Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

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David Redszus
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by David Redszus »

pcnsd wrote: Sat Jun 05, 2021 12:40 am
David Redszus wrote: Thu Jun 03, 2021 12:20 pm
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.
Exhaust noise loudness is primarily a function of mass flow through a variable size orifice (the valve curtain area).
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?
This 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.
Image
Do you have decibel readings? Does your sound meter indicate the frequency spectrum?

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.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by Rick! »

A 2 foot long muffler that is 6" in diameter is a fundamental use of volume to provide attenuation.
Anyone that has has experience in designing, testing and building resonators or mufflers knows this.
The packaging of such a volume is the challenging part as the 2 foot long bazooka really doesn't fit the OP's vehicle.
Also, SAE J1287 is just a free rev test for open course competition vehicles.

pcsnd's exhaust has the necessary ingredients to meet J1287 with an early vintage "thumper."
The key is the use of the Supertrapp stack to control the exit area, IMO.
At 32hp stock, the exhaust may add what, 1-1.5hp at best? I'm sure someone will be able to come up with a number and let us know.
My experience is that Megabombs and Ti tubes and fancy 3 letter manufacturer racing exhausts typically add 1-1.9hp on a 42hp engine on a 250F mx bike. Cams, big bores and headwork provide a bigger bang for the buck in my experience.
Though it's a fun exercise, hitting this vintage bike exhaust with as big a science hammer as has been applied illustrates the boundary where theory and application can diverge. Packaging and weight and resulting handling affects win out over maximum noise attenuation most of the time in the OHV world.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by David Redszus »

Though it's a fun exercise, hitting this vintage bike exhaust with as big a science hammer as has been applied illustrates the boundary where theory and application can diverge. Packaging and weight and resulting handling affects win out over maximum noise attenuation most of the time in the OHV world.
I find no reason to disagree with your observations.

My objective was simply to show that sound intensity and frequency can be determined from basic measurements.

Note that the suggested resonator is only 2" greater in diameter than the SuperTrapp, and only 8.5" longer.
Since the overall length remains unchanged, it can be easily fitted. But packaging and weight are always valid
considerations.

Having built several racing exhausts to meet SCCA noise limits, I was amazed to discover how frequently the original
dimensions were not optimum for performance to begin with. And that no valid sound measurement had been taken to determine the required attenuation. Also, not all the noise comes out the exhaust pipe.

Silencers are not black magic; they are products of science and nothing more.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by pcnsd »

David,
As Rick expressed, I have packaging concerns regarding fitment. I am currently limited to a 4 inch muffler diameter due to swing arm clearance, but have considered a muffler based on a 4 inch radius oval. I choose to go with the adsorption tube model because I have experience with it and knew it could be made to pass sound if there was more of it. They are also easier to construct and have flow rates that fall withing David Vizard's suggested range to prevent horsepower loss from exhaust back pressure. Along the way, I was able to note a 3 inch muffler core works better suppressing sound and weighs less, but the 2.5" core has a cleaner idle while running.

I have a Type1 sound meter and use it on the slow response setting per J1287. I do not have a way to measure frequency spectrum at this time.
Because such things interest me whether or not I actually use them, I would like to learn how to calculate the suppression spread available from a given muffler design similar to your suggested muffler, if you could recommend a source for such information.

Rick,
32 HP stock for these is generous. I think about 29 RWHP. They can be pushed a bit pass 50 RWHP in the right hands on the stock stroke and 90mm bore, but lose reliability in the process. Today they serve mostly as beginner and old guy rides because they are gentle in the general scheme of things.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by David Redszus »

The key is the use of the Supertrapp stack to control the exit area, IMO.
Just curious.
How does a SuperTrapp, with its closed end reflected pressure wave, affect the resonant exhaust tuning of the engine?
As compared to moving the SuperTrapp upstream, and adding a slightly smaller exhaust pipe with an open end?
The lengths would be the same.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by pcnsd »

David,
I'm not sure I understand the question or perhaps you misunderstood the end cap used. Closed end, end caps result in a reversion wave strong enough to blow the fuel out of the carb and prevent rpm above about 3200 rpm. Runs bad well below that. I use an open ended end cap. It allows straight through gas flow...more or less.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by Walter R. Malik »

pcnsd wrote: Sat Jun 05, 2021 12:40 am
David Redszus wrote: Thu Jun 03, 2021 12:20 pm
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.
Exhaust noise loudness is primarily a function of mass flow through a variable size orifice (the valve curtain area).
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?
This 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.
Image
At the track I raced years ago with a Formula Ford the sound limit was 85db at 50 feet drive-by.
A Super Trap similar to yours with the end cap turned around with the taper inward and 12 plates put before the end finally made it under the limit.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by chimpvalet »

