Is exhaust drone caused by cabin resonance?

Carrying on part of the conversation from Plainsman's informative thread on fixing drone with a Borla here: http://www.planet-9.com/987-cayman-...ayman-and-boxster-modifications/139529-simple-basic-cat-back-exhaust-build.html So I suspected that a large factor in drone of exhausts on Caymans (and M3s) is not directly attributable to the geometry of the exhaust because as Plainsman has shown with his exhaust mods, it can be very difficult to make changes to the exhaust that effectively reduce drone. If the cabin has a resonance (an eigenmode) centered around a particular frequency that is excited by the exhaust noise coming from the tailpipe, then a small increase in exhaust noise could lead to a large amplitude "drone" in the cabin. The exhaust does not have to resonate at the same frequency... it just has to be loud enough to excite the mode. Since the Borla is loud at pretty much every frequency, we end up with drone. Or at least that's my working hypothesis. The procedure: to test this, I setup a studio monitor behind the car, tilted up towards the tailpipe, and drove it with a frequency sweep from 50 Hz to 400 Hz, logarithmic in time over 40 sec. I recorded the sound with an Audio-technica ATR6250 shotgun microphone both inside and outside of the car (to get a control point because the system has nowhere near a flat frequency response, even outside of the car). I then normalized and filtered the results in Audacity. I did this with my Cayman and my E92 M3. Ghetto frequency response setup for the M3 (forgot to snap one of the Cayman): Results for Cayman: The image below shows two amplitude traces of the frequency sweep for the Cayman while it was parked inside the garage with the tailpipe pointed out of the garage door opening. The top trace is with the microphone outside the car; bottom is the microphone inside the car held up by my right ear while sitting in the driver's seat: Obviously, the garage itself (3-car, 24ish ft deep) is having a large effect even though the monitor is actually just outside the door. So that was a mistake, and I need to re-do this part outside of the garage. But still... check out that huge lobe centered around 17 seconds! That corresponds to 110-130 Hz. In rpm, that range corresponds to 2200 to 2600 rpm . I don't currently have the Borla, so I can't remember exactly where the drone was, but it was somewhere around there. Results for M3 :I did the same thing on the M3, but this time I measured both the control and the car with the car parked outside of the garage: The trace is noisier due to some wind, but the overall control pattern is smoother when the studio monitor is not inside the garage. On this car, there are resonances down around 75 Hz and up at 200 Hz (at 15.5 s and 27 s, remember it's a log sweep, not linear). The 200 Hz resonance corresponds to drone around 3000 rpm, which is definitely something I hear (this car does have a Borla). I don't ever hear the 75 Hz resonance which would be at 1125 rpm. Here's one more image. This is the raw result of the Cayman (top) vs the raw result of the M3 (bottom). These traces have not been normalized or filtered, so you can see a lot of other stuff going on. This illustrates the relative differences of interior noise of the two cars with the amplification at the same setting. The M3 is a much quieter car (bigger, heavier, and easier to add more sound insulation), especially at these lower frequencies where drone and tire noise occur.

Oh, btw, forgot to say this is a 987.2 , 2.9 L PDK . I'm not suggesting that is relevant; I don't know if any sound deadening was changed with the facelift.

Good work wulfgang. I am kinda in the same boat with resonance and would love to learn and explore more into eliminating it. I currently have a 987.1 CS with a FVD race exhaust . I love the exhaust because its obnoxious and reminds me of a cup car when I step on it, but the drone can be unbearable sometimes, especially on long trips. I just installed (~15 min ago) 3 disk car chemistry inserts which after machining down to make it fit properly, have reduced some drone maybe ~25% (It really "softened the edges"). But I noticed that the car is super quiet under WOT and there is a noticeable loss in power, but highway driving is alot better. I am currently contemplating whether or not I should build my own, something similar to Plainsman. My friend and I have built a custom exhaust before using Ricardo WAVE software and Hemholtz equations for a YFZ-450 which worked well. Im thinking of applying the same methodology to this. It would be ideal to have a valved exhaust ; one side is straight through (like my current FVD) and the other through some sort of baffles or hemholtz resonator. Ive used my phone with a spectrum analyzer and found the excitation frequency, around 125 Hz, just like you found. I would love to help you and Plainsman! This is gonna get interesting! Also anybody consider putting the GT3 rear section on as an exhaust ?

Drone is a result of exhaust valve harmonics and is why the OEM headers are so thick and heavy compared to aftermarket thin walled. typically aluminum, headers . The thinner the wall the louder the drone and it is a sacrifice made when installing even the best of the aftermarket headers . It is the price of admission for lighter weight, better overall exhaust note and some performance gain.

Very interesting Wolfgang. I have replied to Plainsman's thread and won't repeat it on yours as I'm pretty much out of my depth. I would agree that drone maybe caused by cabin resonance but it's very hard to eliminate with soundproofing as the drone remains but all the nice mechanical noises are taken away. Road and tyre noise doesn't help matters and it would be good to know how to cut those down a bit.

wow thats quite impressive wulfgang..... the design of the car makes it more sensitive to resonance but the source elimination works best.

