Using Mic to record the noise of automatic transmission

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Using Mic to record the noise of automatic transmission

Postby mehroz » Sat Jul 02, 2016 5:38 am

I am new user of NVH diagnostic kit, I want to know how can i use/connect mic to record the automatic transmission noise while driving the vehicle.

please let me know where and how should I place the mic in the engine compartment to record the automatic transmission noise.

Following are the equipment details;

Picoscope 4425
NVH diagnostic kit

please provide support at the earliest.........
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Re: Using Mic to record the noise of automatic transmission

Postby Mark Dalton » Mon Jul 04, 2016 4:13 am

The Mic.is not really the tool to use to measure noise from a transmission, especially if it's a whine or similar high pitched noise. The Mic is going to pick up all sorts of noise that will drown out your noise (the mic will not give you an audible record of the noise). The NVH kit is only going to help you if the noise is cyclic / periodic, like a broken gear tooth. Then the NVH kit will tell you how many times a second the noise occurs, but that's only any good to you as long as you've worked out all the ratios inside the box. I think the Mic is primarily for issues like belt squeaks where you can tie the how frequent the squeak occurs to the engine RPM and work out which pulley is causing the squeak.
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Re: Using Mic to record the noise of automatic transmission

Postby mehroz » Fri Jul 15, 2016 10:48 am

Thank you mark for your reply......

Other than mic then what is the best way to capture the automatic transmission noise........
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Re: Using Mic to record the noise of automatic transmission

Postby Steve Smith » Tue Jul 19, 2016 3:28 pm

Hello and thank you both or the posts.

This is an interesting topic as my own vehicle has a whine from the Auto transmission in reverse only.

I have been meaning to apply both the Mic and the accelerometer in order to identify the frequency.

Mark has a valid point about the Mic picking up all forms of noise and possibly "drowning" the noise/whine of interest but this all depends upon the Mic location and specification.

The noise on my vehicle is very faint inside the cabin but would be audible via a contact Mic attached to the transmission case.

Contact Mic's may well be in the pipeline (said too much already) but this does not out rule the existing NVH mic.

With a carefully positioned NVH Mic and vehicle operated in the relevant condition/environment to capture the noise, we can apply filtering during playback to remove the ambient noise Mark has mentioned.

Once we have the noise captured for playback we can home in on the frequency of interest.

The noise will no doubt be fundamentally linked to rotating components within the transmission, the art is identifying what components are rotating at the given time.

The gear ratio in which the whine is present is vital, as is the power flow from the torque converter (Input) through the transmission (output) as numerous components will rotate in a planetary gear set to obtain the ratio of concern/noise

I will have a go at my vehicle using both the accelerometer and the NVH Mic and post the findings here. (Not sure I want to know what is causing the whine!)

I will also have an application note very soon to post regarding NVH/Mic filtering techniques.

I hope this helps, take care......Steve
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Re: Using Mic to record the noise of automatic transmission

Postby Mark Dalton » Wed Jul 20, 2016 7:13 am

Would be very interested to see the article on filtering.
It would be good to see more posts from people who have used the Mic for diagnosis and how they used the data from the Mic to identify the root cause of the issue. it would be interesting to see just how far you could take it.
Apart from the issues with unwanted noise that your Mic is going to pick up as well as the noise your trying to diagnose. The NVH tool is not normally something I'd bother to use for these kinds of noises, because...
-If the noise is really internal to the Transmission, Gearbox, Transfer Case etc. it seems there's no way to repair the issue without removing the component anyway.
- If the noise is in one particular gear eg. reverse, then that already narrows down the possibilities.
- If the noise is cyclic, but to rapid for the human ear to pick up and sounds like a constant whine. if the NVH can show you the occurrence frequency (as apposed to the pitch/tone of the sound), I think you still need more info than you can normally get your hands on about the internals of a transmission to be able to determine the component at fault.

