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

[nicholastanguma]

Is there any way to get this exhaust's decibels down to 95 or lower without utterly destroying good flow characteristics for a wide powerband, maybe using some kind of internal baffles that can work miracles?

No more length can be added to what you see here, nor changes from the shape seen here, nor a muffler added to the end.

However pipe diameter can be changed, and stepped diameters can be included in the length, if somehow this allows for miracle working internal baffling while keeping good flow characteristics across a wide powerband and limiting decibels to 95 tops.

What I'm dreaming of is an exhaust with this ultra shorty I'm-going-to-punch-you-in-your-face appearance and the performance to match...but without the I'm-a-Harley-pirate-pay-attention-to-me sound level.

[David Redszus]

Is there any way to get this exhaust's decibels down to 95 or lower without utterly destroying good flow characteristics for a wide powerband, maybe using some kind of internal baffles that can work miracles?

No more length can be added to what you see here, nor changes from the shape seen here, nor a muffler added to the end.

However pipe diameter can be changed, and stepped diameters can be included in the length, if somehow this allows for miracle working internal baffling while keeping good flow characteristics across a wide powerband and limiting decibels to 95 tops.

What I'm dreaming of is an exhaust with this ultra shorty I'm-going-to-punch-you-in-your-face appearance and the performance to match...but without the I'm-a-Harley-pirate-pay-attention-to-me sound level.

It may be possible to meet your objectives with either a side resonant or diffusion resonator.
Needed information:
Current decibel level (plus frequency range, if known)
exhaust pipe internal pipe diameter
maximum total length of exhaust system.

Why can't the exhaust system be somewhat longer?

[nicholastanguma]

It may be possible to meet your objectives with either a side resonant or diffusion resonator.
Needed information:
Current decibel level (plus frequency range, if known)
exhaust pipe internal pipe diameter
maximum total length of exhaust system.

I simply found these pics online, I don't actually have this moto in my garage. I liked what I saw and wondered if there was any way to make it a streetable exhaust. My question is purely for mental wargaming.

Why can't the exhaust system be somewhat longer?

Aesthetics, nothing but aesthetics.


Anyway, a Helmholtz resonator, hmmm, yes, that might work. Thanks for the idea!

In addition to some kind of minimal internal baffling, perhaps?

[nicholastanguma]

Or might a megabomb header design offer a better solution?

[Rick!]

It's a 500 2 valve air cooled engine. By today's standards, it makes low power, hence, low noise.
Put a stock exhaust on it and call it a day.

The only apparatus I've found to significantly effect noise output on a single cylinder 4 stroke is to decrease pipe diameter, increase the volume of glass packing and decrease the muffler outlet size and throw in a spark arrestor. All reduce flow and potential power and change jetting and gearing requirements.

[pcnsd]

1. What standard is tested to?
2. What RPM is the test is conducted at?

I race that basic machine with some changes and must past sound at each race based on the J1287 off-hwy sound test at 96 db.
I'll first say that observing the results of the testing over time makes me consider the test to be mostly unreliable because the RPM indicating device is demonstrably inaccurate.
I posted 2 designs for this machine, They are both down pipes. Both will pass the above sound if they are packed fresh and correctly, but the larger muffle is quieter through the entire RPM range and is my preference.
https://flattrack.info/threads/two-exha ... ha-500.57/

[David Redszus]

It may be possible to meet your objectives with either a side resonant or diffusion resonator.
Needed information:
Current decibel level (plus frequency range, if known)
exhaust pipe internal pipe diameter
maximum total length of exhaust system.

I simply found these pics online, I don't actually have this moto in my garage. I liked what I saw and wondered if there was any way to make it a streetable exhaust. My question is purely for mental wargaming.

Why can't the exhaust system be somewhat longer?

Aesthetics, nothing but aesthetics.


Anyway, a Helmholtz resonator, hmmm, yes, that might work. Thanks for the idea!

In addition to some kind of minimal internal baffling, perhaps?

We would use a silencing resonator, not a Helmholtz. Think in terms of a pipe within a pipe with holes in the inner pipe.
Somewhat like a firearm suppressor.

RPM has little effect on noise frequency since the engine speed is so low. Throttle opening has no effect on frequency
but greatly affects decibels.

[Rick!]

We would use a silencing resonator, not a Helmholtz. Think in terms of a pipe within a pipe with holes in the inner pipe.
Somewhat like a firearm suppressor.

RPM has little effect on noise frequency since the engine speed is so low. Throttle opening has no effect on frequency
but greatly affects decibels.

Help me out a little bit, David.
The fundamental noise frequency on an ICE is its firing frequency.
On a single cylinder 4 stroke, that would be the RPM/2/60.
With such a low frequency, the wavelength is pretty long, way longer than the exhaust system itself, which is why a single cylinder 4 smoker's exhaust note carries so far. Knowing this, it would be futile to attempt any kind of quarter wave resonators on the OP's subject motorcycle.
So I'm a little confused by your statement that RPM has little effect on noise frequency.
I agree that throttle opening can affect overall sound pressure as overlap noise comes right back out the intake, creating just as loud of noise as out the exhaust on some engines.

[pcnsd]

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.

[pcnsd]

It may be possible to meet your objectives with either a side resonant or diffusion resonator.
Needed information:
Current decibel level (plus frequency range, if known)
exhaust pipe internal pipe diameter
maximum total length of exhaust system.

I simply found these pics online, I don't actually have this moto in my garage. I liked what I saw and wondered if there was any way to make it a streetable exhaust. My question is purely for mental wargaming.

Why can't the exhaust system be somewhat longer?

Aesthetics, nothing but aesthetics.


Anyway, a Helmholtz resonator, hmmm, yes, that might work. Thanks for the idea!

In addition to some kind of minimal internal baffling, perhaps?

Another observation. The volume chambers applied by various manufacturers of 4T single exhaust systems are about producing an exhaust system that is less RPM sensitive than it is about sound suppression. These exhaust must work on a wide range of machine setups from stock to OMG and still provide a happy customer.

[David Redszus]

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?

[frnkeore]

What about the Super Trap mufflers?

https://supertrapp.com/catalog/product/ ... ategory/2/

[David Redszus]

What about the Super Trap mufflers?

https://supertrapp.com/catalog/product/ ... ategory/2/

I don't know about current SuperTrapp designs, but years ago they did not work very well on my Porsche 911S.

[af2]

And still don't I have made a few mufflers for the tt and the xr that were quiet and ran fine. They were basically like a flo master. but using a round pipe at 4" for the muffler.

[pcnsd]

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.