Highway drone and loud idle?

Dave Rosenthal said:

This drone continues to drive me a bit crazy, as I just feel like I'm driving around inside a drum in my new Grenadier. I've owned a good number of cars, but never one with this issue.

So, I've done a bunch of investigating for us. I have a background in acoustics and engineering, so bear with me, this will get a bit technical. TDLR: I have good news and I have bad news: The good news is that I think I know what the problem is (not what anyone thought!), the bad news is I don't have a good way to fix it yet. OK, let's get into it...

First, for the doubters, this is a spectrogram of my Grenadier driving down a road at 65 mph and showing a very strong drone. The Y axis is frequency in Hz and the X axis is time (time is not super important, we're usually looking at like 30 seconds of driving):
View attachment 7869122

Yes, that bright yellow line is a pronounced resonance at ~48Hz. A low bass drone.

Also, this is not only a 65mph thing (though it is worst there for reasons I will explain later). Here is a Grenadier driving down a road at ~25 mph:
View attachment 7869123

As you can see, the same fundamental resonance at ~48 Hz is there even at low speed.

Well, the roads don't have lots of 48Hz noise by themselves--so something in the Grenadier is amplifying this frequency. (I verified this by analyzing interior frequency spectrums produced by banging the frame and suspension in various places with a "impulse hammer", i.e. rubber mallet, and seeing the same effect -- yes this is how they do things in industry.)

So, I set out to figure out what could be causing this 48hz resonance. This is tricky business because sounds this low are basically impossible to localize by ear. My first order of business was testing air resonances.

View attachment 7869124

This shows me doing a bunch of clapping (generating an impulse) and seeing what the resonant frequencies of the cabin are. There might be a bit of a resonance at ~85Hz, but nothing pronounced. So, cabin acoustics are not the main culprit. Next up was investigating structural resonances. I won't bore you with the graphs for the dozens of tests I did, but I analyzed the spectrum of tapping every panel in the inside of the vehicle, searching for the resonance source. Roofs seemed suspicious, but were too high frequency, doors were fine, heat shields were a non-issue, etc. Most tests looked like this one (of the driver's left roof panel), or less interesting. (The vertical lines are individual taps, if you were wondering.)

View attachment 7869125

Looking at all the data I collected, I found only one culprit:
View attachment 7869127

This is the tailgate. It is large, flat, and vibrates strongly at ~45-50 hz. It's basically a giant speaker, and rings at the same exact frequency of the drone we see in the vehicle. The tailgate is causing the resonance. You can experience this yourself by locking yourself in the trunk area and banging the middle of the door. You will hear a pronounced drum effect ringing in your ears. You can also have someone else do it as you sit in the drivers seat--same thing.

Which leaves one last mystery: Why 65 mph? The explanation must involve something on the car that tends to generate 48Hz vibration at that speed (which then gets amplified). Again, long story short, I calculated all of the natural frequencies for the drivetrain at 65. Tires, engine rotation, exhaust, etc., none were even close to the right frequency. Except the driveshafts. Using the axle ratio and Bridgestone's data on KO2 revs/mile, the driveshafts (and associated gears) work out to generate a 48.6 Hz rotation frequency at 65 mph. Yup the same exact frequency.

And that's not the only piece of evidence... If you look closely you can see a "pulsating" effect every few seconds at 65mph that you don't see at lower speeds... (And, just for fun, yes, this graph was taken with the car in 7th gear at 65mph, showing that doesn't change much of anything.
View attachment 7869129

Without going into math, this pulsating is highly indicative of two vibration sources that are vibrating at very close to the same frequency. And to further implicate the already obviously guilty driveshafts, there are indeed two of them... Front and rear.

So, to sum up: The Ineos has a highly resonant rear tailgate that rings like a drum. It resonates all the time, but is most noticeable when going at ~65mph when the driveshaft vibrations stimulate the resonance.

