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Discussion Starter · #1 · (Edited)
I've noticed quite a few posts complaining about noisy exhausts and droning, often asking advice on what to do about the problem. A few of these appeals have popped up quite recently, and I decided to write-up a short overview of silencers, some of the basic types available, their pros and cons, and my own recommendations on dealing with excessive noise and drone.

This write-up is attached to this post as a pdf file. It should be possible to gloss over the technical discussions without missing the main points, and these points are summarized. I hope it will be helpful to some members of the community.
 

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I look forward to reading it.
 

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I remember learning in my undergraduate control theory class that any "system" can be converted to any other "system". One of the examples we did was taking a mechanical system, which was a muffler, and converting it to an electrical system, which was an electrical circuit comprised of reactive and linear elements. Once it was in that form the frequency response was much easier to see. We got to see how a simple muffler was a lowpass filter, blocking high frequency gas movements which are easier to hear, and allowing low frequency gas movements to pass through, which allows the engine to breathe still.

That was when I realized how interconnected everything really is lol
 

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The end result was like

A screen -> a resistor
a long tube -> an inductor
a blocked off chamber -> a capacitor

You can basically make any type of filter you want with these three things, this is why on high end cars the sound is so good, because the designers spend tons of time selecting the components of the exhaust so that the sound is exactly how they want.

An exhaust is a large acoustic filter.
 

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Excellent write-up; it's good to know some people used their time to do things like this during lockdown rather than bingeing Youtube like I did.... Seems like I accidentally did things right when I did my car up, which is good to know.
 

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Discussion Starter · #7 ·
I remember learning in my undergraduate control theory class that any "system" can be converted to any other "system". One of the examples we did was taking a mechanical system, which was a muffler, and converting it to an electrical system, which was an electrical circuit comprised of reactive and linear elements. Once it was in that form the frequency response was much easier to see. We got to see how a simple muffler was a lowpass filter, blocking high frequency gas movements which are easier to hear, and allowing low frequency gas movements to pass through, which allows the engine to breathe still.

That was when I realized how interconnected everything really is lol
Yes, that’s why electro-mechanical analogies are so common in engineering and physics. But it shouldn’t be too surprising that they can work as well as they do. There appears to be an elegant, underlying simplicity--or maybe "economy" would be a better term--in the ordering of the phenomenal world, and if there weren’t, then mathematical models could hardly describe and even predict so many of its relationships.

The 1D mathematical models describing the unsteady gas exchange characteristics of ICEs still in wide use reply on a system of non-linear hyperbolic differential equations, which can complicate analyses, so various simplified methods have been developed that have applications in acoustic modeling of systems, such as the lumped parameter/lumped element method.

I mentioned the electrical analogy to acoustic theory in my post, but only in passing. Thanks for bringing it up in more detail.

Impedance translation and waveguide circuits can be used to model an equivalent circuit of a Helmholtz resonator (for example) which sounds like what you did in control theory class. These can be very good for determining the response of resonators, and a lot of work has been done in this direction.

A HR can also be considered by the analogy of a sprung mass, where fluid mass in the HR neck is represented by a mass and the compressible volume of the flask is the spring. Newton’s second law of motion can be applied (a = Fnet/m, a = x’), mx’ + kx=0, to describe the equilibrium of the forces of the system. The solution to this equation will be f = 1/(2π)(K/m)^1/2, where m is the mass of the fluid in the HR neck and f is the resonance frequency of the resonator. After further manipulation the equation becomes the familiar f = c/(2π) [A/(VL’)]^1/2.

It's acoustic wave frequencies and wavelengths moving through a gas as the supporting medium that are blocked or allowed in these acoustic filters. It's a distinction between energy and matter, but an important one.

In ICE intake and exhaust systems, there i is superposition of acoustic and finite amplitude pressure waves. Acoustic waves have small amplitudes and little effect on the gas particulate medium, but finite waves can be strong enough to develop shock fronts and have significant effect on the medium (e.g. displacement of gas). This is where acoustic theory begins to break down and 1D (and 3D) analysis becomes necessary.
 

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Discussion Starter · #8 ·
Excellent write-up; it's good to know some people used their time to do things like this during lockdown rather than bingeing Youtube like I did.... Seems like I accidentally did things right when I did my car up, which is good to know.
Thanks for the kind words...I saw my fair share of youtube during lockdown too.
 

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Very good write-up. Are you an audio engineer?

I recently added a Vibrant Ultra Quiet resonator to my Thermal exhaust, which already had a resonator (18"?) and a Helmholtz resonator. I didn't know about the different length/diameter thing.
 

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Discussion Starter · #10 ·
Very good write-up. Are you an audio engineer?

I recently added a Vibrant Ultra Quiet resonator to my Thermal exhaust, which already had a resonator (18"?) and a Helmholtz resonator. I didn't know about the different length/diameter thing.
Did the Vibrant quiet things down to a satisfactory level?

No, I'm not an acoustical engineer, although I did study acoustics in school. My interest was primarily fluid mechanics.
 

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Did the Vibrant quiet things down to a satisfactory level?

No, I'm not an acoustical engineer, although I did study acoustics in school. My interest was primarily fluid mechanics.
It definitely tamed it down a little bit more. Maybe 5 more dB. Less raspy too. It's a very tame exhaust now when cruising.
 

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I did the same as @jwuonog but I installed my vibrant ahead of the 18 inch resonator. He installed his between the 18 and the helmholtz. Definitely A substantial difference with this thermal exhaust.
 

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I've noticed quite a few posts complaining about noisy exhausts and droning, often asking advice on what to do about the problem. A few of these appeals have popped up quite recently, and I decided to write-up a short overview of silencers, some of the basic types available, their pros and cons, and my own recommendations on dealing with excessive noise and drone.

This write-up is attached to this post as a pdf file. It should be possible to gloss over the technical discussions without missing the main points, and these points are summarized. I hope it will be helpful to some members of the community.
So basically I just leave my loud exhaust the way it is and wait for texts from my daughter when I'm running around asking if that's me making all the noise. After that text I take it a bit easier. :ROFLMAO:
Small town....Wife always knows where I am.
 
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