Sorting Exhaust Drone'

[ProfDave]

Originally Posted by Simon.h

Hi all,
Well after fitting a full stainless exhaust to my ZT along with a de-cat, here's the exhaust-



and the de-cat-



It sounds great on the outside-
https://www.youtube.com/watch?v=bmp2xbOKxxo

But its got a bad 'drone' from about 1200rpm to 2700rpm, so I popped into a powerflow dealer and they said the drone is because it's running without the cats and it needs a small silencer added to the straight pipe but can't guarantee it will stop the drone 100% and they want £200.

So as i'm not paying £200 for a 'if' so I'm left with the option of buying a silencer from ebay and fitting it myself with clamps (have no welder and can't weld) or removing the system and replacing with std.

Then after reading this-

Drone is the magic range at which you can both hear AND feel the exhaust sound.

Factory systems do loads to prevent you from hearing this. I can't even begin to cover all of that, but most notable examples include mass dampers, valves, and specially chambered devices, which we'll get to.

Sound travels as a wave. Exhaust gas pulses travel like waves through the tubes.

It's about to get tough right here: The sound that creates drone, that pitch 94hz-125hz, these are long waves. You know how waves have wavelengths, and all that jazz? These pitches are long, basically from 9ft to 12ft long. As the pulses accumulate and travel down the exhaust (lets use a straight tube as a reference), the sound created will be a constant drone at that range. Bends and resonators help by breaking up the pathway of that wave, and absorbing the actual volume of the wave... but the low frequency tone is still there.. which is how you can have quiet exhaust that still drones. That sucks. Also, that's sometimes why louder exhaust doesn't seem to drone; you let the high frequency waves cut above it... like back at the concert reference we made before.

The best way to cut down the drone is via destructive interference. What you need to do, is reflect the trouble-causing wave back at itself, 180* out of phase.

This is like two equal people pushing an object in opposite directions- nothing happens!

However, that effectively means you need a chamber that can reflect the wave back in a straight line for 4.5 - 6ft. First up, thats difficult under our cars, to have dead tube space like that. One of the easy ways to do it would be to have a muffler or resonator that had a dead stop in a straight line down the center of the car, but the world will assume it can't flow well enough to work. This is the balance between power/flow and sound/comfort. It's always a balance.

Take APR for example, they use RSC technology. They are licensing this from Corsa, which is awesome, because Corsa has a patent on it. Because it works pretty well. What they do, is create that long length of tubing INSIDE a smaller canister- tough to manufacture as well- but it creates that destructive interference. If you've been in a car with a proper RSC system, you know what this is like- you hear the tone that should cause drone, but it doesnt. You don't feel it.

There are other ways to kick that drone though. Take for example, that listed Stratmosphere exhaust- there's a mini magic trick happening in the center resonator, which works awesome on a B7 A4 because of the long straight length of tube created by the DP/center section. Where the exhaust gas splits to go to duals, its in a perforated "Y" piece.. so the waves can travel through it, hit the back of the resonator, and travel backwards (as a wave, not as exhaust gas pressure) oh, I dunno, from about 4.5-6ft right back towards the source!

So basically, there are ways to cut down drone that you can't see. There are loads of ways to deal with it.

With the mustang, using different size mufflers is just another trick. But it's not always the answer. I know older mustang guys used to claim that having two 26.5" long tubes welded at 90* off a section before the muffler would also eliminate the drone- all sorts of ways.

What further makes things interesting, is how different cylinder configurations create drone at different ranges, but all the same (to an extent).. You can calculate out where 4, 6, 8, 10, 12, 16 cylinder configs will drone. I am trying to remember without digging through documents (As this is all off the top of my head), but 4bangers create the 94-125hz pitches between 3500 and 4500rpm. Add a turbo to the mix, and its more difficult to sit at that RPM range while cruising. So often, you can have a system that is minimal, large diameter, and flows well, without having intrusive drone.
What sucks, is 6 cylinders. They create the 94-125hz tone between roughly 2450 and 3250rpm. Riiiight around the range you'll likely be cruising at on the highway. This also primarily explains why BMW has some of the fanciest tricks for exhaust stuff from the factory, what with a plethora of 6 cylinders along the lineage. It's also why the B8 S4 has light drone BONE STOCK!
8 cylinder motors are easier, as the pitches occur low, like 1850-2450rpm, which in say, the 4.2L, is barely moving along.

No, swapping various Brands mufflers will not likely be the answer. And if it was, it'd be a total fluke. A more interesting scientific test would be to make sure an exhaust system could handle full flow out of only ONE side of a dual exit exhaust.. and then cap off the other side... that'd create your long reflecting chamber..

Anywho- the point is, it is all about waves, and how to bounce them around. You might not see how it's done. And yes, true, not every aftermarket company bothers- but the ones that do, typically are worth the cash.


So after reading the above its just luck if adding a silencer will stop the drone!
The joy of fitting a stainless system to our cars!

What would you do??

cheaper to get some noise cancelling headphones! Its quite difficult to get this out as you point out; also quite a challenge to get a nice outside sound without too much din inside. My Longlife SS one on the ZTT has an extra silencer; even then its still more raucous than the OEM one but not irritatingly so however I am glad I asked for as quiet as possible!!.

[ProfDave]

PS my rear silencer is also quite allot more substantial!!

[Skilrove]

I'm now also considering a THIRD intermediate damper, there seems to be sufficient space, but want to be sure to get something effective -- most such boxes seem to be "free through flow" or some such description. I'd trust something with a baffled construction ...where to buy? any suggestions on whether it should be as far back or as far forward as possible ?

[tareim]

After decatting my car and removing the next box along (front to back) I can tell you I don't get any annoying drone so the main issue lies with either the back box or the oddly shaped box before it

[Anon3]

I'm a bit confused here, as it seems to me any form of extra box or bend or reduction in diameter is only going to cause further restriction.
Which makes removal of the Cat a bit pointless, surely?

PS, that manifold has to be one of the poorest designs I've seen ever.
Doubtless there was little option there.

[Skilrove]

Haha my driveway also has a blue C3 parked alongside!

[Skilrove]

So from Tareims account, He removed the restriction of cat and one more box without noise penalty.
Maybe my attention will move towards the rear part, then. There can be remarkable effects from relatively small changes such as when Kwikfit replaced a back box on a Punto for me a couple of months ago- gave a raspy sound and then I noticed it was not located well, so when they remounted just a couple centimeters further forward it was quiet and sweet once more. So who knows, when a Longlife SS or whatever other catback gets fitted, the dimensions may vary just enough to cause resonance/drone issues. Any ideas what is in that rear "strange shape" box? Kind of triangular section.... Maybe it's baffled rather than straight through, would be interesting to cut an old one open for investigation, any volunteers? If so I will go for another baffled intermediate. Not too worried about the restriction

[tareim]

I wouldn't say there was no noise penalty, it has made it audible (which is wasn't before) I just don't have the boom at about 2k when I previously changed the backbox which is no longer on the car because of it

[Skilrove]

Interesting article copied in above about the resonating tubes etc. if the offending "organ pipe" resonance is about 110hz then a suitable Quarter wave dead pipe would be about 3ft long to kill that frequency. Looks line there is PLENTY of room to place one parallel to that long rear run of pipe.

[Skilrove]

had a damper pupe welded on following earlier described concept

See photo- helped a great deal to quieten things down from 1900-2600rpm. Its a quarter wave length of the pulse frequrncy from the v6