The straight through free flowing megaphone muffler i made is pretty loud, I was OK with that when this bike had a performance oriented engine, now that it is a very mild hop up I want to tame the exhaust sound a bit.
To try apply some science to this completely non scientific venture I bought the cheapest dB meter off ebay to measure results. I doubt it is accurate, but it is probably adequate for testing the differences between the options.
The two options I wanted to try to reduce sound are just inserts so I would not have to modify the muffler. The muffler has an 15" baffle with a 1.75" 16GA perforated tube. It is wrapped with a layer of stainless steel mesh, followed by stainless wool, and then fiberglass.
Option 1: The classic "dB killer" style inserts that i have seen for years. I used a 1.25" tube - 4" long - (20) 3/8" holes. I added some neoprene foam around the flange so i could just tape it in the exhaust and get a good seal.
Option 2: "laminar flow cone" style muffler from Flowmaster. I know the Flowmaster has engineers sciencing out all the parameters for their cone designs to get optimal results, I created mine based on the existing tube diameter, and an arbitrary length of 4.5" I used a cone calculator to draw a template on 20GA stainless perforated sheet with 1/8" holes - 50% open area. I then used a tapered punch and a hammer to form the sheet into the cone before welding the seam and adding a flange. I then riveted the cones on either end of the existing baffle pointing in.
The results were:
Existing muffler - 84 at idle 102 at 4k RPM
Db killer - 81 at idle 99 at 4k RPM
Laminar Cones - 84 at idle 100 at 4k RPM
The Db killer does make a noticeable reduction in sound, however it modifies the tone and sounds tinny and cheap.
The laminar cones also make a noticeable reduction in sound, but retain a very low tone. it actually sounds better than the straight core, more of a rumble.
Both the inserts took away the sharp percussive nature of the straight muffler so I consider that a win, however neither of them are significantly quieter. Its OK for now but i probably will return to this to see if i can quiet this down further. Unfortunately I think this means a major revision to the internals, something i was hoping to avoid, as adding them will probably mean cutting the muffler open which i really don't want to do. I cant seem to find any good info on how to design muffler baffles, so it will probably be mostly trial and error. I think i may try a stock style baffle system.