Dash Cam to NVH

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Steve Smith
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Joined: Sun Aug 25, 2013 7:22 am

Dash Cam to NVH

Post by Steve Smith »

Dash Cam to NVH

Recent testing of a BMW 320d for a vibration/noise highlighted the benefits of a Dash Cam given a microphone is incorporated into the camera.

Before I move on, at this stage I do not have a fix for this vehicle and cannot prove if the complaint is a characteristic of a number of “colliding” factors (which is my gut feeling at present)

Long story short again, the complaint occurs on a particular stretch of road (30-40 MPH) where a momentary vibration can be felt in the floor pan, accompanied with a booming noise inside the cabin. The concern appears to originate from the front axle, but of course this can be deceiving!

The fact the complaint can be felt and heard places the offending frequency between 20 & 200 Hz
Image 1
Image 1
The image below describes the set up used to determine the source of the noise/vibration. Here we utilize 4 x accelerometers positioned at strategic points about the front wheels. The markers in the signal history denote where the monetary noise / vibration occurs. The accelerometer connected to channel C (attached to RH Front upper caliper bolt) clearly indicates the highest amplitude at 111 Hz. Note also that the other accelerometers detect this offending frequency but at a far reduced intensity.
Image 2
Image 2
The momentary sporadic activity you can see in the black rectangle can be largely ignored at this stage based on the fact when the offending frequency of 111 Hz appears in the spectrum, I can hear/feel the noise in the cabin. Given the accelerometers (channel A & C) are attached to the brake caliper, they are subjected to multiple vibrations without damping (unlike channels B & D) that are connected to the brake pipes on the chassis. Imagine accelerometers A & C being dragged along the road surface; think of the multiple vibrations and impacts they would be subjected too and how this would look in the spectrum. (very much like what we see in the black rectangle above)

So why mount the accelerometers in such a fashion as to subject them to impacts that result in sporadic distortion? The answer is to use them as listening devices as we can export the accelerometer data from the NVH software and listen to the results!

The following link will help here
(Export Selected Region of the signal history to .wav)
See “Export Options” of the above forum post

Please listen to all 4 accelerometer recordings below and see if you agree with the data above that channel C appears to demonstrate the noise of concern better than the remaining accelerometers. Please use “full ear, noise cancelling” headphones if possible, in order to insulate your hearing from ambient noise. Concentrate on 5 to 15 seconds from the start of each .wav file as the “boom” is most prominent at this point in the recordings

Channel A
Channel B
Channel C
Channel D

At this stage we can confirm the customer complaint and hopefully agree the RH Front area of the vehicle to be the offending zone. (Ironically directly in front of the driver in the UK ((RHD Vehicle))

It was at this point I thought about how useful it would have been to use a single NVH mic in the cabin rather than 4 x accelerometers (Perhaps 3 accelerometers and one mic)
Then I realised the installed Dash Cam incorporates a microphone (if activated within the camera menu)

The following video contains the Dash Cam road-test captured within image 2 above where the cabin noise can be heard around 55 seconds to 1 minute from the start of the video. This gives an amazing insight into how the vehicle was driven, the road surface & terrain etc.

Dash Cam Footage

Using the technique described here

We can extract the audio from the Dash Cam footage (in .wav format) and import this recording into our NVH software. In theory, the offending frequency captured by the accelerometer at the RH Front caliper should match the frequency captured via the Dash Cam mic! This will reinforce the diagnosis and allow the user to confidently pursue this offending frequency (111 Hz)
Image 3
Image 3
Above we have the entire audio file from the 3-minute Dash Cam recording imported into NVH. When listening to the play back you can hear the lap top announcing the gear positions that you can see in the signal history of image 2 above (3rd, 4th ,5th etc.) This helps to synchronize the Dash Cam audio to the original NVH capture where we have confirmation that the booming sound captured by the camera mic matches our offending frequency of 111 Hz

If we now right click on the frequency view of the NVH file above, we can create a reference waveform of the Dash Cam audio and load this into our original NVH capture to confirm our suspicions
Image 4
Image 4
Once the reference waveform (duplicate) of the data above has been named & created, select “File” and “Load” the original NVH file. This will open and display the original capture (image 2 above) along with the Dash Cam audio reference waveform. N.B. You may have to right click again on the frequency view and select “Auto Scale” to correct the reference waveform scale

The results below confirm the Dash Cam mic and accelerometer have captured the identical offending frequency of 111 Hz and concludes a technique that could be applied to enable more than 4 inputs to be analyzed within the NVH software
Image 5
Image 5
Below I have aligned the original NVH capture with the Dash Cam footage that will help customers, trainers, engineers and support staff gain a through understanding of how the offending frequency manifests itself. As you can see, the vehicle is driven on a straight section of road at 30 mph under minimal load. Synchronizing the Dash Cam video to the NVH capture was achieved thanks to the NVH software announcing the gear position via the lap top speakers! This was captured by the Dash Cam mic so providing a synchronization signal.

