mark_schnell

If I have to go through the trouble of identifying the exact length of the Helmholtz resonator pipe needed to eliminate drone, I'll share with the class as I don't think it should be a secret.

I apologize in advance to Unobtainium's proprietor.

powdrhound

Even though there are no patent issues, I highly doubt any exhaust manufacturer would freely disclose their proprietary information as there was a certain amount of time and effort on their part in getting the right dimensions. I'm sure the length will vary depending on what turbos you have also as the size of the turbines will likely change the resonance frequency slightly. Good luck with the project.

wross996tt

What kind of engineer? You perhaps understand resonant frequencies? Molecules oscillate as a function of vibrational energy. This oscillation can be expressed as different types of resonances, how these are perceived (by humans) can be dependent on the sensory perception of the receiver. Most likely it is perceived as a mechanical or acoustic resonance. This is why some will say...mine has no drone and others will call BS. The acoustic frequencies that lend themselves to irritation also can be dependent on the receiver's sensory perception. By and large, you can not eliminate the natural mechanics of vibrational energy and subsequent resonance. What can be done is to change the frequency to one that is less perceptible, or to create a frequency which is 180º out of phase (difficult in this application). There are a number of variables that can contribute. Which are the most significant is likely a function of the system (in this case, your car) and your measurement system (are you going to actually measure the resonant frequency or just use your ears?)... A short list of variables include:
1. material type
2. tube diameter (by tube I am referring to the exhaust tube)
3. tube length
4. mass (either internal or external to the tube)
5. shape of tube (e.g., cylinder vs. conical)
6. dampening materials and location
7. Interactions of any of the above)

As an engineer you are no doubt familiar with factorial experimental design. This would be the most efficient way to understand what variables and interaction of variables will you be able to manipulate and mange to reduce your perceived "drone".

Third-Reef

I am a mechanical engineer in the Semiconductor capital equipment industry (Silicon Valley). I design vacuum chambers and high temperature wafer processing equipment. I work with a lot of pressure/flow modeling and calculations. My experience tells me that the key to this exhaust resonance calculation is "Garbage in Garbage out". Trying to provide accurate input for modeling a resonance like this is very difficult as the previous post suggests. My attempt at the calculations show that the ideal length was way longer than could be implemented in the space available. I think the key here is try it, change it try it again. UW seems to have done this and gotten good results. I don't blame him for not wanting to share the secret sauce. He has done his homework and produces functional art.

wross996tt

There are much more efficient ways to provide insight to the phenomena (i.e., DOE), and this is a multivariate situation (more than one response variable). For example you don't want to eliminate drone by negatively effecting performance. In any case, the resources necessary to understand how to deal with the phenomena are more available to the manufacturers, at least in theory.

TT Surgeon

One mechanical issue that will drone is any non welded joints post cat,(vbands/bolts etc). Whenever these loosen, and they all do, it will drone like a ******. This is why most one piece systems don't drone as bad, basically an exhaust leak.
In your Kline-like system, I'd replace the gaskets, bolts, use exhaust sealant at the the cat/muffler pipe junction.
Just my personal experience with numerous systems.

s65e90

It's laughable that these guys think they have some sort of IP on the helmholtz chamber design. It's a simple design. Think of when you blow over the top of a bottle and it makes that hummmmmm noise.

That is the definition and exact sound we call DRONE.

Running helmholtz chambers is the solution. It's simple math really. You need to find the frequency you are trying to cancel out and then you can calculate the length of pipe required. Most OEM cars use these too and they are incororated in the piping or the muffler itself. Honda usually stuffs them at right angles to the piping and they are easily recognizable. If you don't know the exact dimensions as you don't know the frequency you are trying to kill a good idea is to run a slip fit helmholtz chamber. You can then adjust the length until the desired output is reached. At that point you can weld them so they wont move or just clamp them.

There's plenty of calculators online to figure the length.

mark_schnell

Links please? I've been looking.


The best write-up I've seen so far was here:
http://www.corner-carvers.com/forums...&postcount=111


Looks like step 1 is get an Decibel meter, step 2 is get exhaust gas temperature, step 3 test for peak dB, step 4 compute the wavelength of sound at that frequency and temp, and step 5 attach a 1/4 wavelength pipe pre-muffler. Length will likely be in the ballpark of 2-3 feet.