So, the next step is figuring out how to reduce the problem. I worked on that a bit, but it seems like it will be tricky. Ideally, Ineos would have built a tuned mass damper into the tailgate to mitigate this. (Porsche did this with early Caymans which suffered a similar issue.) Anyway, at least we have reasonably high confidence about what is going on. So, no, bending your heat shields will not do anything about this, and no, sadly 7th gear at 65 won't either. Driving around with the tailgate open will (It's possible that applying a bunch of constrained layer damping material on the tailgate could help, but it's hard to say as it's a low frequency we're dealing with.)

I'll let you know if I figure anything else out. - Dave

Click to expand...

Quartermaster said:

@Dave Rosenthal Brilliant work! Why not change the mass of the door? If it is like you elaborated, the drone should go away if you drive without spar wheel.

Click to expand...

Dave Rosenthal said:

This drone continues to drive me a bit crazy, as I just feel like I'm driving around inside a drum in my new Grenadier. I've owned a good number of cars, but never one with this issue.

So, I've done a bunch of investigating for us. I have a background in acoustics and engineering, so bear with me, this will get a bit technical. TDLR: I have good news and I have bad news: The good news is that I think I know what the problem is (not what anyone thought!), the bad news is I don't have a good way to fix it yet. OK, let's get into it...

First, for the doubters, this is a spectrogram of my Grenadier driving down a road at 65 mph and showing a very strong drone. The Y axis is frequency in Hz and the X axis is time (time is not super important, we're usually looking at like 30 seconds of driving):
View attachment 7869122

Yes, that bright yellow line is a pronounced resonance at ~48Hz. A low bass drone.

Also, this is not only a 65mph thing (though it is worst there for reasons I will explain later). Here is a Grenadier driving down a road at ~25 mph:
View attachment 7869123

As you can see, the same fundamental resonance at ~48 Hz is there even at low speed.

Well, the roads don't have lots of 48Hz noise by themselves--so something in the Grenadier is amplifying this frequency. (I verified this by analyzing interior frequency spectrums produced by banging the frame and suspension in various places with a "impulse hammer", i.e. rubber mallet, and seeing the same effect -- yes this is how they do things in industry.)

So, I set out to figure out what could be causing this 48hz resonance. This is tricky business because sounds this low are basically impossible to localize by ear. My first order of business was testing air resonances.

View attachment 7869124

This shows me doing a bunch of clapping (generating an impulse) and seeing what the resonant frequencies of the cabin are. There might be a bit of a resonance at ~85Hz, but nothing pronounced. So, cabin acoustics are not the main culprit. Next up was investigating structural resonances. I won't bore you with the graphs for the dozens of tests I did, but I analyzed the spectrum of tapping every panel in the inside of the vehicle, searching for the resonance source. Roofs seemed suspicious, but were too high frequency, doors were fine, heat shields were a non-issue, etc. Most tests looked like this one (of the driver's left roof panel), or less interesting. (The vertical lines are individual taps, if you were wondering.)

View attachment 7869125

Looking at all the data I collected, I found only one culprit:
View attachment 7869127

This is the tailgate. It is large, flat, and vibrates strongly at ~45-50 hz. It's basically a giant speaker, and rings at the same exact frequency of the drone we see in the vehicle. The tailgate is causing the resonance. You can experience this yourself by locking yourself in the trunk area and banging the middle of the door. You will hear a pronounced drum effect ringing in your ears. You can also have someone else do it as you sit in the drivers seat--same thing.

Which leaves one last mystery: Why 65 mph? The explanation must involve something on the car that tends to generate 48Hz vibration at that speed (which then gets amplified). Again, long story short, I calculated all of the natural frequencies for the drivetrain at 65. Tires, engine rotation, exhaust, etc., none were even close to the right frequency. Except the driveshafts. Using the axle ratio and Bridgestone's data on KO2 revs/mile, the driveshafts (and associated gears) work out to generate a 48.6 Hz rotation frequency at 65 mph. Yup the same exact frequency.