Based on my results (where the engine was not even operating), I would have to strongly disagree. The cabin does indeed have a natural resonance, and if you look at the excitation volume, none of the peaks in the excitation match the drone. Or in other words... even if you design your exhaust to NOT resonate at 2500 rpm, your cabin still will drone.

So as I understand it, you're saying that the Cayman cabin is like a bell that resonates at certain frequencies. If so, could that frequency (or the resonant energy) be changed by identifying the most resonant areas (rather than the entire interior), and dampening the vibration or the offending panel(s)? Even applying spaced beads of some mastic/caulk might dampen the vibration enough to be effective - without adding much mass. But this also raises the question - what about drone in a Boxster ? I know it exists. But the acoustics are totally different.

I think its complicated. Cabin design definitely has something to do with it, but its not the whole story. I've experimented with my car and determine a simple fact that my exhaust is just loud. Its not drone...its just that it is loud and just the sound that it creates. The bottom of the 981 car is basically a big open cavity with a 'drum like' thermal lining. When the car is on pavement, it might as well be a subwoofer. This sound bounces and makes its way to the driver, who is also inside a space that is similar to a subwoofer. Now if you park in a garage and test you get even more amplification. You can test in a flat field of grass and get very different results. End of the day, I really think its NOT the cabin, its simply the exhaust vibrating and the sound reaches the driver. If the exhaust has some low bass to it, you'll hear it depending on the environment. Loud exhaust is loud...only solution is to not use a loud exhaust if you don't like a loud exhaust and IMO everything else is really over thinking. To add something, I actually have a boxster and get more 'drone' with the top down and a paved road (parking lot being the worst). If take the top up, I hear LESS drone, but its not that obvious on the highway because wind noise masks exhaust noise.

Interesting theory Minthral. I like a loud exhaust but there are loud exhausts and loud exhausts. My brother has a huge exhaust on his Subaru and that is a constant drone. My friend has an even louder exhaust on his Ferrari and that is really crisp and clear throughout the whole rev range. It's loud in the cabin all the time but a beautiful tuneful sound. That's the sound that I want. I had a 986 Boxster with an exhaust bypass and that was beautifully crisp and clear, and loud. No drone and sublime with the roof down, it sounded like a playstation game.

Well the bottom line is the deeper & louder the muffler the more energy is transferred to the cabin and chassis in the form of sound(vibration) A good example of the chassis is car with massive woofers vibrating both your car and their rear license plate! Anyway, all cars are setup to block as much of the acoustic waves the stock muffler and motor makes at mild RPM's. In our Cayman 's once you hit 3500 RPM then the engine/muffler sounds somewhat overcomes the sound deadening. Hope this helps.

So why do the Ferraris with the loud exhausts sound so good in the cabin?

Because the interior is acousticly tuned to allow more exhaust sound waves into the cabin-now why no drone? Well, the exhaust is tuned to be a higher tenor pitch rather than the V8 boom box mufflers.

So if a manufacturer were to produce a higher tenor pitched exhaust for the Cayman , could that eliminate the drone?

Yea but the highest pitch exhaust is.... stock. Aftermarket is all about the bass.

You and I are in total agreement then. I've never heard a Cayman exhaust that sounds good inside the cabin apart from maybe a Capristo exhaust that is $6000. I don't understand why manufacturers and customers want Bass.

This is why I have a Capristo! Oh and you can get it for less than $6k, be sure and tell them you are coming from Planet-9 to get the Planet-9 discounted price, whatever that currently is...

So...any drone at all on your system?

Bass is cheap to make. Why people like the deep V8 bass, go figure. The most awesome exhaust comes on the older Ferrari's. Now does the Ferrari drone; sure maybe to you but to me it has an awesome growl. Can the Cayman be drone free? Sure, but nobody really makes a exotic sounding muffler for the Cayman that doesn't hyper the low RPM db. The tubi sounds awesome, but only at full wail, kinda of boomie at normal driving RPM. Now I think the best OEM sounding exhaust is the Infiniti G35/G37 Coupe/Vert. Sounds great even when you pull away, but no drone while motoring along.

Nope, not at all unless I purposefully open the valves at idle and punch it you get a little initial drone but that goes away as rpm climbs, if you leave the valves in auto mode you never get any drone because valves don't typically open until say 3000-3500 rpm at the earliest depending upon throttle and back pressure. You must not have heard a Capristo, it makes the Cayman sound very Ferrari-like. Not boomy at all!

Another Capristo owner. Zero drone when run in auto mode or valves closed at low power settings. Even running with the valves open I was never able to get more than a transient drone from the system doing very slow accels between 2 and 3 krpm. Maybe if we had some longs hills around here. But it just doesn't seem to happen even trying to cause it on a consistent basis. I don't know how pricing is currently but when I bought if one skipped the headers the cat back portion was similar to PSE price wise.

