What is exhaust drone?

[SierraBronco]

Drone is caused by resonant sound in the exhaust system. It can be "tuned" to have various audible qualities. This tuning process can be done scientifically and with attention to detail (much more necessary with smaller displacement engines to achieve a desired sound) or relatively carelessly (open up the exhaust and let it do whatever it wants). The point of a tuned exhaust system is to improve the efficiency of the engine and enhance performance. With smaller displacement engines, it is less about performance improvement and more about modifying the sound. There is a misconception that only a large engine can sound good. This may be because of the number of aftermarket exhaust companies that do relatively less engineering and analysis, resulting in systems that are only capable of sounding good specifically because they are attached to larger displacement engines. There are some companies that understand both the esthetics and performance issues at play and have produced systems with these variables in mind. MRT in Plymouth, Michigan is one of these companies. It is owned and managed by someone with a clue regarding what people say they want vs. what they really want (everyone is hoping for a system that produces a bad*** sound). To achieve the right sound without producing the relentlessly annoying drone takes more work than just throwing together some pipes and increasing the diameter from stock, and the people at MRT have both the knowledge and understanding to accomplish this potentially daunting series of associated tasks.

My 1260cc Ducati L-Twin sounded

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[Umichigan1]

Some correct, a lot not.

Drone is primarily determined to be present or not (and how loud it is) by the exhaust system itself. It happens when the engine exhaust excitation frequency (output pressure waves exiting the exhaust valves) excites a standing acoustic wave in the exhaust system. And at such a resonance, the volume is much louder relative to nearby rpms.

It looks like this (with the caveat that this is for a net zero flow - same concept applies). Top shows the pressure wave, next graph show particle displacements and last shows the pressure.



The primary variable just being the effective length of the exhaust system. It will happen at an engine rpm when f, the engine frequency

f = rpm/60 * N/2 (N is number of cylinders)

f ≈ c/4L , 3c/4L, 5c/4L, etc.

c = speed of sound in hot exhaust gas
L = effective exhaust pipe length

This is an approximation for a simple, uniform exhaust system with no chambers, resonators or branches.

Straight pipes and large diameters, often done in the pursuit of power are likely to cause or worsen drone.

At the risk of being perceived as a white knight for MRT, they were providing an answer to an ordinary person and not someone with a PhD in engineering.

 

[ThunderFlash]

I think some people consider drone to be any form of exhaust note. I have the Borla Cat Back Climber Touring exhaust and it is about as quiet as an aftermarket exhaust is going to be, yet I see people complain about drone with it. Mine does NOT have drone. You'll know if you do, though. On a two hour freeway drive, first you'll be like "Why am I feeling so agitated?", then probably start wondering "Is some else in here with me...why am I hearing voices?", then at some point later you will become convinced aliens are sucking your brains out through your ears.

I'm happy with my Borla. The stock exhaust was so quiet with the less than hushed Bronco cabin, you couldn't tell what the engine was doing, even on slow speed trails. With the Borla there is just enough report to know how it is reving. The climber series also moves the main can between the frame rails in front of the axle. Sport has no resonator, but the touring has a small resonator behind the axle.

 

[swamp2]

At the risk of being perceived as a white knight for MRT, they were providing an answer to an ordinary person and not someone with a PhD in engineering.

And, at the risk of being perceived as argumentative, I was trying to provide multiple things:

1. A more accurate definition of what is actually happening.
2. Pictures to help with those who are more visual learners.
3. A very small bit of math for the "nerds".
4. Corrections to incorrect information

All in a reasonably short explanation that doesn't require any degrees to get a lot out of.

 

[Umichigan1]

And, at the risk of being perceived as argumentative, I was trying to provide multiple things:

1. A more accurate definition of what is actually happening.
2. Pictures to help with those who are more visual learners.
3. A very small bit of math for the "nerds".
4. Corrections to incorrect information

All in a reasonably short explanation that doesn't require any degrees to get a lot out of.

What is that word "argumentative"? My limit is 3 syllables per word.

 

[helifino16]

As an alternative you could zip-tie a Harmonica under the Bronco allowing air to pass through and give an annoying hum/whine.

There's also this annoyance:

(if you know, you know). lol

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