And that's not the only piece of evidence... If you look closely you can see a "pulsating" effect every few seconds at 65mph that you don't see at lower speeds... (And, just for fun, yes, this graph was taken with the car in 7th gear at 65mph, showing that doesn't change much of anything.
View attachment 7869129

Without going into math, this pulsating is highly indicative of two vibration sources that are vibrating at very close to the same frequency. And to further implicate the already obviously guilty driveshafts, there are indeed two of them... Front and rear.

So, to sum up: The Ineos has a highly resonant rear tailgate that rings like a drum. It resonates all the time, but is most noticeable when going at ~65mph when the driveshaft vibrations stimulate the resonance.

So, the next step is figuring out how to reduce the problem. I worked on that a bit, but it seems like it will be tricky. Ideally, Ineos would have built a tuned mass damper into the tailgate to mitigate this. (Porsche did this with early Caymans which suffered a similar issue.) Anyway, at least we have reasonably high confidence about what is going on. So, no, bending your heat shields will not do anything about this, and no, sadly 7th gear at 65 won't either. Driving around with the tailgate open will (It's possible that applying a bunch of constrained layer damping material on the tailgate could help, but it's hard to say as it's a low frequency we're dealing with.)

I'll let you know if I figure anything else out. - Dave

Click to expand...

zvakanaka said:

I posted something about this previously, a good friend who worked for Honda 22 years is convinced it harmonic distortion as exhaust gas flows through the exhaust system. Other manufacturers have this issues also and is resolved by adding magnets along the exhaust until resonance disappears then welding a suitable steel plate in that area. I cannot verify but certainly plausible diagnosis and solution.

Click to expand...

Dave Rosenthal said:

This drone continues to drive me a bit crazy, as I just feel like I'm driving around inside a drum in my new Grenadier. I've owned a good number of cars, but never one with this issue.

So, I've done a bunch of investigating for us. I have a background in acoustics and engineering, so bear with me, this will get a bit technical. TDLR: I have good news and I have bad news: The good news is that I think I know what the problem is (not what anyone thought!), the bad news is I don't have a good way to fix it yet. OK, let's get into it...

First, for the doubters, this is a spectrogram of my Grenadier driving down a road at 65 mph and showing a very strong drone. The Y axis is frequency in Hz and the X axis is time (time is not super important, we're usually looking at like 30 seconds of driving):
View attachment 7869122

Yes, that bright yellow line is a pronounced resonance at ~48Hz. A low bass drone.

Also, this is not only a 65mph thing (though it is worst there for reasons I will explain later). Here is a Grenadier driving down a road at ~25 mph:
View attachment 7869123

As you can see, the same fundamental resonance at ~48 Hz is there even at low speed.

Well, the roads don't have lots of 48Hz noise by themselves--so something in the Grenadier is amplifying this frequency. (I verified this by analyzing interior frequency spectrums produced by banging the frame and suspension in various places with a "impulse hammer", i.e. rubber mallet, and seeing the same effect -- yes this is how they do things in industry.)

So, I set out to figure out what could be causing this 48hz resonance. This is tricky business because sounds this low are basically impossible to localize by ear. My first order of business was testing air resonances.

View attachment 7869124

This shows me doing a bunch of clapping (generating an impulse) and seeing what the resonant frequencies of the cabin are. There might be a bit of a resonance at ~85Hz, but nothing pronounced. So, cabin acoustics are not the main culprit. Next up was investigating structural resonances. I won't bore you with the graphs for the dozens of tests I did, but I analyzed the spectrum of tapping every panel in the inside of the vehicle, searching for the resonance source. Roofs seemed suspicious, but were too high frequency, doors were fine, heat shields were a non-issue, etc. Most tests looked like this one (of the driver's left roof panel), or less interesting. (The vertical lines are individual taps, if you were wondering.)

