Hemholtz resonator - design and implementation ideas (1 Viewer)

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Thank you gents for your excellent feedback, makes me want to go work on this NOW but the temps haven't hit above freezing yet so I'll have to wait another month or so.

I mentioned on my first post that I tried four different mufflers and one of them was this Magnaflow unit, which is basically a higher caliber Cherry Bomb :). I kid you not, I wanted to BLOW my brains out for the two hours I drove around with it. Even though it was a nice package, didn't impact the boost in any way, it did very little to quiet the freqs but with a Hemholtz resonator, it may not be so bad now. Not that I actually need to install it again, it's good to have options. V band clamps make swapping out parts quite lovely.

The first pic shows the space I have for J pipe Hemholtz resonator above the muffler, or to the side. Second pic shows another potential space to put a J pipe perpendicular to the exhaust flow.

Since I'm not an experienced exhaust pipe welder, I opted to use 2.5 to 2.5 union everywhere I had to butt weld the pipes together. This prevented blowing through the pipe as well as prevented goobers in the air stream.

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So, I went to this site to do some calcs Drone Calculator - http://super-null.com/drone.html

I inputted 100deg F exhaust temp, 353 for the speed of sound and got the following. I see that I need a chamber that's 28" long but I don't see a diam, which I find to be puzzling!

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Second set of calcs comes from the spreadsheet that I posted on #1. I wanted an adjustable J pipe and not knowing exactly which freq I want to tune out, I started with what Doc stated below, which is 140hz. My thinking is that I wanted to be able to adjust plus or min 140hz using the spreadsheet calcs.

Quoting @doc_random above:
LX450 2800 RPM * 3pulses/rev / 60 sec/min = 140 Hz resonant frequency. YMMV. (Doc is using speed of sound = 360)
It's not the exhaust temperature, it's the gas temperature in the closed tube that goes into the sound reflection calculation.

Option 1 and Option 1 below are the min and max of the length adjustments using the slip joint that is shown previously. If you look at the rows corresponding to the freq@125f, this line corresponds to the speed of sound of 361. So, based on the length of the chamber, I should be able to correct freqs 117hz to 153hz.

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@alia176

Quarter wave reflection (J-pipe) is setting a path length for sound reflection at a frequency. Diameter doesn't come in to the equation, just needs to be similar to the main pipe. 60% of the main exhaust diameter is mentioned above, mine is 66%.

Reading up on Helmholtz resonance, it seems to be creating the sound that cancels noise vs reflecting it. The two examples they site are blowing over the opening of a bottle and cracking a car window at speed to get woo-woo-woo sound. The shape of the resonator determines the frequency, not the length.
Same result by different methods.
The calculator for the Helmholtz above uses a 1.5" neck and 3" chamber, and you could vary the length of the 3" to vary the volume and tone.

thus, the resonance frequency is: [formula is black on grey if you have the dark motif in Mud].


f_{H} = \frac{v}{2\pi}\sqrt{\frac{A}{V_0 L_{eq}}}
.

"Automotive[edit]​

Helmholtz resonance sometimes occurs when a slightly open single car window makes a very loud sound, also called side window buffeting or wind throb.[7]

Helmholtz resonance finds application in internal combustion engines (see airbox), subwoofers and acoustics. Intake systems described as 'Helmholtz Systems' have been used in the Chrysler V10 engine built for both the Dodge Viper and the Ram pickup truck, and several of the Buell tube-frame series of motorcycles.

The theory of Helmholtz resonators is used in motorcycle and car exhausts to alter the sound of the exhaust note and for differences in power delivery by adding chambers to the exhaust. Exhaust resonators are also used to reduce potentially loud and obnoxious engine noise where the dimensions are calculated so that the waves reflected by the resonator help cancel out certain frequencies of sound in the exhaust."


Really long thread that talks about J-pipe, Helmholtz, and something new-to-me that is just a small (6") capped pipe horizontal to the exhaust.

"The reason for opting for a Helmholtz resonator instead of a 1/4 wave tube is that the Helmholtz covers a much wider range"

J=pipe data
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Not sure how this works . . .
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I have an app that I got for decibel readings that also has a spectrum analyzer. There are a ton of them available, this is just the one I use.
Physics Toolbox Suite. Pro is $2.99 and 4.9 stars. If anyone is going to go down this path it would be good to get a baseline spectrum recorded getting on a freeway and moderate acceleration to 70mph.

Ulysse Speedometer Pro has a racing timer for 0-60, 0-100 and 1/4 mile. Again would be good to baseline (but I did not :(). You set it up when stopped and it waits until you move to start the timers. All three run together and stop independently when each goal is reached. I have not hit the 100mph timer in my LX450 . . .
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Some old info in the dark recesses of my brain was nagging at me a little, regarding this thread - was wondering if it would come up...
We may be nibbling on the technical terminology definition of "helmholz resonator" going way back to the guy named helmholz & what he invented. The bottom line is that there are at least two, maybe three separate physical phenomenon available to work with here - So - don't get caught up in the definition thing, instead, focus on what kind of resonance is being tuned.

