I'm at it again - fixing drone on a brand new MBRP

[Clint Beastwood]

I've recently thought about looking into making a particle damper. It works at a much wider range of frequencies than a J pipe, and isn't as sensitive to temperature changes. Though to be honest, this exhaust bangs around enough, I'd rather not add any more weight the thing to make that problem worse. I'd think it'd take a good amount of filler to make that particle damper work too. Maybe someone can give that a try? From what I can find, particle damping tuning is more of a guess-and-check process than a calculation to find the right masses, but there are white papers out there that will lead you the right direction

I don’t think anyone uses mass dampers on passenger vehicles anymore, just on big trucks I think, calculating and targeting specific frequencies is just waaaaaaay more efficient.

I’d really like to finish my real helmholtz chamber but I have exceeded my math skills at this point. I know how to calculate everything out but I am lost when it comes to chamber size vs neck diameter vs neck length.

 

[The_Ryan]

I don’t think anyone uses mass dampers on passenger vehicles anymore, just on big trucks I think, calculating and targeting specific frequencies is just waaaaaaay more efficient.

I’d really like to finish my real helmholtz chamber but I have exceeded my math skills at this point. I know how to calculate everything out but I am lost when it comes to chamber size vs neck diameter vs neck length.

Mass damper =/= particle damper. particle damper is like the sand in a deadblow hammer. a mass damper is well, a weight tuned to that frequency. But either way, you're right in that it's way more time efficient to calculate and make a j-pipe. I was just hoping to find a solution that's not a permanent weld-in-place part. something reversible


As for your helmholtz chamber there's likely to be a calculator somewhere online. I know I used one at one point for designing loudspeakers. Unless you enjoy doing the math

 

[AZSP33D]

The center tunnel hanger is mounted on thinner sheet metal that transmits vibration a bit more than other areas, so I didn’t use the stiffer Whoosh hanger for this location, but all the others. This cut the drone and exhaust noise in the cabin overall.

 

[Clint Beastwood]

Mass damper =/= particle damper. particle damper is like the sand in a deadblow hammer. a mass damper is well, a weight tuned to that frequency. But either way, you're right in that it's way more time efficient to calculate and make a j-pipe. I was just hoping to find a solution that's not a permanent weld-in-place part. something reversible


As for your helmholtz chamber there's likely to be a calculator somewhere online. I know I used one at one point for designing loudspeakers. Unless you enjoy doing the math

I enjoyed the project for a long time, but I have put too many hours and like $3k dollars into building exhaust systems
It's fun, sure, but I'm a little burned out. I've been looking on yelp for an "enginerd" for hire to spec out the chamber and then find a fabricator.

 

[Clint Beastwood]

Actually the drone is at about 2600 rpm so it might be the Mishimoto downpipe as you said. I made extra sure that the exhaust isn't hitting that center brace and it doesn't show signs of friction. Perhaps adding some less stiff hangers would help and extra bit. I'll keep doing tests.

can you toss a screenshot of your spectrum graph up so I can see it?

So I pulled some new recordings/spectrum graphs. I had to turn my scaling way down and I see the 85ish hz peak you’re taking about. I think you might be trying to fight the overall volume of the exhaust, because there’s no 20+dB “spike”, just a “loudness”. It’s not really , drone, just overall volume.

sidebar - I *HATE* the automatic ad injection on the forum now, it’s constantly taking words I type and turning them into links :/

 

[Clint Beastwood]

If you try to cancel 86hz the wave is like 13.3ish feet, so you’d need a chamber 39 inches long.,. But then you’ll probably unleash the 106hz death drone

 

[CrystalWhite]

I bought a used aluminized MBRP locally for around half price. Finally installed it after hours of messing around with the already crushed clamps(I'm never buying a slip fit exhaust again) and noticed the horrendous drone everyone talks about. I then deleted the symposer which helped slightly, adjusted the middle section of the exhaust and added washers on the midbrace to add some clearance. It was ear shatteringly loud around 3.2k rpm and would definitely cause hearing loss if left at that frequency. I love loud cars and have driven them for years but it was something about the boominess and tone which was like nails on a chalkboard.

I decided enough was enough and added a Vibrant muffler to the end of the cat back. I bucked off the dual tips with a bandsaw and welded in new hangers and the muffler to the pipe. It was 6 inches short so I just put one of the 3 inch MBRP tips on the 3 inch outlet of the muffler and voila, job done. After the muffler break in its still loud, which I like, and makes noise around that rpm band, but all of the ear raping drone has been reduced right down to a level that is acceptable for my ears. The tone of the muffler is great and I can't wait to do a catless downpipe. If you still want it loud but not to become deaf I recommend the Vibrant muffler.

