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Discussion Starter #1
A couple years ago I dug the stock exhaust system out of the attic over the garage and opened it up. I had been running a Akrapovic header and Urban brawlers but that was way too loud. High 90 DBs on start up and mid 90s at idle. That mod can be found here.


At that time it crossed my mind to go to a under belly only but I just couldnt see it. Something about the swing arm being completely exposed bothered me then.
Well I think I'm going to take another look at it and also make a change inside the belly can to see if I can eliminate a crackle in the exhaust note that I hear between 2 and 5K rpm. Before I do that though I took some sound readings to document what's there now. Here they are.
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Ambient noise was very low, never saw more then 50 on the meter. Love my neighborhood.

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Akrapovic header 4 into two section used on all setups.
Cold start with modified belly can and urban brawlers.

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Modified belly can and brawlers after warm up.

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Modified belly can and OEM slip ons coldish start.

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Modified belly can and OEM slip ons after warm up.

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Open header

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Akrapovic header Y pipe and OEM slip ons. This one surprised me as I didnt think it was this loud. It's very deep though so maybe it's the frequency.

Now comes opening the OEM can up and getting rid of the remaining section of 1 1/2 pipe and possibly the insertion of a Flow Master ish delta baffle. Then remove and patch the left side port and either enlarge or add an additional right side port plus fabricat a tip. If you know of a good article on muffler design please send it my way.
 

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Discussion Starter #3
Its something I remember from an add back years ago. Looks something like three Chevrons or Vs stacked in line . Sorta of like these two but with three and in increasing lengths from short to long.
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Nice SPL meter. I think I still have my Radio Shack digital meter tucked away somewhere.

FWIW, I'd do the test over an open field with a soft surface like a nice thick lawn. Not too many of of have access to an anechoic chamber, so whatever you can do to cut down on reflective gain, the more accurate the reading. That concrete will net you a 3 dB gain. If there are any wall near by, the meter could pick another 1-3 dB.
 

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Discussion Starter #5
@Volfy Thanks. It was on sale at Sears one day and scoped it up.
Your absolutely correct about reflective noise interfering and or contaminating a sound test. I had wondered if some one might bring up the bucket even.
In the past I've had to take classes so that I could be certified/competent to take and record sound level readings in and around our work environments to make sure the level of hearing protection being used exceeded industry and NIOSH standards. Now days their even starting to pay attention to the frequency of sound and I think that is way over due.

The one thing that both instructors and quite a few of the articles on the subject refered to though was this.
The best true sound level.... test is one given in the exact environment the subjects will be In.
 

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Discussion Starter #6 (Edited)
Time to fire it up and play welder.
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Absolute measurements are less important than the relative values here, I think, and comparing to stock is perfect because everyone knows what stock sounds like. But Avintage69 also touched on the point that frequency plays as much of a role in the "irritation factor" of motorcycle exhausts that a simple SPL meter can't give you. Typically, lower frequencies are tolerated much better than higher frequencies because us humans are more sensitive to that higher range of frequencies. Indoor measurements will worse because the higher frequencies reflect off hard surfaces better than the lower frequencies so they sound louder.
I've been thinking a lot about getting a header and keeping the stock cans for sound control so this thread gave me a lot of information on what to expect. Thanks!
 

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The problem with taking measurement with a "live" surface nearby is that not only do you get a reflective gain, you also get wave interference from the reflected sound waves - just like the ripples on water surface. This could mean that if you move the microphone slightly, you end up with +dB one direction, and -dB another direction. How much this happens depends on the frequency you are measuring. Different wavelengths will have different interference patterns.

If you park the bike in the middle of a large opening, and the ground surface is covered with acoustically dead material (like thick grass over soft soil), the SPL meter will mostly just pick up the direct sound wave. The session to session results can be much more accurately compared, without having to place the mic/meter at the EXACT same location every time.

That said, motorcycle can be ridden anywhere, and the perceived exhaust sounds can be quite dramatic from place to place. I remember riding in NYC years ago, cruising down one of the major avenue when it's relatively quiet in the wee hours in the morning, the sound differences between open side street and the block buildings was mesmerizing. These days, I don't much care about exhaust note any more and prefer the peace and quiet. But every once in a while, I do get a bit of kick revving the bike when I go through tunnels or under narrow overpasses. :devilish:
 

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BTW, reflective interference is exactly how those V baffles in that Flow Master Delta muffler help reduce the amplitude of the exhaust sound pressure pulses.

