Do you have decibel readings? Does your sound meter indicate the frequency spectrum?pcnsd wrote: ↑Sat Jun 05, 2021 12:40 amThis is what I'm running now. Of three attempts, it works the best and is quietest. I am using the open ended Supertrapp end cap with 8 disc to lower the target rpm to ~7500.David Redszus wrote: ↑Thu Jun 03, 2021 12:20 pmExhaust noise loudness is primarily a function of mass flow through a variable size orifice (the valve curtain area).pcnsd wrote: ↑Wed Jun 02, 2021 7:38 pm All the testing I've been subjected to was based on distance and rpm. I bought a decibel meter to confirm I could pass the testing. I can see that rpm and throttle position are related, but question with Rick the proposition that rpm is not a factor in sound generation level. On a practical observation, absorption mufflers are effective across a wide range of frequency. Canceling schemes address only a narrow range of frequency. Big mufflers work on large singles. JMO.
A wide open throttle will increase the mass flow as will higher engine speed (increased mass flow).
But noise is the result of combination of amplitude (pressure ratio) and frequency.
From the moment the exhaust valve opens, the exhaust gas will flow through an area changing orifice,
changing its frequency. The frequency range is typically from about 100Hz to 4000Hz. A four stroke engine running at 9000 rpm will produce an exhaust pulse at 75 Hz; a pulse consisting of the entire frequency spectrum. The frequency range will also be affected by the exhaust gas temperature.
The exhaust gas pulse does not produce a pure tone. In addition to the frequencies produced at the valve curtain,
the gas pressure waves will cause metal surfaces to vibrate at various frequencies and there will be assorted harmonics produced due to reflections or pipe bends and obstacles.
A straight pipe will not attenuate any frequencies and will rely on air friction to reduce noise.
A noise cancellation silencer such as a diffusion type can reduce the decibels across a wide range of
frequencies (250Hz to 1750Hz), if designed properly for the application. It will however allow pass bands at certain frequencies while cancelling most others.
A side resonanttype silencer will cancel specific frequencies typically centered about a single frequency.
In addition, absorption type silencers have used glass wool packing as a means of reducing certain frequencies.
A well designed silencer may even use a combination design that makes use of all three types to not just reduce
decibels, but to create a certain agreeable sound.
Given a specific exhaust pipe diameter, maximum permissible length and resonator diameter, we can predict the decibel attenuation level and the specific frequencies. And the band pass frequencies where the silencer does not work.
Do you have any dimensions?
Based on the dimensions of your SuperTrapp, (with an open end), I have calculated the sound attenuation as follows.
20 decibels from 750 to 2000Hz
40+ decibels from 1100 to 1250Hz
with a 55 decibel spike at1175Hz
The exact frequency attenuation will depend on the size (3 to 6mm) and number (50 to 65) of holes in the resonator section. You can move the location of the SuperTrapp resonator to any position in your exhaust system to improve
resonant exhaust tuning.
A alternative silencer can be constructed very easily.
Using your existing pipe (1 3/4" dia) by 40" length.
Build a sheet metal resonator cylinder 6" dia by 24" long.
Insert your downpipe into the resonator to a depth of 12".
Use an exit tailpipe, (1 /3/8" x 12" long), inserted 6" into the resonator.
The overall length should be the same as you now have.
The sound attenuation will be:
20+ decibels from 300 to 1700Hz
35+decibels at 1100Hz.
These calcs assume a gas temperature of 1400degF.