Diagnosis of quiet cabin noises

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

Diagnosis of quiet cabin noises

Post by Steve Smith »

NVH Mic use pros and cons

A question that comes up regularly via the Help desk and Auto forum with NVH is “Why can I hear the cabin noise with my ears but cannot hear the noise within the NVH software during playback”?

We have touched upon this topic several times with one such example here viewtopic.php?p=69031#p69031

I recently had the perfect opportunity to demonstrate this exact phenomena with my own vehicle that exhibits a cabin whine around 65 to 70 mph during light acceleration

The symptom is so pronounced (in my opinion) that when driving without monitoring the speedometer, when the whine appears and I glance down at the speedo, my road speed will always be between 65 to 70 mph

Based on experience and given the noise clears when cruising and during over-run, this sounds like a differential whine!

Interestingly enough when I ask passengers “can you hear that noise?” the reply is often “what noise?”

This brings me onto two very important items to note.

1. The cabin position of those complaining of the noise is of vital importance. I have been told by irate customers that “if you can’t hear the noise, you must be deaf!” When I think back, this may have been down to the fact that I was a front seat passenger whilst allowing the customer to drive the vehicle until the noise occurred (We are in different cabin locations)
Let us also bear-in-mind the age, hearing and lifestyle factor of the driver (we are all different)

2. We as humans have an inherent ability (a God given gift) to subconsciously filter out environmental noise and tune into (or focus upon) only the offending noise. This does take a little time of which the owner/driver/complainer has acquired over hours spent in the cabin. (something of which a technician fresh from the workshop cannot initially possess) Ask yourself, how many times you have had to get yourself “tuned to the fault” be that a noise, hesitation, misfire or vibration?

So how do we tackle the challenges above?

Using our NVH software we use Multiple Sensor Mode which will allow for the positioning of several microphones (4 Max) about the cabin/vehicle in order to capture and zone into the offending noise

Try not to laugh at the picture below where I have chosen to wear two NVH microphones (whilst driving) one of which points to the center of the cabin (Blue Channel A) the other to the OSF drop glass (Red Channel B) Note: This is a RHD, RWD vehicle (2016 BMW 535d GT)
In addition to the above microphones, I have placed another mic under the vehicle aimed at the rear differential and an accelerometer (configured as a microphone) to the rear differential oil level check plug
Below is our NVH Set-up screen prior to road test using Multiple sensor mode (4 x sensors).

Note: The correct configuration for each sensor is vital to a successful measurement and ultimately a diagnosis. Note how the two microphones I am wearing are configured for the “Passenger compartment” and their location clearly labeled in the “Notes” field

Likewise, those sensors under the vehicle are labeled and configured for “Engine compartment”

Consider “Passenger compartment” as inside the vehicle and “Engine compartment” as outside the vehicle
So why configure the accelerometer attached to the rear differential as a microphone? (Channel D)

This is a bit of a cheat and we have discussed the benefits here viewtopic.php?p=91701#p91701

The long and short of this technique is that it allows the user to listen to the data captured by the accelerometer within the NVH software without having to export the accelerometer data to .wav format and then listen in a 3rd party media player.

In effect, our accelerometer attached to the rear differential is acting like a contact microphone (listening device) immune from ambient/environmental noise, capturing structure-borne noise (unlike the mic which captures airborne noise)

Let us assume at this stage we are chasing a cabin whine with no preconception that it is linked to the differential. All we know is that the whine sounds like transmission/drivetrain noise!

Based on the above, add as much information as possible to the Vehicle information window and Advanced vehicle settings dialog box
Note above we have included the transmission ratios and rear differential as a minimum. This will help identify if our whine is linked to a specific gear position. Had this been an AWD vehicle, information around the front and center differentials would also be invaluable

A word to the wise here re the countless differential ratio options with today’s vehicles:

Several VM’s readily provide differential ratio information with ease; others provide tooth count (e.g., 48/18 where the differential ring gear has 48 teeth divided by a pinion of 18 teeth = 2.67 :1 ratio) Note: Tooth count is often revealed by “Parts” supply as opposed to workshop technical data

Finally, other options include a label attached to the differential (if you are lucky) or the NVH method of “calculation” if you suspect the vehicle under test has been modified

In order to calculate the differential ratio with NVH we require engine & road speed (E1 & T1) obtained via OBD and accurate knowledge of the transmission ratios.

