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)
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 belowpost49441.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