NVH (Noise Vibration & Harmonics Tests)
VW Golf/Rabbit

Note: The unit of measure used throughout this case study is mg. This refers to 1 thousandth of a g, where g is the gravity of Earth and refers to the acceleration that the Earth imparts to objects on or near its surface.

It is no secret by now that amongst the numerous accessories to accompany PicoScope, the NVH kit (Noise Vibration Harshness) introduces a whole new dimension to diagnosis in which (as technicians) we are only scratching the surface.

When I read through our forums, it never ceases to amaze me how scope users have adapted the scope for their own measurement needs using custom probes and maths channels etc.

The possibilities are endless and now with a means to measure and more importantly, identify noise/vibration, we have a whole new area of study to embrace and techniques to apply.

A recent question raised on our automotive forum asked “What can I diagnose with the NVH kit?”

See https://www.picoauto.com/support/topic10137.html

I guess the easy answer would be, “Anything that rotates, shakes, vibrates, or makes a noise” which pretty much covers everything mechanical/automotive!

The reality is slightly different given we need to apply the theories before we dive in and condemn components with real conviction.

The forum link above refers to the beginning of this case study:

A MK5 VW Golf 2.0 TDI with a distinct vibration throughout the cabin at a very specific road speed range of 67- 73 mph, presented a real nuisance at cruising speed. This problem resulted in the customer driving at 65 or 75 mph to avoid the vibration.

The vibration was not evident through the steering wheel and the customer confirmed all 4 wheels and tyres had been balanced.

With any form of diagnosis, confirmation of the symptom with the customer is critical and never before has this been more essential than when investigating NVH. I mention this now because our perceptions of noise and vibration differ from one individual to the next.

What might appear a nuisance to the customer may in fact be a vehicle characteristic based on the experience of the technician. Until now, proving this fact to a customer was dependent on the rapport and trust of the technician with the customer.

Now, with Pico’s NVH kit we can take a physical measurement of the vibration level and present the customer with factual evidence from their vehicle or even from a donor vehicle (when a comparison is required). This enables you to explain whether the vibration level is a characteristic of that vehicle or requires a component to be replaced or repaired, identified from the frequency of the vibrations detected.

So how does the NVH kit detect and identify the numerous vibrations present in a vehicle?

To help answer this, the NVH kit was installed to the aforementioned VW Golf.

The NVH kit contains a magnetic base accelerometer that is generally mounted onto the driver’s seat rail mounting bolt. This is due to the mounting bolts direct location to the chassis and the area of reported vibration by the driver. This location provides an initial bench mark for vibration measurements (the NVH kit also includes a microphone for measurements of audible noise).

The accelerometer is then connected to the NVH interface box which in turn is connected to the PicoScope via a BNC lead to Channel B. Any vibration detected by the accelerometer is then sent to Channel B via the interface.

The final connection is the generic scan tool USB to OBD lead (not supplied with the NVH kit) connecting the PC to the vehicle OBD system. The scan tool lead provides the NVH software with engine speed and road speed data (if available) enabling the software to calculate the rotational speeds (frequencies) of numerous components.

N.B The NVH software will allow other forms of engine speed detection to be used if a scan tool or live data is not available.

With all connections made, the NVH software is launched and the all-important Setup procedure followed. At this point I cannot stress how important it is to enter as much valid information as possible during the set up process, as all calculations made by the software regarding vibration frequencies are dependent upon accurate vehicle specifications.

The NVH software uses the engine speed and entered Vehicle Information (gear/drive ratios) to calculate the rotational speeds of the drive train. As technicians we always refer to rotational speed in revolutions per minute (RPM), however RPM can also be expressed as revolutions per second if we divide any RPM figure by sixty. E.g. 3000 RPM / 60, gives 50 revolutions per second. We know 1 Hz is one cycle per second, so we can express 50 revolutions per second as 50 Hz.

Our engine is therefore running at 3000 RPM or 50 Hz, the speed and frequency of a component are one and the same.

An engine rotating at a speed of 3000 RPM is rotating at a frequency of 50 Hz (50 rotations per second) and this formula goes for any rotating component on our test vehicle.

PicoScope NVH Analyser Setup Wizard
Setup Tab

Below is a typical Setup screen confirming the engine speed detection method, accelerometer type, location and function.

