In general, and I must generalize, since I don't have much specific information on the dynamic relationship between exhaust gas velocity, pressure and temperature. The following is my understanding from reading of technical information and a career spent flying airplanes which were certified to progressively stricter requirements for noise abatement.
Turbojet engines were very noisy. They had hot, high velocity exhaust gasses. Turbofan engines were much more efficient producers of useful thrust. Turbofans were also noticeably quieter due to the cool fan exhaust flowing around the hot, high velocity core flow. With an increasing emphasis on noise abatement, the trend was toward higher bypass ratios which have a larger percentage of the thrust coming from the fan duct. As regulations tightened, the exhaust duct was frequently shaped with "petals" or "flutes" to force greater mixing of the fan and core exhaust. Also, the inlet ducts were acoustically treated to absorb fan noise.
From this, I am of the opinion that heat, pressure and velocity are the principal cause of engine exhaust noise. We can lower perceived noise by cooling, slowing, and reducing the pressure of the exhaust flow. The very action of allowing the high pressure exhaust to expand rapidly as it leaves the manifold into an expansion chamber (muffler body) should cause a sudden loss of heat and noise energy. Noise can also be reduced by baffling the exhaust flow to create acoustic interference.
Basically, I rely on slowing, mixing to cool, and reducing the pressure of the exhaust stream. I think this is the safest way to quiet the exhaust without causing back pressure which might overheat our engines or harm the character of our stunt run.
Almost no one ran mufflers until they became required for the 1971 competition season. I would have preferred not to run a muffler, but, since it was required, would use a flow through type muffler which should cause less back pressure. My 1971 airplane was the NATs winning 1972 and 1973 Sea Fury. The airplane was chosen, in part, for its radial cowling which could enclose a muffler, but, only a custom designed muffler. I designed a short, fat, flow through muffler which would direct the exhaust gasses down an internal bypass to a scale opening in the side of the airplane.
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I was never really happy with the available power for the Sea Fury so I designed the molded Mustang. "Snaggletooth" would have a better power loading with a ST 46 than the Sea Fury with a ST 60. The "Snaggletooth" series of molded Mustangs would also require a very challenging and innovative design to get an effective muffler into a scale like fuselage. At the time, stunt timed rear exhaust engines were not available. By "canting" the engine and hand carving a very close fitting exhaust manifold elbow it would be possible. "Canting" the engine was risky but necessary for the concept of a very scale like competition airplane. No one knew how this might affect the run. I was lucky. Excepting an unusual tank height, the run was unaffected.
This muffler design is also a flow-though design but, this time, with an annular inlet around the manifold instead of a center inlet location. It worked well, both as a muffler, and as an "augmented exhaust system". An "augmented exhaust" would use induced flow through the muffler to essentially "pump" air from the engine compartment, mix it with engine exhaust in the muffler body expansion chamber, and dump the cooler, lower pressure, slower moving, exhaust overboard. This type augmenter was used on the Cessna 310 and the Convair 340 to increase cooling airflow across the engine. The nearer of the three muffler bodies was the original configuration. The second muffler body has louvers to try to introduce more cooling air into the expansion chamber. the third body has an extra exhaust tube and larger louvers to farther reduce back pressure in the muffler body. They all worked fine, but stains on the last body louvers makes me think that they were acting as supplemental exits instead of augmenting inlets. The only machining on the manifolds was the inlet where the manifold mounted on the engine. The rest was about 8 hours of carving. Effort aside, the really important feature of this muffler was that it worked very well in a very competitive environment.
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The problem with this insistence on internal mufflers means that it might be necessary to essentially "carve" muffler parts such as this manifold essentially from a block of aluminum with a Dremel Tool. It sorta depends on how bad you want it, or how necessary you think it is. Since the muffler worked well and helped the airplane to win the 1977 NATs and place 2nd in the 1978 World Championship, I thought it worth the effort.
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My latest design for two stroke, rear exhaust engines, has been to add airframe mounted expansion chambers to a standard, but shortened, rear exhaust manifold. Shown are the mufflers for the new "Snaggletooth" and the original Classic Bearcat. They rely, for effectiveness, on the concept of cooling the exhaust, mixing it, and reducing its pressure and velocity in the expansion chamber to reduce the heat energy and noise. I hope for, rather than rely on, the augmented muffler effect to provide some "pumping" through the chamber. They work well, but aren't the quietest of mufflers. Regardless of the apparent difference between these and the Mustang mufflers above, they are functionally the same. This manifold/expansion chamber muffler configuration does lend itself to being additionally baffled, when necessary, to meet required noise measurements. I will do this, if necessary, by adding converging and diverging vanes behind the manifold through the muffler body.
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My final effort has been to muffle, as required, my four stroke powered Bearcat and Critical Mass. I originally designed and built a simple muffler from sheet aluminum and aluminum tubing held together with JB Weld, and lots of CA'ed thread. It was rigidly mounted to the airframe and coupled to the engine by the flexible yellow silicon tube. This should protect a lightly constructed muffler from the engine's vibration and, to some extent, isolate the muffler from the engine's exhaust heat. I think it might have worked for a two stroke engine, but the extra heat of four stroke exhaust blew the thing apart. Fortunately, a stock muffler could be adapted to work in the absence of the original design. To salvage the original design, I considered welding it together, but finally decided it would be cheaper and easier to pop rivet the thing. I did, and think it will work fine now. I'll try it for a couple of hours on the test stand while breaking in a new engine. If there are no problems, I'll retrofit it into the BBQB Bearcat and make a new one for the Critical Mass. It will be slightly lighter than the stock muffler, and I wouldn't be surprised if it is noticeably quieter.
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The second photo shows the internal configuration of my four stroke muffler design. The exhaust flows aft through the first tube where it dumps into the muffler body and migrates forward to the front. From there it enters the second tube and flows aft again to exit under the leading edge of the wing. The balsa holds the tubes in position while the exterior aluminum sheet is wrapped and attached, then removed before the ends are added. I would post photos of the riveted result but, today, I can't find it. What the heck, it looks the same. It is the same excepting the pop rivets. I guess I'll have to clean up the workshop.
All the best stuff does not necessarily come from the local hobby shop. Neat things are possible if one will work enough to achieve the dream.
Al