I like the idea of just attaching a pipe that slides like a trombone, so it can be adjusted to the desired length and then clamped/welded. That makes it so the science isn't even needed...

NoogaSparrow

Pretty easy to determine the frequency you're trying to 'move' away to another.

Best way is to measure the frequencies through the RPM band, then identify where (in the RPM band) it's most intrusive to you, then review the frequencies that were recorded at that RPM range where it bothered you. You'll find a spike in the frequency that bothers you where it isn't prevalent anywhere else... (assuming the exhaust doesn't bother you anywhere else in the RPM range)

wross996tt

Unfortunately you guys act like the vibrational energy and a host of other variables are constant. Not to lecture, but have you heard of the concept of variation? Within car and between car, etc. If all you use is the means of the frequencies, then on average you will reduce the drone. If you use the spike, then only that spike may be eliminated. What if the spike frequency changes...If the problem is variability fixing the mean or a single data point is the wrong action. If it were that easy, why hasn't it been done....everywhere?

s65e90

it is done everywhere. Get underneath any car and the OEM set up most likely has one. EGT is also prevalent. If you can measure all these then it's just math.

Theres literally hundreds if not thousands of pages of information on this. It may be new to you but it's not to others. Many A/M don't bother with it. It's also due to packaging constraints as A/M mufflers are made to be loud and thus the piping for the chamber would then be real long. It's easy to retrofit. Many just don't care.

mark_schnell

Again I ask, can you please point us to specifics? We can all benefit from knowledge sharing. If you have links to the best, most useful calculator and the most concise materials to bring DIY enthusiasts up to speed, please share.

996tnz

The world can never have too many exhausts for the 996T, but for those not into fabricating their own exhausts or resonators, I will just restate that I have had years of pleasure already from an aftermarket exhaust on my 996T - but only after putting a few concentrically stacked layers of Dynamat on the big unsupported metal panel surfaces of the firewall and parcel shelf.

Before we go further, REMUS aren't alone in it of course, but they really know how to make exhausts. I believe they do many of the better German OE ones - think Ms and AMGs - right up to and including one for the Dauer/Porsche 962. But they specifically say their 996T exhaust is for the manual only, and I found out why. I got a great deal on one for my Tip, so couldn't refuse a good stainless with 100 cell race cats. It was fitted alongside a tune and upgrade I had planned, so I can't guarantee it would behave exactly the same on a stock tuned Tip, but what I remember most about it is:

1 - How awesome she sounded as the workshop delivery guy drove her back into our street (yes, my upgraded car called me out onto the balcony even before she got home).

2 - How terribly ill and queasy I felt within a mile or two of driving her. She had an awful in-cabin drone at cruising revs that seemed to shake my innards and made me seasick in the same way that some of those warbling home alarms set out to do - almost to the point of wanting to vomit.

and,

3 - How great she's been to drive ever since adding that Dynamat.

I probably covered some of this one page 1 already (along with the fitting details), but we were booked to take my family on a 100 mile plus Porsche Club run to an out of town restaurant the following Sunday so emergency measures were needed. With a lightweight battery and deleted rear wiper I obviously hate adding weight anywhere - but that 10 odd pounds (half a door kit) of drone killing Dynamat is worth its weight in gold every day I drive her.

Would similar sound deadening work for every droning exhaust? Maybe, maybe not. At an educated guess, yes, at least for any with an awful low-frequency booming drone somewhere near a 2000 odd rpm cruising speed.

Third-Reef

996tnz, exactly where did you put the dynamat? Is it on the horizontal surface below the DME and the other black boxes back there under the sub woofer? or on the vertical surface behind these just under the window? Or both? I put a layer of the thick fabric sound insulation in there and it had no appreciable effect. i understand the dynamat works by damping the surface vibrations rather that trying to insulate the sound created by the surface (a first order fix) i want to give it a try as my 02 is an occasional long haul commute vehicle and quiet is nice.

SteveMFr

Go hang out at Breitbart or something - or talk exhausts. Not your take on politics.