Another thing I noticed is that your Craftsman SPL meter seems to be set to dbA or A-weighed, so it is already accounting for the low frequency roll-off in human hearing. I think a lot of us are just conditioned to prefer a lower tone exhaust.

Thanks for taking the time to take the pics/notes and post your results. It's another subject I find fascinating. I used to be heavily involved in auto sound systems. Played a lot with spectrum analyzers and built a lot of subwoofers in my day. Fond memories.
 

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Discussion Starter #10
Storm rolled in and I lost power so heres were I'm at.

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The outer layer peeled back. You can see the sound deadening material.

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Sorry I didnt take a picture of the perforated screen before cutting it out. Think I should have just used a cut off wheel. Need practice with the plasma cutter I bought.

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I kept calling it a 1 1/2" pipe inside the can, turns out I lied It's a 1.685.
What is that nearly 1 11/16". Or 42 mm.

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As you can see it looks like they bounced the exhaust off the back wall.

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No sound deadening material in the back section.

Sensitive content, not recommended for those under 18 Show Content
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Heres a good look at where I made the previous changes from the inside and the relation of our exit ports.

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Lost power while hacking on the pipe and baffle. Glad I'm decent with a grinder.
 

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Discussion Starter #11 (Edited)
Is everyone seeing a sensitive content tag on the second to last picture of the muffler? Guess they thought I butchered the poor thing enough.
 
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Yeah, you have to be careful when you post muffler porn. I mean, who knows if you had consent to strip that poor baffle in front of a camera? And then to dissect it right down to it's core! OMG, have you no decency? I wonder what bit of AI content detection software you triggered. Maybe the intake pipe, bent in the middle and swelling on the end sent their software into a tizzy.
Thanks for showing that, BTW. I've always wondered what the can contained. Rapping on the outside told me it wasn't just a hollow shell.
 

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Yeah, you have to be careful when you post muffler porn. I mean, who knows if you had consent to strip that poor baffle in front of a camera? And then to dissect it right down to it's core! OMG, have you no decency?
OK... this was some of the funniest sh!t I've heard in awhile. Thanks for the laugh.
 

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I feel like Avintage could keister his sound meter. As long as he does it , for each test, it's not going to matter.

Volfy, this is a test where the specific number is much less important than the relationship between the various mufflers. And if hes off by 15%...or 50%, what are you going to do to fix it, or to help? Build him a room? Hes not substituting or working for CARB during covid 19. This is just a basic test. Avintage, thanks for taking the time to do this. Think about Volfy's reaction next time you hear something like, "All opinions matter" or, "there are no stupid questions".

It would be I interesting to use a good, larger diameter header , and weld it to the stock box. It would be nice to know what the dimensions were for the cheap, ebay headers. If they copied Akra, you could do this for 150.00, all in.
 

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Sweet write up avintage, you might come up with a patent pending on the modification of the bread box, looks like you might as well hit it all the way and start with a blank slate, they use to put plates with holes in the silencers on our racing karts to attenuate the sound, it was like a series of three plates with holes.
 

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Avintage, throw that mutilated box away and change out the brawlers lol, I bet you could sell them fast.
 

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The last time I looked at my stock header, I remember seeing g how tall those welds were where the head pipes meet the head. I know you can take some material off of those and help it flow better.

Avintage is not a puss. The info he posts is his. Volfy, july 4th will be here, soon. On that day, when you walk out to the front of your trailer to post your rainbow flag, I want you to think of Avintage...well, not in the rainbow context, but you need to remember there are good, hard working americans who are not afraid to rip apart Japans best and improve on it.
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The last time I looked at my stock header, I remember seeing g how tall those welds were where the head pipes meet the head. I know you can take some material off of those and help it flow better.

Avintage is not a puss. The info he posts is his. Volfy, july 4th will be here, soon. On that day, when you walk out to the front of your trailer to post your rainbow flag, I want you to think of Avintage...well, not in the rainbow context, but you need to remember there are good, hard working americans who are not afraid to rip apart Japans best and improve on it. View attachment 29328
Easily post of the year 😄 😄
 
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