Vehicle Information Tab

The Vehicle Information Wizard allows for vital drive line information to be entered including, engine configuration with number of cylinders, transmission style, gear count and ratios (optional if road speed is available from OBD II), drivetrain arrangement, final drive ratio, tyre size, and finally accelerometer type, and location.

The image below displays a pictogram containing a summary of key information entered into the Vehicle Information Wizard.

Whilst I have mentioned above that entering gear ratios are optional I would always recommend when looking for road speed related vibrations that as much information is entered into the software as possible. Entering the gear ratios allows the software to calculate the gear selected and will automatically display and announce the gear shift during acceleration/deceleration allowing for easy auto capture of data with a clear reference of the gear selected at any given time during the road test.

Record and Analyse Tab

Once all the data had been entered a road-test could be carried out at 70 mph to Record and Analyse the data obtained.

At this point it is well worth discussing how the NVH software can calculate driveline component frequencies and identifies specific vibration levels based upon the engine and road speed data provided via the OBD II connection and the entered Vehicle information (this will most certainly help with interpretation of the up and coming results).

Example of NVH software frequency calculation based upon the following vehicle information:

• Engine at 50 Hz
• Transmission 3rd gear ratio 2:1
• Final Drive ratio 4:1

With engine at 50 Hz into transmission, the engine, flywheel, clutch and transmission input shaft rotate at 50 Hz and any vibrations occurring at 50 Hz are related to these components.

Input into transmission at 50 Hz / 2:1 (3rd gear ratio) = 25 Hz. Therefore transmission output shaft and propshaft rotate at 25 Hz and any vibrations occurring at 25 Hz are related to these components.

Propshaft at 25 Hz / 4:1 (differential ratio) = 6.25 Hz. Therefore differential crown wheel, drive shafts and road wheels rotate at 6.25 Hz and any vibrations occurring at 6.25 Hz are related to these components. 

The software can identify these key frequencies and provide the user with identification abbreviations to assist with diagnosis. E.g. E1 First order engine vibration T1 First order tyre speed related vibration.

Results below were obtained from the VW Golf with vibration evident throughout the cabin at 70 Mph.

PD data for the VW Golf in the fault condition (Click to download PS Data)

Looking at the screenshot above we were able to clearly measure a vibration level of 21.4 mg from the accelerometer attached to the seat rail at 69 mph (speed reported by the customer). The NVH software is able to process the numerous vibrations detected by the accelerometer and display them in order of their frequencies, so identifying specific components based on engine speed and the Vehicle Information entered. In the case above a T1 (First order tyre speed related vibration) is responsible for the complaint.

So how did the software identify T1?

Using the vehicle information the formula follows the path below to identify T1:

T1 vibration level is excessive compared to all other vibrations (21.4 mg at 15.68 Hz)
Average engine speed: 2000 rpm / 60 = 33 Hz (approx.)
33 Hz into 6th gear (ratio 0.638:1) 33 Hz / 0.638 = 51 Hz (approx.)
Transmission output shaft 51 Hz into differential (ratio 3.238:1) 51 Hz / 3.238 = 15.75 Hz - the approximate frequency of our wheels/tyres.

What is invaluable at this point is not what is causing the vibration but more “What is not?” How many times have we balanced four wheels/tyres to find the vibration is still evident? In this scenario without a vibration analysis solution we would have to assume the vibration level is a result of propshaft or engine mounts and had a tough decision to make as to what component to replace or balance with no supporting evidence.

So, now time to recap on the original symptom and evaluate the test results.
We know the vibration is evident through the cabin and not through the steering at 70 mph (hence this is not a front tyre issue).

Our test results confirm a vibration level of 21.4 mg at 15.68 Hz and this aligns perfectly with T1 First order tyre speed related vibration.

So now we know that whatever is the cause of vibration, it is not engine or transmission up to the crown wheel of the differential. This information in itself has saved hours of turmoil and guess work.

We therefore know that whatever is rotating at a frequency of approx. 15.68 Hz is the cause of our vibration. This includes wheels, tyres, drive shafts, and differential/crown wheel.

Keeping it very simple, a close inspection of the tyre run-out revealed an ovality on the inner edge of the OSR tyre. See video below:

Whilst the tyre wear pattern in the video suggested excessive negative camber, further inspection confirmed a broken coil spring which of course has the side effect of lowering the suspension and increasing negative camber. A combination of a continually loaded boot space, a broken coil spring and daily motorway journeys of over 100 miles have taken their toll not only on the suspension but the structure of the tyre resulting in the ovality generating a flat spot about the tyre circumference.