View attachment 7869125

Looking at all the data I collected, I found only one culprit:
View attachment 7869127

This is the tailgate. It is large, flat, and vibrates strongly at ~45-50 hz. It's basically a giant speaker, and rings at the same exact frequency of the drone we see in the vehicle. The tailgate is causing the resonance. You can experience this yourself by locking yourself in the trunk area and banging the middle of the door. You will hear a pronounced drum effect ringing in your ears. You can also have someone else do it as you sit in the drivers seat--same thing.

Which leaves one last mystery: Why 65 mph? The explanation must involve something on the car that tends to generate 48Hz vibration at that speed (which then gets amplified). Again, long story short, I calculated all of the natural frequencies for the drivetrain at 65. Tires, engine rotation, exhaust, etc., none were even close to the right frequency. Except the driveshafts. Using the axle ratio and Bridgestone's data on KO2 revs/mile, the driveshafts (and associated gears) work out to generate a 48.6 Hz rotation frequency at 65 mph. Yup the same exact frequency.

And that's not the only piece of evidence... If you look closely you can see a "pulsating" effect every few seconds at 65mph that you don't see at lower speeds... (And, just for fun, yes, this graph was taken with the car in 7th gear at 65mph, showing that doesn't change much of anything.
View attachment 7869129

Without going into math, this pulsating is highly indicative of two vibration sources that are vibrating at very close to the same frequency. And to further implicate the already obviously guilty driveshafts, there are indeed two of them... Front and rear.

So, to sum up: The Ineos has a highly resonant rear tailgate that rings like a drum. It resonates all the time, but is most noticeable when going at ~65mph when the driveshaft vibrations stimulate the resonance.

So, the next step is figuring out how to reduce the problem. I worked on that a bit, but it seems like it will be tricky. Ideally, Ineos would have built a tuned mass damper into the tailgate to mitigate this. (Porsche did this with early Caymans which suffered a similar issue.) Anyway, at least we have reasonably high confidence about what is going on. So, no, bending your heat shields will not do anything about this, and no, sadly 7th gear at 65 won't either. Driving around with the tailgate open will (It's possible that applying a bunch of constrained layer damping material on the tailgate could help, but it's hard to say as it's a low frequency we're dealing with.)

I'll let you know if I figure anything else out. - Dave

Click to expand...

Dave Rosenthal said:

This drone continues to drive me a bit crazy, as I just feel like I'm driving around inside a drum in my new Grenadier. I've owned a good number of cars, but never one with this issue.

So, I've done a bunch of investigating for us. I have a background in acoustics and engineering, so bear with me, this will get a bit technical. TDLR: I have good news and I have bad news: The good news is that I think I know what the problem is (not what anyone thought!), the bad news is I don't have a good way to fix it yet. OK, let's get into it...

First, for the doubters, this is a spectrogram of my Grenadier driving down a road at 65 mph and showing a very strong drone. The Y axis is frequency in Hz and the X axis is time (time is not super important, we're usually looking at like 30 seconds of driving):
View attachment 7869122

Yes, that bright yellow line is a pronounced resonance at ~48Hz. A low bass drone.

Also, this is not only a 65mph thing (though it is worst there for reasons I will explain later). Here is a Grenadier driving down a road at ~25 mph:
View attachment 7869123

As you can see, the same fundamental resonance at ~48 Hz is there even at low speed.

Well, the roads don't have lots of 48Hz noise by themselves--so something in the Grenadier is amplifying this frequency. (I verified this by analyzing interior frequency spectrums produced by banging the frame and suspension in various places with a "impulse hammer", i.e. rubber mallet, and seeing the same effect -- yes this is how they do things in industry.)

So, I set out to figure out what could be causing this 48hz resonance. This is tricky business because sounds this low are basically impossible to localize by ear. My first order of business was testing air resonances.

