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Quarter Wave Resonator vs Drone

General engine tech -- Drag Racing to Circle Track

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Dust Buster
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Re: Quarter Wave Resonator vs Drone

Post by Dust Buster » Thu Jul 23, 2020 2:17 am

Thanks @ptuomov – your first pic looks more like a anti-reversion header to prevent drawing exhaust into the piston chamber (or cheap EGR), which I haven't seen on turbo set-ups. Fig 24 would provide a better reflection to cancel the shorter waves. What's interesting to me is that my in place, thick-walled, but gutted CAT / DPF did seem to act like its own acoustic system (chamber). I removed it due to the pipe size restrictions surrounding it and its weight.
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Re: Quarter Wave Resonator vs Drone

Post by ptuomov » Thu Jul 23, 2020 4:54 am

Dust Buster wrote:
Thu Jul 23, 2020 2:17 am
Thanks @ptuomov – your first pic looks more like a anti-reversion header to prevent drawing exhaust into the piston chamber (or cheap EGR), which I haven't seen on turbo set-ups. Fig 24 would provide a better reflection to cancel the shorter waves. What's interesting to me is that my in place, thick-walled, but gutted CAT / DPF did seem to act like its own acoustic system (chamber). I removed it due to the pipe size restrictions surrounding it and its weight.
The point I was trying to make is that the side branch resonator can be fabricated by placing a larger pipe around the smaller main pipe that has a gap in it. Both sides of such basic can act as side branch resonators.

I’m not a big believer in anti-reversion chambers for the purposes of preventing reversion. However, they will alter the sound because they amount to a combined side branch resonator and expansion chamber.
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Re: Quarter Wave Resonator vs Drone

Post by Dust Buster » Thu Jul 23, 2020 12:38 pm

@ptuomov – ah! noted

My concern is the flow rate. I have gone two sizes larger than OEM. I know the absorptive silencers' perforated sides creates eddies, but was hoping the middle of the pipe would follow laminar flow principles.

Open window test

Opening the window does increase the white noise, probably mainly due to wind and tyre noises. Between 1750 and 2000 rpm one can hear the 170 – 176 Hz drone, but it is now just background noise. However, closing the window in this rpm range doubled the sound energy.

So, I'd say that I definitely have a sympathetic cabin resonance and am still hoping that a correctly calculated length Quarter Wave Resonator will solve the root problem. Surely the optimal length will attenuate other nearby frequencies as reported in other fora.
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Re: Quarter Wave Resonator vs Drone

Post by ptuomov » Thu Jul 23, 2020 12:43 pm

First, have you computed the Reynolds number in the pipe now? I’m skeptical that you’re in the laminar flow regime, I suspect you are in the turbulent flow regime anyway. Second, whatever turbulence the pipe-in-pipe resonator will cause, the side pipe junction will probably also cause a similar magnitude turbulence.
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Re: Quarter Wave Resonator vs Drone

Post by Dust Buster » Thu Jul 23, 2020 1:02 pm

Probably wishful thinking on my part – perfect world and all.

I haven't seen any other sites that have employed Quarter Wave Resonators complain about loss of flow like presented by a baffled style muffler. They often report that from outside of the vehicle, it sounds nearly identical as before the install.
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Re: Quarter Wave Resonator vs Drone

Post by ptuomov » Thu Jul 23, 2020 7:08 pm

Dust Buster wrote:
Thu Jul 23, 2020 1:02 pm
Probably wishful thinking on my part – perfect world and all.

I haven't seen any other sites that have employed Quarter Wave Resonators complain about loss of flow like presented by a baffled style muffler. They often report that from outside of the vehicle, it sounds nearly identical as before the install.
For sure, the pipe-in-pipe “side-branch” resonator that I have in mind would not be the same as a baffled muffler. You can see thru it, and drop a golf or tennis ball (or soccer ball, depending your pipe size) thru. The pipe-in-pipe side branch resonator will also allow you to tube out two frequencies, one for each end of the dead larger pipe, by choosing the location of the cut section on the inner pipe.