Seems that the ST with end cap stack of plates should act in a diode fashion, with resonant tuning to that point as it's open air beyond that.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by David Redszus »

pcnsd wrote: Tue Jun 08, 2021 8:40 pm David,
I'm not sure I understand the question or perhaps you misunderstood the end cap used. Closed end, end caps result in a reversion wave strong enough to blow the fuel out of the carb and prevent rpm above about 3200 rpm. Runs bad well below that. I use an open ended end cap. It allows straight through gas flow...more or less.
As I indicated earlier, I have not used a SuperTrapp for many years. When we did use them, they had closed end caps which required a large number of plates to pass sufficient exhaust gas without excessive back pressure. Back then (late 70's), we did not use open end caps.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by jsgarage »

How close to 95 dB are you right now? I assume you own a dB meter so you can test small changes yourself. I also assume someone else picked 95 dB as the cut-off and it was not a grab-outa-the-sky number. The accepted std meter set up is a calibrated dB meter (often a Radio Shack unit) set on 'B' scale, 'fast' response, 50 ft from the edge of a track at right angles to the vehicle (NOT pointing straight up the tail pipe(s). dB meters are also typically set up at the bottom of a climb so racers cannot sandbag their throttle openings effectively and still set a decent lap time.

A turn-down end on most exhaust pipes drops the measured sound level by about 1 dB, and pointing the resulting outlet away from the dB meter also helps. On cars, dual exhausts are often both routed on the opposite side to the meter placement, pointing away from the vehicle (and meter) at a 45-75 degree angle. I realize both of these last are not exactly reducing your machine's actual sound level but if it improve the measured amount of noise the thing generates.....

One big problem you have with your thumper is, a significant proportion of the noise you hear comes right thru the cyiinder wall, cyiinder head and exhaust pipe wall. A water jacketed engine with a radiator will be measurably quieter than an equivalent air cooled machine which has sympathetically ringing cylinder & head cooling fins, gear drive noise etc. Double-walling the exhaust may help and you can sort-of-check this by temporarily wrapping the whole exhaust for an easy before & after dB check. If 95 dB is not a track-or police-required sound level but a self-imposed goal, be satisfied with 1 dB improvements as sound levels are logarithmic so 1 dB is a significant change.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by BirdMan »

Just to give you an idea. I have made 2 sets of collector mufflers/noise suppressors for our drag race cars. On my old headers 7/8" tube to collector which is 3" DB A scale 96 or 98, new ones are 3 step 1-5/8, 1-3/4, 1-7/8 into 3" collector DB A scale 85.

Headers made for sons 412W are 3 step 1-7/8, 2, 2-1/8 into 3-1/2" collector. The outer shell is 5" dia. Only set I made for it and don't remember DBA but can do burnouts and tb stall 6000 without hurting ears.
Uncoated headers 021.jpg
Uncoated headers 008.jpg
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by David Redszus »

BirdMan wrote: Thu Jun 24, 2021 10:31 am Just to give you an idea. I have made 2 sets of collector mufflers/noise suppressors for our drag race cars. On my old headers 7/8" tube to collector which is 3" DB A scale 96 or 98, new ones are 3 step 1-5/8, 1-3/4, 1-7/8 into 3" collector DB A scale 85.

Headers made for sons 412W are 3 step 1-7/8, 2, 2-1/8 into 3-1/2" collector. The outer shell is 5" dia. Only set I made for it and don't remember DBA but can do burnouts and tb stall 6000 without hurting ears.
Uncoated headers 021.jpgUncoated headers 008.jpg
Consider replacing the one large diameter exit orifice with two smaller pipes of equal combined area.
You may find a noise reduction without a flow loss.
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Re: Any Way To Get This Exhaust Down To 95 Decibels Or Lower Without Destroying Good Flow?

Post by BirdMan »

Dave, I don't care if there is a flow loss as car runs faster than I intended it to and noise level is very tolerable.

My car went from 98 down to 85 DBA and it didn't run any different, slower or faster.
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