But I'm sure there's a lot of science out there that may allow us to work this kind of stuff out and take the diagnosis much further than I can fathom out.
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Re: Using Mic to record the noise of automatic transmission

Postby Steve Smith » Thu Jul 21, 2016 11:48 am

Hello and thank you for the continued posts and interest.
I have to agree with you Mark from a technician point of view.

When we have noise such as whines etc. we know from experience if it whines only in reverse gear that is our target upon disassembly. Often disassembly may not be economically viable and a replacement transmission is called for.

However, should disassembly be necessary the fact we have captured data prior to fix, we know conclusively the set of components to replace or adjustments to make to reduce the noise level beforehand, as with all diagnosis, once we commit to a repair we cannot go back in time.
The diagnosis must be actioned before overhaul, especially with Auto Transmissions.

Once the repair has been actioned the noise level can be measured again to qualify the repair where we hope and pray it has improved!

This brings me onto characteristic noise as the Mic is incredibly useful in objectively reassuring customers their vehicle is just like others of the same specification. Once we attempt to rectify characteristic noise we are in for a whole heap of pain!

My line of thinking here is the scenario I have with my vehicle where the transmission whine is most certainly characteristic in reverse and not detrimental to serviceability

Using the Mic we can measure the amplitude against other vehicles, present our customer with the evidence and explain why the noise occurs at the specific frequency they can hear. (Internal knowledge of the power flow through the transmission gear train will be required)

Once again Mark I agree with your concerns surrounding Mic use and unwanted noise.
Positioning of the NVH Mic is paramount and the operational environment of the vehicle must be considered for background noise. My first attempt at capturing my transmission whine with the Mic was unsuccessful when positioned at the firewall in the engine bay. (I could hear the recorded whine but could not conclusively identity the frequency of concern)

Repositioning the Mic at the underside of the vehicle aimed at the transmission sump pan has proved far more successful. (See video below of my transmission whine)

NVH FUTURE BETA TRANSMISSION WHINE.mp4
Transmission whine in reverse
(4.43 MiB) Downloaded 15 times


I have not posted the Pddata file as the software will not run on the current beta PicoScope software as I have been using a test build to demonstrate the filter techniques. (As soon as the new beta is released I will post the file here)

Notice how both the accelerometer and the Mic capture the whine and how the frequency changes in relation to road speed (transmission output speed)

If anyone has knowledge of the BMW/ZF 8 Speed transmission power flow in reverse that would help explain the frequency obtained (1200 Hz approx.) this would be most helpful. I will make some inquires here too and post anything I can find.

Another example of Mic use from one of our VM customers helped to locate a differential whine at very high road speeds from a high performance 4WD car.

A high amplitude “Unknown” frequency at 811 Hz was identified within the NVH software
Front Differential has a Crown wheel with 37 teeth and Pinion with 14 teeth
Rear Differential has a Crown Wheel with 41 teeth and Pinion with 17 teeth
Tyre frequency calculated at 21. 8 Hz “Unknown” frequency at 811 Hz
21.8 Hz x 37 Number of teeth of front crown wheel = 806.6Hz
21.8 Hz x 41 (Number of teeth of rear crown wheel) = 893.9 Hz
Front differential at fault- Tooth contact and pre-load issue

The differentials on this vehicle were not cheap and being able to measure before replacement correctly identified the relevant differential preventing unnecessary expense and I am sure tears for the warranty team

To conclude, using the Mic is challenging but certainly essential for complaints of noise:

Position the Mic relative to the noise

Be aware of the operational environment about the mic and shield where necessary

Be aware of the operational condition of the vehicle in which to create the noise. (Use this to your advantage)

The new filtering techniques available very shortly in beta form will certainly open up the use of the existing NVH Mic see the link below
post49441.html#p49441

Future microphone developments will add to the existing Mic applications.

There is a nice example of Mic use here at
post33991.html?sid=93562130d5f7f01f31fcc70fc16dc768#p33991

I hope this helps, take care…….Steve
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