I went thru a lot of these considerations in this thread: viewtopic.php?f=1&t=49628
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Re: Quarter Wave Resonator vs Drone

Post by Dust Buster » Wed Jul 29, 2020 11:08 am

@ptuomov - thanks for your “Making a muffler” thread of which I have read all 22 pages and YouTu.be links, but still have some of the articles to read.

I'm curious as to why for your infinity pipe, you didn't split a pipe into two halves and make a large oval pipe. It would have required four long welds instead of two to attach a long rectangular plate on the top and bottom, but would have avoided the tumbling between the two partially opened pipes.

So, taking from your "out of phase" wastegate pipe, your summary using unequal pipe lengths and somewhat from your infinity pipe, to avoid turbulence, then I should split the flow (possibly two smaller, but equal sized pipes). The length of B should be the length of the A pipe plus either 3.94m (being twice 1.97m) or 3.46m (being two times 1.73m). The doubling of the quarter wave being required (or half the full wave) to return to the A pipe 180 degrees out of phase. Pardon my rough drawing.
Half_Wave.PNG
My problem is that I don't have nearly 4m of space even doubled back several times.

What do you mean by pipe-in-pipe resonator? Was this the Helmholtz chamber used in the BMW box or similarly, the truck muffler with several chambers with “Ts” of different length.
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Re: Quarter Wave Resonator vs Drone

Post by ptuomov » Wed Jul 29, 2020 1:16 pm

“ I'm curious as to why for your infinity pipe, you didn't split a pipe into two halves and make a large oval pipe. It would have required four long welds instead of two to attach a long rectangular plate on the top and bottom, but would have avoided the tumbling between the two partially opened pipes.”

The infinity pipe flows very well, there’s no turbulence problem. The pressure measurements back that up.

We also constructed an oval pipe but it’s much harder to make it in a way that will last. The infinity pipe with rod supports works well. In fact, one could just cut half the distance from each pipe and add some holes to make an even stronger infinity pipe.
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Re: Quarter Wave Resonator vs Drone

Post by ptuomov » Wed Jul 29, 2020 1:25 pm

“ What do you mean by pipe-in-pipe resonator? Was this the Helmholtz chamber used in the BMW box or similarly, the truck muffler with several chambers with “Ts” of different length.”

If you have a 2” exhaust pipe, put it inside a 3” inch pipe. Fabricate end caps to the 3” pipe. At the one end of the section of the 2” pipe that is inside the capped 3” pipe, add 2” section of 25% porosity perforated section. Say you just drill it since it’s a shirt section. This 3” resonator has a cavity between the 3” outer shell and 2” inner pipe that works approximately the same way as a 2” branch resonator of the same length as the 3 inch pipe section (minus about half the perforated section length).
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Re: Quarter Wave Resonator vs Drone

Post by ptuomov » Wed Jul 29, 2020 4:20 pm

An artfully crafted illustration of the pipe-in-pipe side branch resonator:
3D0B7E50-23FD-443B-8692-11DF79C38A3D.jpeg
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Re: Quarter Wave Resonator vs Drone

Post by Dust Buster » Thu Jul 30, 2020 2:05 am

haha – “artfully crafted illustration” possibly similar to your “acceptable” 90 dB exhaust noise.

Indulge me for a moment.

If one were to generate all human audio range frequencies equally, let's call this a white noise generator, and then used a spectrum analyser to take samples in several parts of the room, then one would find that certain soft furnishings absorb some frequencies while hard furnishings bounce certain frequencies creating shadows behind them. Then with a graphic equaliser, one could increase or decrease certain frequencies depending upon where they normally listen to create a “balanced” room or even accentuate more bass, if that was one's preference.

A vehicle cabin and its exhaust system are akin to two steel drums next to each other with one vibrating in sympathy (harmonics) with the struck drum.

