Exhaust drone & exhaust noisewhile both are different by definition, both are created as a result ofimpactors/promoters. Exhaust drone is that which is caused by harmonics- a soundwave generated which in essence takes the exhaust sound and sends it back upthe exhaust system (towards the engine) and is released or transferred into thepassenger compartment. This transfer typically happens where large contactareas exist that have little to no sound dampening qualities and/or materials-as a result of the muffler construction or vehicle manufacturer.
Exhaust Noise (in passenger compartment) is what most people actually complainabout. While harmonics do play a contributive role, it is not the primarycause.
Impactors/Promoters
I would love to tell everyone that “x” is the cause 99% of the time for exhaustnoise complaints, but it is typically a series or combination of numerousfactors which leads or develops the level of noise which is uncomfortable tothe ear. The first “rule” of an exhaust system is that the noise should begenerated at the point of discharge (i.e. tailpipe tip). Obviously, if the tipterminates at your door or immediately under the passenger compartment, thesound level will be much greater at the exhaust discharge point than if the tipexits the rear of the vehicle, but this is common sense.
Assuming that there are no exhaust system leaks and isolators are correctlyinstalled at all mounting points, what is not acceptable by industry standards,is when an exhaust system has been installed correctly, exiting well past thepassenger compartment to have substantial exhaust sound transfer into thepassenger compartment at a level which does not allow a person to talk withoutraising their voice to well above ambient noise levels. Whether you love tohear your engine roar (which I do), stereo blast or have a conversation, this interfereswith your ability to enjoy what you like.
Now for the Causes
The first problem is most muffler shop staff today have little to no knowledgeof exhaust systems, the “tricks” to installing a system that will “please” thecustomer, and simply verbally repeat the marketing materials provided to themby the system manufacturer. As previously noted by several others, there are 3basic muffler designs, Absorption, Diffusion, and Chamber (Resonator), nomatter which one suits your ear best, the criteria for promoting unnecessaryin-cabin noise is the same.
Impactors:
Exhaust system material thickness should be no less than 16 gauge-double-walled pipe was commonly available through the 1970’s untilmanufacturers began seeking ways to reduce costs. Many of the “Muffler Chains”who marketed lifetime warranties always used double-walled pipe before“stainless steel” became the “in-crowd”. If a shop had it available, I wouldchoose double-walled aluminized coated steel or 14 gauge stainless steel(depending on environmental conditions or personal tastes) - it is very quiet,solid and in terms of costs, pipe is pretty cheap, depending on length, 16gauge aluminum coated runs about $11 to $20 wholesale (depending on diameter),stainless about double for the same gauge.
The muffler should be located as close to the axle (far away from the passengercompartment) as possible, allowing at least 3” of straight run pipe before abend occurring (if installed to the forward position of the axle), with theoverall length as long as possible.
Duel Tailpipes should be limited to no greater than 2.00” (although 1.5 is best) inside diameter, tips which are larger cancertainly be installed but should be limited to 12” in length and have adischarge point past any material that may “catch” a sound wave or exhaust.When you exceed these diameters, it now becomes a contributor.
Tailpipe length in general should be as short as possible, minimal bends, etc, terminatingpast the axle, 1.5" past the body or bumper. Lengths greater than 4 feetpromote “drone” (i.e. harmonics), typically about 3 ½ feet past the muffler.
Pipe diameter reduction should occur before the muffler. While many mufflerinstallers like to use mufflers which reduce pipe diameter or split the exhaustat the point of discharge at the muffler, or install reducers to connect thetailpipes after the muffler induces “noise”.
Ultimately, it is the quality of the material and the installer which 90% ofthe time leads to a successful installation. I would like to also emphasizethat it is not necessary to spend a fortune on a muffler ($100+) or system tohave an excellent sounding, performing and long lasting result.
If you decide to change mufflers, I would recommend saving a lot ofmoney & go with Allied resonators......available is any muffler shopnationwide, are of excellent quality, 1/2 the price of “performance Mufflers”,available in any length from 10" to 38" (IIRR), flow as good as Borlaor most any of the "Race Mufflers”, tone is deeper and you get a nicerumble (low) with no drone and none of the tinny "ricer" echo. Alliedhas been made for at least 30 years and available in just about any diameter youwould require (I have personally run these for since the 1970's....nice lowrumble on acceleration & deceloration, no drone, deep tone.