View attachment 7869124

This shows me doing a bunch of clapping (generating an impulse) and seeing what the resonant frequencies of the cabin are. There might be a bit of a resonance at ~85Hz, but nothing pronounced. So, cabin acoustics are not the main culprit. Next up was investigating structural resonances. I won't bore you with the graphs for the dozens of tests I did, but I analyzed the spectrum of tapping every panel in the inside of the vehicle, searching for the resonance source. Roofs seemed suspicious, but were too high frequency, doors were fine, heat shields were a non-issue, etc. Most tests looked like this one (of the driver's left roof panel), or less interesting. (The vertical lines are individual taps, if you were wondering.)

View attachment 7869125

Looking at all the data I collected, I found only one culprit:
View attachment 7869127

This is the tailgate. It is large, flat, and vibrates strongly at ~45-50 hz. It's basically a giant speaker, and rings at the same exact frequency of the drone we see in the vehicle. The tailgate is causing the resonance. You can experience this yourself by locking yourself in the trunk area and banging the middle of the door. You will hear a pronounced drum effect ringing in your ears. You can also have someone else do it as you sit in the drivers seat--same thing.

Which leaves one last mystery: Why 65 mph? The explanation must involve something on the car that tends to generate 48Hz vibration at that speed (which then gets amplified). Again, long story short, I calculated all of the natural frequencies for the drivetrain at 65. Tires, engine rotation, exhaust, etc., none were even close to the right frequency. Except the driveshafts. Using the axle ratio and Bridgestone's data on KO2 revs/mile, the driveshafts (and associated gears) work out to generate a 48.6 Hz rotation frequency at 65 mph. Yup the same exact frequency.

And that's not the only piece of evidence... If you look closely you can see a "pulsating" effect every few seconds at 65mph that you don't see at lower speeds... (And, just for fun, yes, this graph was taken with the car in 7th gear at 65mph, showing that doesn't change much of anything.
View attachment 7869129

Without going into math, this pulsating is highly indicative of two vibration sources that are vibrating at very close to the same frequency. And to further implicate the already obviously guilty driveshafts, there are indeed two of them... Front and rear.

So, to sum up: The Ineos has a highly resonant rear tailgate that rings like a drum. It resonates all the time, but is most noticeable when going at ~65mph when the driveshaft vibrations stimulate the resonance.

So, the next step is figuring out how to reduce the problem. I worked on that a bit, but it seems like it will be tricky. Ideally, Ineos would have built a tuned mass damper into the tailgate to mitigate this. (Porsche did this with early Caymans which suffered a similar issue.) Anyway, at least we have reasonably high confidence about what is going on. So, no, bending your heat shields will not do anything about this, and no, sadly 7th gear at 65 won't either. Driving around with the tailgate open will (It's possible that applying a bunch of constrained layer damping material on the tailgate could help, but it's hard to say as it's a low frequency we're dealing with.)

I'll let you know if I figure anything else out. - Dave

Click to expand...

Stu_Barnes said:

And the mvp award goes to….. @Dave Rosenthal

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Catpaw4x4 said:

Yep same "zone" here - just at the repair at RDS the other week. My independent mechanic spotted the issue at ~5500 miles. Thank you!

Click to expand...

Jean Mercier said:

Mvp? ;-)

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Jean Mercier said:

Mvp? ;-)

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Stu_Barnes said:

I think for us it can be most valuable post.
One for the list @Jean Mercier

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Mike Morrison said:

Wow - this is what forums are all about!
If one piled a whole lot of gear against the back doors would the resonance be diminished?
If there is a vibration between the inner and outer skin of the doors leading to the resonance could one not just pump the gap between with something like a spray foam?

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Eric said:

Most valuable player, it is a US acronym, not used in UK and EU I believe.

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Andiamo said:

Funny enough I think the Grenadier is the same or a little quieter on the highway than my new Defender was (with the expedition roof rack and KO2 tires). I’ve found that I gauge my speed more by sound than the speedometer and have noticed that I’m on average 5-8ish faster in the Grenadier so I’ve been making a conscious effort to watch my speed around town. Have around 800 miles on our Grenadier now.

Click to expand...

Jean Mercier said:

There are 0 countries in the European Union where English is the official language, therefore it is indeed not used

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