Third-Reef

I was wondering if anyone has any actual knowledge if it is from the surfaces of the exhaust system or is it all coming out the tail pipes? Depending on where it originates from I would employ very different changes to eleminate it. I like my exhaust but hate the drone. I intend to engineer it out. Hope to not have to mount sensors to identify the actual source.

DaveCarrera4

damn good question. +1. Hope someone knowledgeable chimes in.

Third-Reef

DaveC, you are probably the most qualified to answer my other question. My spoiler always goes up at 75 but only sometimes goes down when I park the car. I have no warnings like I had when it did nothing. The button moves it up and down no problem. Any ideas?

DaveCarrera4

PM sent.

mxracer

Exhaust drone is caused by resonance initiated from the exhaust thru the car. Meaning, sound waves at "x" frequency (usually around 135hz) which resonate thru the entire car.

No idea what exhaust you have but, an exhaust resonator (really designed to cancel the resonance/sound waves), redesign the exhaust, or get another exhaust are the only real answers.

If you google exhaust drone you can read for days on the subject. It's been around in cars since they were invented, and way before that in nature.

Buz914

Good description. Most things have a resonating frequency or multiple frequencies. My truck for example might shake at 70, but go slower or faster and it goes away. Airplane control surfaces can resonate(flutter) and tear themselves off the aircraft. Its kinda like a magic frequency, its kinda neat if you really think about whats happening at that moment. Engineers try to design things to not resonate at frequencies that would be experienced during normal operation.


My .02

autobonrun

As others have stated, it occurs when a system vibrates or oscillates around it's natural frequency. Everything has a natural frequency. It can be reduced or eliminated by changing the internals or even the way it's designed, in the case of an exhaust even changing the mounts. Anything that moves the normal operating frequency away from it's natural frequency will help reduce the effect. If you want to read some technical stuff, search "overdamped, underdamped, and critically damped" systems.

One example is when the volume on an amplifier is increased, it will reach a point where it reaches it's natural frequency and starts to oscillate; and we hear the loud screeching sound.

For an exhaust, we just get the annoying drone, but worse case scenario, the oscillations at the natural frequency continues to grow until the system destroys itself. Here's one of the worse case examples everyone has seen of oscillation at the natural frequency.

http://www.bing.com/videos/search?q=...851C8E64E85361

One other thing, there's usually more than one frequency at which a system will oscillate. Usually one or two are areas of concern and others are outside the region where the system operates. In terms of an exhaust, say one occurs at 3000 rpm and the other at 12,000 rpm. Our concern is the one at 3000 rpm and a system designer would try to redesign the system to shift that frequency out to say 5000 rpm. Either you want it outside the area where you spend significant time or you want it at a place where you go through it as fast as possible. I'm no exhaust designer so I don't know how easy it is to shift exhaust natural frequencies but in theory, that's how it would be avoided.

Third-Reef

I guess i should have been more clear with my title. I really want to know if anyone knows where it originates from physically. I am an engineer and deal with control system theory and understand the basics of resonance.

My exhaust is a knock off of the kline?



It has no markings except for the laser marking of the fittings as 304 sst 2 1/2". I am wondering if my drone could be coming from the walls of the cans resonating and emitting the sound into the engine compartment/car. Or if it is the resonant pulses exiting the exhaust tips and transmitting back. One way i could probably differentiate it is to put some big floppy rubber hoses on the tip of the exhaust that would (look like hell) and effectively damp the resonant pulses exiting the tips. This works great on 2 stroke dirt bikes with a piece of inner tube. If this had no effect then i would assume that it is being emitted from the surfaces of my cans and plumbing. If this was the case and extra cover of sheet metal with a thin layer of sound deadening (fiberglass) and some metal straps to hold it all on might eliminate the drone. If it is coming from the tips then i would need to change the flow path and or volume to some how damp or shift the drone out normal operating speeds. I have some ideas and i will get rid of my drone, one way or another.

This can't be nearly as hard as filling a 15 manometer 20:1 via with tungsten.

Just remembered that several people said that swapping headers from stock eliminated the drone, makes me think it is the tuning of the pulses coming out the tips not the cans themselves.

Buz914

Maybe it varies with different systems because of the slight differences in individual header lengths, ect. The engine pulses coming together and amplifying things? If that's the case external changes might not be that effective. Interesting topic. Please share your results.

autobonrun

My apology, I just went back and re-read your question. I would think it is coming from both the body/mounting and out the tail pipe. You could do an experiment where you stuff some fire retardant material temporarily around the exhaust body to act as sound deadening see if the frequency changes.

Or if you have access to the mounts/hangers, change the material where the body is connected to the hanger.

Just a suggestion.

Third-Reef

Or i might try disconnecting the mounts completely for a quick drone around the block and see if there is a change in the drone.

Thanks for the suggestions. i will update when i get any significant data.

996tnz

Good on you if you finally manage to fix it at source. For my part, I focussed on how it was transmitted/induced in the passenger cell. The problem is that the relatively unbraced firewall becomes a resonant bass driver, creating that nauseating drone at certain rpms (was about 2200 in my case). Hated adding weight to my car but researched the problem and just half of a door kit's worth of dynamat applied to the cell side of the firewall made a big difference and passengers stopped complaining. Suspect you may already know this but it works by mass damping rather than acoustic insulation so putting more layers on in the middle of the vibrating panels and little or none around the edges has the best effect.

For further surety I also added some dynamat to the metal surface of the parcel shelf (another potential bass driver) and even a bit on the alu covers under the Bose (just to preempt any buzzing).

Easy job, done in an hour or two - would post pics but can't find them now. Couldn't stand the half hour drive to work before sorting that - would leave me queasy.

32krazy!

on a 3" exhaust the use of hemholtz tubes as used on the unobtanium welding all but eliminates the drone. the tubes simply move the resonance to a higher rpm range away from the 3000 rpm area that most drive in.

the design of the tubes has been changed numerous times as different shops attempt to copy the original design

Third-Reef

996tnz, what type of dynamat did you use? From looking at their web site i would assume it was the dynamat extreme. I am going to give this a try.

Thanks for the suggestion.

rmc1148

Completely different application as far as on firewall but I have tried dynamat a few times with marginal success= worth a try.