1) easiest to understand, and to calculate - The J-pipe is simple - the length of the tube resonates at an integer number of 1/2 and/or 1/4 wavelengths. It's easy - change the length, and you tune to a different set of frequencies (more than one, it's a "comb" as @doc_random mentioned. The diameter is not involved - only length. Think of Trumpets & French Horns with their 3 or 4 valves switching in different side tubes with different lengths - each length making a different set of harmonic notes available to resonate very cleanly. Trombones too, but with continuously variable tube length. A long tube will "excite" at a larger number of harmonic frequencies, a short tube - only at a few lower harmonics.

2) similar to 1) but 2 or 3 "resonance modes" may be at work simultaneously - integer number of wavelengths - in 3 directions - length, width, and height. So - a more stubby chamber resonator is possible. But it's still about wavelengths. Can you sing in the shower and hit different notes?

3) A different phenomenon, based on the elastic compressability of the gas in the chamber, so this isn't about wavelengths, but it's about "spring constant" of the gas, and the volume of gas being compressed, and the friction of the size of the opening might have an influence, with gas rushing in and out. This might be what the guy named Helmholz invented, way back.

2) and 3) at the same time could be made to work. But complicated - hard to visualize or calculate what's going on - ton's of trial & error experiments needed.

I think I remember reading that Mercedes? had valve switched frequency tuned mufflers. And I know for sure that my '99 lexus GS300 has dual tuned valve switched intake manifold, more for powerband than sound.
 
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I have Sunday coffee with a friend of mine every week and there is some car show on TV that runs in the background that we half ass watch. Today the guys were putting on an exhaust system on something and I noticed that at the tips was a round canister with two pipes T'd into the exhaust pipe. I immediately interupted the conversation and started telling him what they were and their purpose, which the host eventually got to. I find all of this fascinating. 👍 :beer:
 
Some old info in the dark recesses of my brain was nagging at me a little, regarding this thread - was wondering if it would come up...
We may be nibbling on the technical terminology definition of "helmholz resonator" going way back to the guy named helmholz & what he invented. The bottom line is that there are at least two, maybe three separate physical phenomenon available to work with here - So - don't get caught up in the definition thing, instead, focus on what kind of resonance is being tuned.

1) easiest to understand, and to calculate - The J-pipe is simple - the length of the tube resonates at an integer number of 1/2 and/or 1/4 wavelengths. It's easy - change the length, and you tune to a different set of frequencies (more than one, it's a "comb" as @doc_random mentioned. The diameter is not involved - only length. Think of Trumpets & French Horns with their 3 or 4 valves switching in different side tubes with different lengths - each length making a different set of harmonic notes available to resonate very cleanly. Trombones too, but with continuously variable tube length. A long tube will "excite" at a larger number of harmonic frequencies, a short tube - only at a few lower harmonics.

2) similar to 1) but 2 or 3 "resonance modes" may be at work simultaneously - integer number of wavelengths - in 3 directions - length, width, and height. So - a more stubby chamber resonator is possible. But it's still about wavelengths. Can you sing in the shower and hit different notes?

3) A different phenomenon, based on the elastic compressability of the gas in the chamber, so this isn't about wavelengths, but it's about "spring constant" of the gas, and the volume of gas being compressed, and the friction of the size of the opening might have an influence, with gas rushing in and out. This might be what the guy named Helmholz invented, way back.

2) and 3) at the same time could be made to work. But complicated - hard to visualize or calculate what's going on - ton's of trial & error experiments needed.

I think I remember reading that Mercedes? had valve switched frequency tuned mufflers. And I know for sure that my '99 lexus GS300 has dual tuned valve switched intake manifold, more for powerband than sound.

Excellent explanation amigo, thank you!! I'll go about designing a J pipe dealio with variability built in.
 
This is a Volkswagen /Audi style suitcase muffler where the right side is a built in Helmholtz resonator. I had read that you can strengthen the design by putting some of the neck into the chamber so you don't have a large mass hanging on the end of a long tube.
And the left side is essentially a glass pack.

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Another one that shows two smaller chambers are design for "destructive interference" (10:00) as well as a built in Helmholtz resonator (19:10). This may be what the small perpendicular tube at the end of post #23 is.
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I've been studying up on my drone and it seems like I have two freqs that I need to address. One is at 80hz and the other one is somewhere higher up, maybe in the 120-150hz, can't quite get an accurate #. The higher frequencies are when I'm on the hwy but my phone app is also picking up wind noise, which is mucking up the analyzer a bit. I'll see if I can make two resonators under the cargo area, or maybe one under the cargo and one just in front of the axle area.

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EDIT;

check out this solution! I grabbed the pic from here

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Well, I made my first Hemholtz resonator this weekend but haven't gone for a drive yet. It lives above the muffler and runs parallel to it. Currently have a slip joint clamp and able to adjust the length + or - 1.5". So, a max of 29" total length is the max, which I'm hoping to cancel out 130 - 140 hz but we shall see.

When I order the step exhaust pipe band clamp thing from Amazon, i'll be able to slip inside the current 2.5" pipe another 2.25" pipe for a trombone affect. This will allow me the greatest amount of adjustment but until then, this will have to do. I got a trip to UT coming up on Wed and need to get other projects.



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