side with 3 holes .410".The spacing is to get sound waves going in opposite directions when they can cancel out.
And that's it for now,