 

[The_Ryan]

I don’t think anyone uses mass dampers on passenger vehicles anymore, just on big trucks I think, calculating and targeting specific frequencies is just waaaaaaay more efficient.

I’d really like to finish my real helmholtz chamber but I have exceeded my math skills at this point. I know how to calculate everything out but I am lost when it comes to chamber size vs neck diameter vs neck length.

Looks like the Crown Vix/Grand Marquis did, and maybe some BMWs? that was was a heafty 3lbs, and there's 3 of them on that system, but I think we may be able to get away with something a bit smaller.

https://www.crownvic.net/ubbthreads/ubbthreads.php?ubb=showflat&Number=2852372

 

[JNLimon]

can you toss a screenshot of your spectrum graph up so I can see it?
View attachment 25839
So I pulled some new recordings/spectrum graphs. I had to turn my scaling way down and I see the 85ish hz peak you’re taking about. I think you might be trying to fight the overall volume of the exhaust, because there’s no 20+dB “spike”, just a “loudness”. It’s not really , drone, just overall volume.

sidebar - I *HATE* the automatic ad injection on the forum now, it’s constantly taking words I type and turning them into links :/

It has been some unusually cold days here in CDMX so I waited for a more average temprature. I think I need to add some lenght due to your comment regarding you needed 5" longer for the MBRP vs your custom exhaust.

 

[JNLimon]

can you toss a screenshot of your spectrum graph up so I can see it?
View attachment 25839
So I pulled some new recordings/spectrum graphs. I had to turn my scaling way down and I see the 85ish hz peak you’re taking about. I think you might be trying to fight the overall volume of the exhaust, because there’s no 20+dB “spike”, just a “loudness”. It’s not really , drone, just overall volume.

sidebar - I *HATE* the automatic ad injection on the forum now, it’s constantly taking words I type and turning them into links :/

Another idea... what if I put the OEM resonator as a Helmholtz for the 107Hz drone of death and then add the extra inches to the 1/4 wave?

 

[Clint Beastwood]

Another idea... what if I put the OEM resonator as a Helmholtz for the 107Hz drone of death and then add the extra inches to the 1/4 wave?

Well by design the oem resonator would be a terrible helmholtz as it absorbs/dissipates frequencies instead of reflecting them back in an inverse wave to cancel out the original noise.

 

[AZSP33D]

So if the Thermal system is well designed to eliminate the drone... what exactly would be the effect if one eliminated the front resonator?

 

[Clint Beastwood]

So if the Thermal system is well designed to eliminate the drone... what exactly would be the effect if one eliminated the front resonator?

you’d be upsetting a well engineered system

 

[AZSP33D]

you’d be upsetting a well engineered system

I understand... exactly how? Or approximately how? I have two ST's and three exhaust systems including the Thermal, and I'm thinking one of the other ones may need the resonator more and the Thermal is very heavy... and I can cut weld. Not going to do it unless I understand what the resonator does, but it seems to just quiet things down a notch.

 

[TyphoonFiST]

That is exactly what it does is resonates the sound down so its not higher pitched drone.

 

[JNLimon]

Well by design the oem resonator would be a terrible helmholtz as it absorbs/dissipates frequencies instead of reflecting them back in an inverse wave to cancel out the original noise.

Thanks for the input... what was the final lenght of your pipe with the MBRP?

 

[Clint Beastwood]

I understand... exactly how? Or approximately how? I have two ST's and three exhaust systems including the Thermal, and I'm thinking one of the other ones may need the resonator more and the Thermal is very heavy... and I can cut weld. Not going to do it unless I understand what the resonator does, but it seems to just quiet things down a notch.

Most exhaust systems I have seen for the FiST are "built" for the FiST, but the thermal is the only one I have seen (not that others don't exist, but that I have personally seen - AND they share their engineering process which is awesome) that was acoustically *engineered* for the fist. The quarter-wave chamber hidden in the thermal's rear resonator only attenuates a spread of like 15hz, so they use the front resonator to strip away any higher frequencies that make it sound raspy, and some of the higher frequencies that could result in... what I'll call "sympathetic oscillations" (I'm not an audiologist or audio engineer so my terminology is likely off, but sympathetic oscillations is a valid enough descriptor). I suspect if you removed the Thermal's front resonator, you would notice 3 things. One, a general increase in loudness, two, more vibration from the system itself, and three, a much "muddier" sound and less efficient attenuation, as now the highly targeted quarter-wave resonating chamber is trying to cope with a bunch of sympathetic and half frequencies, creating the aforementioned "sympathetic oscillations". At constant speed it would likely make a wah-wah-wah type oscillating sound at ~3200rpm, where the quarter wave resonator falls in and out of effectiveness.