My theory is that the drone frequency heard in the cabin is unrelated to Method One revs and number of cylinders. Sure, the sympathetic frequency must have increased in volume to now cause the drone in the cabin, but perhaps not the frequency suggested by this method. The actual frequency as discovered by the spectrum analyser app is the one that needs to be attenuated. The volume increased as a result of the new exhaust system, not that more cylinders have been added nor a different rpm is being used.

So, for space consideration, I need to revisit Helmholtz's volume formula to create an appropriately sized box. I already have two perforated pipes in larger casings in series.
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Re: Quarter Wave Resonator vs Drone

Post by ptuomov » Thu Jul 30, 2020 6:48 am

With your two drum analogy, the first drum (exhaust) by my logic has to resonate at a frequency that excites the second drum (cabin). That to me means that the first drum needs to generate noise at an integer multiple of the resonant frequency of the second drum.

The exhaust generates noise at many engine orders, some fractional, which then get multiplied by the rpm to get the frequency. By my logic, you want to suppress the frequencies in the exhaust that excite the cabin. i am guessing that the factory did that with their exhaust. Your new exhaust doesn't do that, so you want to add a component that "takes out" those problem frequencies.

What the muffler box looks like on the outside doesn't really mean much. Even if you have two identical looking muffler cans in the exhaust, this doesn't mean that a third identical looking can couldn't perform a completely different function. The side-branch resonator that I sketched for you hits a specific tuned frequency and is unlikely to behave the same way as your two existing muffler cans, even if both have a perforated pipe section in them.
Dust Buster wrote:
Thu Jul 30, 2020 2:05 am
Indulge me for a moment.

If one were to generate all human audio range frequencies equally, let's call this a white noise generator, and then used a spectrum analyser to take samples in several parts of the room, then one would find that certain soft furnishings absorb some frequencies while hard furnishings bounce certain frequencies creating shadows behind them. Then with a graphic equaliser, one could increase or decrease certain frequencies depending upon where they normally listen to create a “balanced” room or even accentuate more bass, if that was one's preference.

A vehicle cabin and its exhaust system are akin to two steel drums next to each other with one vibrating in sympathy (harmonics) with the struck drum.

My theory is that the drone frequency heard in the cabin is unrelated to Method One revs and number of cylinders. Sure, the sympathetic frequency must have increased in volume to now cause the drone in the cabin, but perhaps not the frequency suggested by this method. The actual frequency as discovered by the spectrum analyser app is the one that needs to be attenuated. The volume increased as a result of the new exhaust system, not that more cylinders have been added nor a different rpm is being used.

So, for space consideration, I need to revisit Helmholtz's volume formula to create an appropriately sized box. I already have two perforated pipes in larger casings in series.
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Re: Quarter Wave Resonator vs Drone

Post by Dust Buster » Thu Jul 30, 2020 11:17 am

Agreed

Correct

Which brings me back to my original dilemma - sizing the side branch resonator. I'm running 2 1/4" pipe - btw. I would think that pre-perforated SS pipe was available, but what length? Also, what diameter should the larger pipe be, presumable over nearly the same length with no packing to allow maximum reflection for cancellation?
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Re: Quarter Wave Resonator vs Drone

Post by ptuomov » Thu Jul 30, 2020 12:43 pm

You want the area of the gap between the inner pipe and outer pipe to be about the same as the area of the inner pipe to get the side branch math to work out. Ballparking it in my kind, 3” would be the smallest that would work and it would probably be pretty close to ideal. Is (3^2-2.25^2)/2.25^2 about 1? 3.25” maybe better?
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Re: Quarter Wave Resonator vs Drone

Post by Dust Buster » Fri Jul 31, 2020 2:33 am

If one is the magic number, then =(3.182^2-2.25^2)/2.25^2 [or =((3.182*3.182)-(2.25*2.25))/(2.25*2.25)] gets close.

So, the length of the side branch is irrelevant or longer increases the attenuation? If this is so, then there must be a formula for this relationship.
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