Dash Cam with NVH

So, with all the above in place, what is causing the noise/vibration?

In a nutshell I don’t know at the moment! What I can say is the suspension, wheels and tires are all fine and I have also tested another set of wheels/ tires that unfortunately were an identical brand to the original fitment!

I say unfortunately because I would have liked to test a different brand tire with a completely different tread pattern on this very same road surface!

I have a gut feeling this may be a combination of road surface and tire construction/characteristics that compound to generate a momentary vibrating force. (The tires in question are Run Flat 19 inch 45 series Bridgestone Potenza “Star” rated)

I can reproduce this noise during every trip along this particular road and very occasionally on similar road surfaces in other locations!

Looking back at image 2 above, notice how the 17th harmonic rulers align perfectly with T1 @ 6.5 Hz? (6.52 Hz x 17 = 110.84 Hz) See the following link for more information on harmonic rulers: Click here

This could be renamed T17 @ 111 Hz approx. or 17 disturbances for one revolution of the tire. (Think of 17 high spots about the circumference of the tire)

For a description of harmonic and vibration orders the following link should help
Click here

Another theory is “Resonance” from a suspension component where the momentary interaction between this style of tire and road surface generates a sufficient disturbance (force) that matches the natural frequency of a suspension component, therefore resulting in a momentary nuisance resonance

More information on resonance and natural frequency can be found here

Coming back to the Dash Cam for a moment, those cameras that enable "AV Out" could potentially link direct to a PicoScope channel as we have proved here
My concern however would be the accuracy of the data in terms of both amplitude and frequency. (A project for another day)

It’s all food for thought and discussion at this stage for sure and I will feedback when I discover more.

I hope this helps, take care……..Steve

martinr
OneWave
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Re: Dash Cam to NVH

Post by martinr »

Plenty of food for thought, Steve. Nevertheless, it’s still quieter than a Rover 100 with its engine off and stationary.

Was there anything noticeably different about the road surface at that point? Was that “resonance” sensitive to speed, in that, if you hit the same patch of road say 5mph up or down, did it make any difference?

I presume you were using your 4425; any reason you didn’t opt for your 4823, or was it done deliberately to look at capitalising on the dash cam audio?

Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 1150
Joined: Sun Aug 25, 2013 7:22 am

Re: Dash Cam to NVH

Post by Steve Smith »

Hello and thank you for the feedback

The road surface is most certainly playing a key role in the generation of this nuisance vibration
First impression of the road surface; “it is nothing out of the ordinary”, I guess I should inspect closer.

Great point about the resonance at higher road speeds, what I have noticed is that it remains between 111 & 113 Hz regardless of road speed and I have tested at 30, 40 & 50 mph!

I really do need to return to this study as I would like to get to the bottom of how this vibration manifests itself and is transferred into the cabin

The 4425 was used purely because it was available, the 4823 could have been used but at present would only allow the use of 4 channels with NVH

I hope to follow this up in the future as I am sure others will have experienced something similar

Take care…….Steve

martinr
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Re: Dash Cam to NVH

Post by martinr »

what I have noticed is that it remains between 111 & 113 Hz regardless of road speed and I have tested at 30, 40 & 50 mph!”


Steve,

That’s really interesting. I must go back and listen with earphones this time, but, on the face of it, that suggests it’s not a true resonance, although I have in the past knowingly misused the term “resonance” because it summed up a noise so well.


Having said that, does the magnitude of the 111Hz peak change with speed?

Two things I wondered might be worth doing, Firstly, can you get access to a different car and set up everything the same and then drive over the same patch of road? I’ve no idea what to expect but I wonder if it might throw up a “that’s interesting “ moment. At least it might establish a base line. And if, by some miracle, you also had access to a car of the same model car, that too would be very interesting.

The other thought, if you can pinpoint on the road where the noise starts, there might be value in walking by the side of it, as long as it’s perfectly safe, to see if you notice anything and also to listen to the road noise happens when other vehicles pass that point. That might prompt a thought or two.

martinr
OneWave
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Posts: 45
Joined: Tue Mar 06, 2018 4:06 pm

Re: Dash Cam to NVH

Post by martinr »

Steve,

Looking at the footage again and listening through earphones, I take back what I said above: I don’t see the point of using a different car. (However, if you could get your hands on an identical model ....)

The road surface doesn’t look any different in the camera footage: no sign of different tarmac. What happens if you travel in the opposite direction?


Is there a way you can connect a signal generator to the car’s entertainment system and see (ie listen to) which part of the floor pan is resonating at 111Hz and if there’s a simple fix?

Resonance is definitely the perfect word, though, but suppose it’s not coming from the road surface? There isn’t some huge generator humming away behind that hedge is there? ;). Provided it’s safe to do so, is there a minimum speed below which there’s no resonance?

Jfixmycar
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Re: Dash Cam to NVH

Post by Jfixmycar »

Steve, I don't know how I missed this write up, but.... Awesome!

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