A new OSR tyre was therefore installed, along with two rear coil springs.

T1 is described as a First order tyre speed related vibration and so therefore it goes without saying T2 would be a Second order tyre speed related vibration, generating two vibrations per revolution of the wheel.

This information will prove invaluable when looking at various vibration conditions that highlight excessive T2 levels allowing the technician to look for specific tyre characteristics or shape. Remember T1 and T2 refer to tyre speed related vibrations which include not only the tyre but components rotating at tyre speeds such as driveshafts and differential/crown wheel.

A road-test was carried out after the fix matching the results obtained from the initial capture and the conditions mentioned by the customer. See below:

PD data for the VW Golf (after fix)

From the image above we can see how the vibration level has fallen by 76% as a result of installing the new tyre and coil springs. This presents undeniable evidence to customers of before and after fix test results that demonstrate the efficiency of your workshop, best practice followed, the professionalism of the technicians involved and not only confirmation of a repair well done, but a historical vibration overview of the vehicle condition at the time of completion. This covers not just the complaints reported by the customer but all components across the frequency spectrum (engine/driveline components and unknown vibrations that may require further attention with customer permission).

The ability to rapidly analyse and identify vibration levels presents opportunities to indicate to customers potential areas of concern that require attention or monitoring. Dual mass flywheels spring to mind given the potential for failure depending on driving conditions. Routine maintenance work could also incorporate an NVH overview/assessment that can be presented to customers on completion of service work with the results based upon a data base built by the garage in question and their product knowledge of the vehicles involved.

This completes the case study of the vehicle above along with a general overview of the NVH software used to diagnose the customer’s complaint. I would like however to carry on a little here to mention some additional features and characteristics of the NVH software.

Additional information

Like all PicoScope software, NVH is free to download and run in demo mode (only the Beta version at present has the NVH software). Once installed you can open the included NVH files within this case study and look at the various options in which we can display vibration levels, see below.

The three images below are all from the VW Golf case study, displaying identical results in various formats

The vertical (order) markers in the image below indicate the vibration orders or harmonics associated with any frequency identified by the user. Clicking on, and dragging the order markers to a point of interest in the Frequency graph will allow the technician to identify additional vibrations relative to the source of the fundamental vibration.

Remember how we mentioned T1 and T2 are 1st and 2nd order vibrations of a tyre speed related component. A tyre with one flat spot (T1) generates one disturbance every single revolution of the wheel whereas a tyre with two opposing flat spots (T2) generates two disturbances every single revolution of the wheel at twice the frequency of T1 (the fundamental frequency)

E.g. 
A tyre rotating at 10 Hz with a single flat spot (T1) appears on the frequency chart at 10 Hz.
A tyre rotating at 10 Hz with two flat spots (T2) appears on the frequency chart at 20 Hz.
T2 is a harmonic (second order) vibration of T1.

The same can be said for a 4 cylinder four stroke engine rotating at 50 Hz. 
E1 indicates the vibration level relative to the engine speed and any single imbalance within the engine (likened to T1 single flat spot of a tyre) 
E1 is displayed on the frequency chart at 50 Hz.

E1 will normally be much lower than E2 given the engine produces two firing strokes per revolution two x imbalances.
E2 is displayed on the frequency chart at 100 Hz as this is a 2nd order vibration (harmonic) of E1.

The NVH software also has help files on hand ready to read and digest at your leisure

There are two NVH help files included with the software!
The first help file is available by opening the PicoDiagnostics program and clicking on Help before selecting any tests. Click on Help, then Contents, then select Tests and click on NVH. Here the help file will guide you through the setup of the tool and how to interpret your readings.

Possibly the most valuable help file that contains all the theory of NVH is available by selecting the NVH test when opening PicoDiagnostics, cancel the PicoScope NVH Analyser Setup Wizard, then click on Helpagain. 
Here you will now find Show vibrate help which will then take you through the principles of NVH and accompanying theories.

Many thanks for taking the time out to read this case study/tutorial as I am sure it has been hard going at times. As I mentioned at the start of this article we are only just beginning to scratch the surface of what the NVH tool/software can reveal and the techniques we can now apply in a non-intrusive fashion to provide accurate and concise diagnosis for our customers.
I look forward to the knowledge to be shared on the forum as the kit becomes integrated into our diagnostic procedures.

For any further information, please contact [email protected]