If you just need a resonator for another car, Vibrant makes some great ones - not just the ultra quiet, but they have great bottle resonators as well. I might be a bit of a Vibrant fanboy, but comparing them side by side to some of their market competition they seem to use thicker steel and the construction feels more solid. Full disclosure, I *do* think I killed a Vibrant bottle resonator, I think the interior tube came loose, but I'm pretty sure it was all my fault and not Vibrant's.

I've emailed a bunch back and forth with the Thermal guys - once I indicated the research and testing I had done and praised their attention to the acoustic details they talked a little more freely, because it was obvious I already *had* the information and wasn't trying to clone their product. The rep I communicated was engaging and interested in my "hobbyist's journey". The willingness to communicate with a potential customer and show a genuine interest did a lot to win me over, in addition to them actually publishing the work they put into engineering systems.

If you're looking for another muffler that does the internal quarter-wave thing, so far I have found indications that Billy Boat mufflers and Corsa both use similar technologies, though I could never get an email response from either on what frequencies they attenuate, just that they use "patented quarter-wave technology".

 

[Clint Beastwood]

Thanks for the input... what was the final lenght of your pipe with the MBRP?

I think it was 33.27", but I'm realizing now I should not have done all my testing and calculations at an ambient temp of over 90 degrees f, as now that it's in the 40's the resonator does not work very well. I should have tested at the high average temp, low average temp, and adjusted my speed of sound/temperature calculation to address the midpoint. I did notice, while playing with different pipe diameters and whatnot, that 3" mandrel bent pipe seems to be the tipping point with the fist - in order to clear parts under the body at 3" the curves straighten out enough that the 106hz wavelength (10.6ish feet I think) can propagate in the pipe, causing "drone" and creating secondary resonance. Smaller diameter, or non mandrel-bent would break up the wave. I think thats why 2.5" and 2.75" systems for the fist, while just as free flowing as the 3" ones, don't really get as much hate for droning.

For what it's worth, I almost always wear quality musician's earplugs on long drives whether I am in a loud car or not - I find I arrive at my destination much less fatigued regardless of which vehicle I've chosen to take. The normal "foam" earplugs can be a little dangerous to drive with because they attenuate all frequencies, but musician's earplugs strip away harmful frequencies and leave you with certain ranges (human speech, etc.) completely open. My current favorites are some etymotic er20x's, though the "eargasm" ones are growing on me. With the eargasms, I put in the plugs and my exhaust noise is gone, and I can hear all the mechanicals of the car, the gears engaging, the wastegate opening and closing, etc. - it's kinda neat

 

[AZSP33D]

Most exhaust systems I have seen for the FiST are "built" for the FiST, but the thermal is the only one I have seen (not that others don't exist, but that I have personally seen - AND they share their engineering process which is awesome) that was acoustically *engineered* for the fist. The quarter-wave chamber hidden in the thermal's rear resonator only attenuates a spread of like 15hz, so they use the front resonator to strip away any higher frequencies that make it sound raspy, and some of the higher frequencies that could result in... what I'll call "sympathetic oscillations" (I'm not an audiologist or audio engineer so my terminology is likely off, but sympathetic oscillations is a valid enough descriptor). I suspect if you removed the Thermal's front resonator, you would notice 3 things. One, a general increase in loudness, two, more vibration from the system itself, and three, a much "muddier" sound and less efficient attenuation, as now the highly targeted quarter-wave resonating chamber is trying to cope with a bunch of sympathetic and half frequencies, creating the aforementioned "sympathetic oscillations". At constant speed it would likely make a wah-wah-wah type oscillating sound at ~3200rpm, where the quarter wave resonator falls in and out of effectiveness.

Thanks, that's a great response, good analyses. Little more raspy would be fine, less weight would be fine (it's as heavy as stock, shipping weight 50lbs), but oscillating noise would be unwelcome. Good thing that the car likes the weight down low.

 

[Clint Beastwood]

Thanks, that's a great response, good analyses. Little more raspy would be fine, less weight would be fine (it's as heavy as stock, shipping weight 50lbs), but oscillating noise would be unwelcome. Good thing that the car likes the weight down low.

Yeah at the end of the day, my big recommendation would be "don't cut up a well engineered 700 dollar exhaust to save 90 bucks"

If the weight really worries you, it's all relative - most of us could stand to lose 10-12lbs which is enough to offset the exhaust weight if the thermal tubing was thinner and lighter, it would buzz more, so :shrug: necessary evil I say.