I have no measured database, but I made and modified many silencers during more than half century, as I play with engines.
First of all, practically every usual silencers can fulfill the requirements of FAI Sporting code. (As I know by heart it is below 94 dB (A), measured from 1 meter, (no reflective object at the area).
All we want lower, furthermore not too sharp, irritating sound. 
As I observed, there are some antagonistic limits:
-when the chamber is smaller than 5 x swept volume of the engine, the sound will be too loud and sharp, or the silencer makes too high counter-pression for the engine, resulting power loss, and poor sensitivity in manoeuvres.
-when the chamber is 10 x or more than swept volume, we can get affordable silent and "round" sound, with good responsibility of the engine, but its weight and dimensions will be unaffordably big...
-one thing pro memoria: usually the model needs some silicone tube at the end of the silencer's outlet: it is very easy to make some unwanted Helmholtz resonator, which works differently at different rpm's, for this reason there must be implemented some baffle etc. to eliminate all resonances.
Istvan

Even in a ' box ' chamber , you get shock wave reflections .

But the suck it & see , would be easiest . Seldom are all factors envisaged , so Calcs'd be a GUIDE rather set in concrete .

The stack length also .

HP 40 I had two short ' extentions ' tween motor and a 3/4 ? tube . But used about ten 2 m.m. holes in the side .
the Theory being it broadens the powerband ! ( Hope springs eternal    ) AND is quiter - as the shock is spread acoss ,
so a ruble rather'n a CRACK . THAT seemed to work , and the engine ran well .

Its kind of a ' Brooklands Can ' ( Copying the fish tail drilled outlets ) which were ' race sinlencers .



SO , if your putting a ' flat washer ' plate , in the back . You can get out the black & decker .

Use about the same area as would be the tailpipe .

it is very easy to make some unwanted Helmholtz resonator, which works differently at different rpm's, for this reason there must be implemented some baffle etc. to eliminate all resonances.

  Or, crazy idea, you could make an intentional Helmholtz resonator.

   Brett

-when the chamber is smaller than 5 x swept volume of the engine, the sound will be too loud and sharp, or the silencer makes too high counter-pression for the engine, resulting power loss, and poor sensitivity in manoeuvres.
-when the chamber is 10 x or more than swept volume, we can get affordable silent and "round" sound, with good responsibility of the engine, but its weight and dimensions will be unaffordably big...

I agree with the 10X estimate for four-stoke.  I believe in 7X for two-stroke because the bottom 30% if the displacement does not contribute to the combustion volume. 

The ear-splitting pulse only occupied about 2% of the time.  An expansion chamber of 70% of the nominal displacement will allow this pulse to expand with being heard from the outside while also allowing the engine to function without loss of power.

I got a question about my comment about 70% of displacement on a two-stroke engine.

The ear-splitting, damaging part of the sound occurs only as the exhaust port opens.  This is confirmed on a sound analyzing app such as "engine RPM" on an I Phone.  The engine does not compress air and create power and noise with the exhaust port open, the bottom 30% of the stroke.

An expansion allows the exhaust to lose it's bite while expanding within the chamber.  The volume of the chamber reduces the back pressure.  Opening up the ultimate exhaust bigger than 20% of the piston's exhaust opening defeats the muffler and allows the pulse to travel through without loss of impact.

Remember, the damaging noise only occurs 2% of the time, the ultimate muffler opening is open 100% of the time.  Even 20% is generous.  The function of the muffler is convert the series of loud pulses into a steady flow averages-down the peak energy.

I'm not a huge muffler expert, but from the few that I've had in my hands I've noted a large variation in weight and a small variation in chamber volume.

If you're chasing weight reduction, before you go on a quest to make the chamber smaller, consider making the walls judiciously thinner.  Either sacrifice some durability with ordinary aluminum, or sacrifice on low cost with some expensive aluminum (you're taking the effort to weld stuff up anyway!)

Or -- make a real tongue muffler, roughly the shape of, and (for a 40) a bit bigger than a human tongue.  Because that's more honest than the things people usually call "tongue mufflers".  The pictured example has more volume than yours, and less visual impact aside from the fact that someone deliberately shaped it to look like a body part.

If your brother can weld good enough for the color to be even, anodize it pink when he's done.

https://www.fai.org/sites/default/files/documents/f2c_design_development_of_silencers_for_f2c_rm_dec_2013.pdf

And if youve got a bad hair problem ,




or a STALKER , the can diameter matches . My one does, anyway - Nail onto the exhaust fitting . Dosnt need a rear end , as it's got one already .

Bit thin for welding unless youre midgiterised , and the gear . But rather light & also inexpensive . Particularly when its on special . Make your hair stand on end .

It worked.  Better sound than tongue muffler.
We will see how it flies.  Still breaking in engine.