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Exhaust drone causes, and reduction techniques

Exhaust drone causes, and reduction techniques

Exhaust drone causes, and reduction techniques

Hey guys & gals of the ENG-TIPS forums! I've been working trying my hand at finding good exhaust drone cancelation solutions, because I'm trying to further my understanding of sound waves and automotive engineering. I work in R&D, and I took a project on for someone who bought an aftermarket exhaust and immediately had a huge drone! Some back drop:

2017 Camaro SS V8
Legato performance exhaust

Originally the exhaust was so bad it would shake the whole car, causing the whole body to vibrate and really hurt your ears. Anyways. My co-worker who speaks english as a second language already typed up a report, which I'll post below. I am looking for additional solutions as we want to completely resolve this issue. I have a few in mind, but more on those after the report.

Due to the replacement of aftermarket mufflers, the crew is suffering from an intensive acoustic drone throughout each gear between 1500 to 2500 RPM under medium to high engine load. In order to reduce this effect, several methods were tested.

Original exhaust sound data logging
With several test drive we are able to collect the data and visualize the sound wave. The overlay of the sound wave file (chart1) indicates that there is very strong peak between 120 to 150Hz at
about -25 to -22 Db with a strong noise level across between 200 and 7500Hz. Based on the theory of the most common range for human hearing is around 1000Hz. Therefore, we made our assumption that we should aiming to lower the noise around the 1000Hz area while focused reducing the 120~150Hz range.

Collecting more information

With the known value of the noise inside cabin, we decided to test the resonating frequency of the exhaust pipe and trunk. The reason we did this is because
1, we are trying to find out if the drone is originally generated from the engine firing order or the resonance of the exhaust pipe.
2, the trunk space (as well as the dual layer trunk lid) are acting as an acoustical chamber that amplifiers the exhaust noise.
Based on the testing we find out that the resonating frequency of the trunk lid is around 220hz and the exhaust pipe is around 120Hz.

Exhaust J pipe

The first step we utilize is to create an acoustical pass the run along the exhaust pipe before the mufflers. These acoustic passes are also referred as the J pipe to the public due to its shape. The theory behind this device is to use the pausing wave from the original exhaust flow to travel a certain distance and bounce back to the main exhaust gas flow to cancel the original exhaust sound wave. In theory this method will work but it does have its own draw backs, the length of the path has to be calculated and the ideal length can only cancel the noise at certain Hz but not across the wide range. At this point we invited a sound engineer to help us doing the analysis and decided to focus the targeting frequency range to 120Hz and based on the calculation, we generated our target path length should land at 27.5 inch length.

We perfoemced several testing after the Jpipe installed with the length around 28 inch and finally confirmed the 27.5 inch provided the best result for this application.

Chart 2 reveals the different results that different length pipe creates.we notice that the J pipe effect not only reduced certain frequency but also boost the level of other frequencies as well. Therefore, we are trying to find out the most balanced point (27.5inch) and to further reduce the niose by apply sound deadening material.

Sound dedening installed with exhaust servo disconnected
With the sound deadening material applied to the trunk area, we did the test again with the exhaust servo disconnected (constantly open position) in order to eliminate all the potential varieties the ECU controlled servo will provide. Then we got a testing result like this (Chart 3).


It is worth to point out that at although the chart shows the sound wave in quite similar shape as before, we were actually trying to hunt this peak level by porously loads the engine in a very low RPM range, and we barely got any drone through the 1-5th gear (7 gears total). However, according to chart 3, although the low frequency level reduced to around -30Db, the noise across the whole range actually goes up. This was within our expectation that the exhaust servo was put on from factory for noise reduction propose. Therefor we reconnected the servo and did the test again and get the result like this (chart 4)


Chart 4
Now, it is very clearly to see with the servo connected, the noise level reduces dramatically across the whole range. The peak at around 100Hz reaches lower than -40Hz level, and the most sensitive area (around 1000hz) is dramatically reduced to under -70Hz. Please be note that this is the freeze screen on the low speed, 7th gear, and the highest engine load condition.
Weighting the exhaust system
With the already great result, we are trying to test if we can achieve an even better result by put extra weight on the exhaust in order to reduce the vibration resonance of the exhaust system itself. We put 5 pounds weight at the very end of the exhaust system and did the test again. However, the noise level actually boosted instead of reduced. We can see the value from chart 4


Extra weight would seem impractical. It seemed to make the drone happen at a lower RPM.


We are evaluating resonant chamber designs, and other possible changes to reduce the drone. The muffler kit seems to use chambered exhausts. if these are designed improperly will they produduce this problem?

RE: Exhaust drone causes, and reduction techniques

Here are some solutions we are currently considering but I have not yet found access to the equations, and my math skills only go as far as calculus 2.

RE: Exhaust drone causes, and reduction techniques

All the picture links are broken for me

RE: Exhaust drone causes, and reduction techniques

I'd be very surprised if anyone described 1000 Hz as a drone. I would say the only problem of interest in a noise exposure related way is the 120 hz. Are the spectra shown A weighted?