The physics of sound! :banana: Nice job documenting cabin resonance Wulfgang and I agree that the cabin plays a significant role in it. 120hz is roughly B2 on a piano. Go play that note and it will sound very familiar. Tesla demonstrated 100 years ago that absolutely everything has a resonant frequency. Every piece of tubing, every box, every piece of metal, wood, plastic... even your neighbors cat. They all combine to create a system of resonance in our car that either amplifies or cancels each other. Think of an exhaust system as a trombone with the valve lash equalling the players lips, the exhaust tubing length and diameter equals the brass tubing and bell, and your cars cabin is like a "one note" tuned woofer projecting sound to the audience. Change the RPM, length or diameter of the tubing and you change the resonant frequency of the exhaust . If the exhaust resonant frequency is close to the cabin frequency at common cruising RPM (3000ish) you get pretty unbearable drone. It's all a resonant system that works well together... or not. Adding an aftermarket exhaust , most drivers want BIGGER tubes and more free flowing. This lowers the resonant frequency and increases drone at cruising speeds. What to do? Porsche engineers spent a ton of hours meeting a ton of compromises to get an exhaust that sounds good, is quiet enough for Laguna Seca, and will last 100k miles. No small feat. If you want louder and lighter with no drone it will be expensive because somebody else put in a LOT of hours in R&D and wants to get paid. Cheap and light won't get it done. The other option is to add bass traps tuned to cancel cabin resonance at 120 hz. The wavelength is about 10' long so this may be tricky but carefully placed 1/4 wave bass traps might show promise. Time for further sound experiments Wulfgang?

Plainsman and I were discussing active noise canceling, but that is a longer term experiment. If by "bass trap" you mean a Helmholz resonator + duct placed in the trunk, then yes, I could probably do that in less time.... except I don't know how to design that yet. I don't have Ricardo WAVE, but I do have access to FDTD codes for EM that can be used for acoustics. Not sure I want to tackle modeling the shape of the Cayman's cabin, but if someone were to send me a CAD file, I could probably start there.

Speed of sound calculator: Speed of Sound Calculator Good cavity resonance page w/ Helmholtz calculator at the bottom: Cavity Resonance

Speed of sound calculator: Speed of Sound Calculator Good cavity resonance page w/ Helmholtz calculator at the bottom: Cavity Resonance

Something as simple as this bass trap, sized to fit behind the seats on the engine cover might be enough to absorb and neutralize cabin drone. http://www.gsacoustics.com/Products - Wall - Bass Trap BT.pdf

That's interesting. FWIW, I've tried with the same sort of material ($200 worth) in different areas of the cabin and behind the seats, but I found that when I got rid of the drone in one area, it popped up in another area. I was/am chasing it all around the cabin. I'm not convinced (yet) that soundproofing the inside can get rid of the drone. The problem I've found with sound deadening material is that it gets rid of the nice mechanical noises too. Mind you, the bass one above must just work. Is it expensive?

I have a Fabspeed Supercup Race on my 987.2 base manual and it's one of the most notorious droners. I have to say that the drone totally stopped after about 7k miles. I hear nothing but what should be heard now. I'm a happy man. Ironically I had an E92 M3 as well with an Eisenmann Race that I bought used and there was very little to no drone there either. Especially by the time I sold the car. I highly recommend BOTH exhausts. Both are works of art and sound amazing. Sent from my StarTrek communicator using Tapatalk

My 2008 CS OEM exhaust would drone around 2600 or was it 2800 rpm and a little load. A downshift or speeding up slightly would get ride of it. So not purely aftermarket.

Not made for my model, but a good idea, thanks.

The bass trap listed above is not common automotive "sound deadening" but designed to absorb LF wavelengths specifically. It will absorb 20db of 120hz cabin resonance which is the nasty drone stuff. It rolls off above 200 hz so most of the high rev noises should come through. I have not tried this one and have not priced it in a custom size to fit the Cayman but I do use similar bass traps in my studio and can confirm that the technology works, making your listening area far more ear-friendly by greatly reducing LF nodes and resonance. I suspect the price would be around $150 for one custom built to Cayman specs. A small price when shopping $5k exhaust systems.

My response was intended to be tongue-in-cheek, but I suppose you could rig up something similar if you were so inclined. IIRC, the "sound symposer" ports intake sounds into the cabin, not exhaust sounds.

The rearmost hatch corners and across the back really get my vote. For grins I'm going to roll and bind a spare quilt or moving blanket into a tight cylinder and try it at the back of the hatch. It's been noted that the overall shape/profile of the rear hatch is that of a mega-[really mega]-phone. can't change how reflective the glass is, but maybe take a bite out of the rear bass. Maybe?

Interesting. I have the same Fabspeed exhaust , with stock headers on my 06 987.1 CS. Awesome exhaust and beautiful quality build. And yeah it drones. What could've resulted in the drone totally stopping with yours? I wouldn't have expected it to change over time. I've only driven the car for about 500 miles, the previous owner installed the exhaust and put limited miles on it. Have considered Fabspeed 's headers , expensive but also beautiful.