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Exhaust drone causes, and reduction techniques

I apologize about that, I tried repairing the first few which appeared broken for me on another browser due to me moving hte pictures in my photobucket.

I do not believe my android phone is capable of producing A weighted charts with the free frequenseeapp. I agree, the 1000 hz is not the "drone" but oddly enough when we try to focus on reducing the 120 hz noise it takes away noise in other frequencies as well. I am supposing these other frequencies that are reduced are multiples of the same frequency, although I'm not entirely sure.

I know I do not have the best test equipment available, but it's all I've got glasses
Also the car will be back with us for development again on monday so I can gather different data or perform other tests then

Here's what we tried with the side branch resonator:

I assume some of the trouble getting it to work right is we had to put bends in place to package it under the car. One of my theories is that the bends scatter some sound waves back and not all reach the end and the path for them to get back into the exhaust is not perfectly straight either. We'll see.

RE: Exhaust drone causes, and reduction techniques

I see a problem now. I used photobucket but don't have a subscription. If none of the pictures are showing up I'll have to use this website to host them.

RE: Exhaust drone causes, and reduction techniques

I realized photobucket was the problem I've uploaded the pictures to this website instead. see first post. now to update the rest!

RE: Exhaust drone causes, and reduction techniques

I would not expect much difference based on what gear the transmission was it. The main contributor is the rate of pressure pulses released into the exhaust and those depend directly on engine RPM with a minor change in amplitude due to engine loading.

It's my impression that aftermarket exhausts were chosen to amplify engine sound - or so my experience with every day cars would suggest. At least if they weren't to amplify, they were to eliminate the sound deadening characteristics the original exhaust systems were carefully created to have.

If you are interested in experimenting, you could make a variable length pipe. It works for slide trombones. It doesn't have to clear the ground as long as the car can be safely operated when elevated.

RE: Exhaust drone causes, and reduction techniques

You might want to look into the formulae for Helmholtz resonators too, since your device looks more like one (it has a neck and a volume).


RE: Exhaust drone causes, and reduction techniques

This is a post I am going to follow with great interest, I have a noise on a ford explorer that sounds very much like what you are investigating.
At around 2300 RPM to 3000 RPM a loud drone starts then goes away when you go outside of that RPM band . I have suspected a collapsed baffle in a resonator on the exhaust . Maybe your work will enable me to get a closer handle on this noise.

You are judged not by what you know, but by what you can do.

RE: Exhaust drone causes, and reduction techniques

I'm peeved that I may need to get involved with engineering drone out of exhaust systems. I shouldn't need to, but driving vehicles that are at least 50 years old places me at the mercy of retailers of replacement mufflers. There is no alternative but trial and error- which is both expensive and time consuming (and frustrating). On my 6-cylinder '55 Plymouth I got lucky on the third purchase. I'm on my third attempt with my 6-cylinder '67 Falcon with no success yet. This one has a loud resonance at 1,300-1,500 RPM (30-35 MPH, irritating as can be). With this one I also replaced the complete system- new manifold and all new original-spec pipes- to no avail.

RE: Exhaust drone causes, and reduction techniques

Ah, well that can be explained, if not cured.

Hmm, I suspect you are fighting (a) physics and (b) incompetence.

I suspect you'll find that's a big peak at 70 hz, and that is likely to be a combination of a bending mode in the structure of the exhaust excited by the acoustic resonance of a higher harmonic of the organ pipe length of the total exhaust system.

30-40 years ago I plotted the noise of lots of cars vs rpm, and the number of them that had exhaust booms at around 1600 rpm was astonishing, my only explanation was that before then it was not common practice to lug the engine.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Exhaust drone causes, and reduction techniques

Not wishing to hijack this guys thread ,Pontiacjack and Greg,
But you guys are describing the problem I started having on my Ford explorer 3 weeks ago
Since I do not do this for a living I am going to start a post on engineers with hobbies and see if we can pick it up there.
Then we may need to redflag the last 3 posts.

You are judged not by what you know, but by what you can do.

RE: Exhaust drone causes, and reduction techniques

I'm confused. Are you are measuring sound, or vibration, in Pic 2 ?

When it is "droning" can you feel noticeably vibration in the trunk floor etc with your fingertips that is not there when the droning is not noticeable?

If you can find one of the Radio Shack sound level meters shown on the left here, it offers A weighting and a microphone output for processing on a PC using free FFT software that used to be all over the web -

RE: Exhaust drone causes, and reduction techniques

My android SLM app has A weighting, it is not rocket science. You don't generally want A weighting if you are interested in exhaust drones as the 20-100 Hz region is most important.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Exhaust drone causes, and reduction techniques

have you considered an equilizing tube.

RE: Exhaust drone causes, and reduction techniques

I actually like a bit of a "drone" rather than have the engine perfectly silent. I have been in some cars - (a certain Cooper S comes to mind) where the drone was so loud that it was literally painful.

RE: Exhaust drone causes, and reduction techniques

PontiackJack, not sure why you have to try to eliminate drone via trial and error. The method shown above provides a calculation for building a 1/4 wave resonator to target and cancel drone.

I could get so much done if I didn't have to go to work

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