Here is where I struck gold with the ZF 8HP transmission installed into the above BMW.

The following link provides generic information around this widely used transmission of which we have a 1st Generation 8HP 70 https://en.wikipedia.org/wiki/ZF_8HP_transmission (This link also includes Sun and Ring gear tooth count of the epicyclic gear sets involved in generating the ratio for each of the 8 forward gears; how cool is that!) See below
Returning to calculating the rear differential ratio, below we can see we have a road speed of 68 mph @ 1487 rpm in 8th gear and a first order rear tyre frequency (RT1) of 14.2 Hz
With our engine speed at 1487 rpm (Divide by 60 = 24.78 Hz) in 8th gear (0.667:1) ………

Engine frequency = Gearbox input shaft frequency 24.78 Hz / 0.667 = 37.15 Hz Output shaft/Propshaft frequency (“P1”)

Now divide the propshaft frequency by the tyre frequency to find the differential ratio

P1 = 37.15 Hz / T1 = 14.2 Hz = 2.62 :1 approx. Differential ratio

The label found on the differential indicates 2.65:1 suggesting a small error between the two values which we can attribute to deviations in OBD road speed against actual road speed and “Tire correction factor”

More on “Tire correction factor” can be found here viewtopic.php?p=36541#p36541

Moving onto NVH analysis of captured data

Where do you start when presented with information like we have below?
Pressing the playback button is likely to annoy you given the level of wind noise present on channel C (Green) which dominates the frequency view at an approx. peak of 102.2 dB (NVH will playback the recording from all sensors simultaneously)

Step 1. Drag the frequency axis to a maximum of approx. 2 kHz and hide channels C & D. We only wish to listen to our cabin whine (our customer complaint)

If you are unsure what to select as the maximum frequency of interest (2 kHz below) use the NVH function generator (viewtopic.php?p=49681#p49681 ) to generate a 2 kHz fixed frequency Sine wave, listen to the generated sound/pitch and compare to the whine you can hear with your ears in the cabin. If the generated sound is higher pitch, this is will confirm your offending whine will be captured somewhere in your spectrum of 0 – 2 kHz.

Should the generated sound be a lower pitch than the offending whine, you need to increase the frequency of interest
Step 2. Use the Auto scale function to improve resolution of the capture data

Step 3. Apply noise canceling (over-the-ear) headphones and listen carefully to the audio playback during periods of light acceleration from 65 to 70 mph. Whilst “playback” is active look for peaks appearing in the spectrum during the 65 to 70 mph window (See below where channel B reveals a peak of concern)
Note above the “suspect peak” (@ approx. 600 Hz, 48.3 dB) that appears during our cabin whine during light acceleration between 65 to 70 mph, is difficult to hear during playback given the peak cabin noise level captured by the cabin mics is 78.5 dB

Step 4. Use the zoom feature to focus upon the “suspect peak”; note how the mic at the drivers RH ear has captured a higher peak sound level than the mic at the drivers left ear! (8.1 dB difference) suggesting this “suspect peak” maybe more prominent or intruding into the cabin via the RH drop glass seal? This could be why passengers claim they cannot hear the noise (See below)
Note: A 3 dB reduction in sound pressure level is the typical value by which humans can perceive a detectable change in sound/volume (try this on your TV/Audio system if dB units are displayed when using the volume control) In addition, we perceive sounds to be twice as loud or quiet when they are roughly 10 dB apart not 3 dB. (our delta value of 8.1 dB is therefore very significant indeed)

Step 5. Use the NVH function generator to create a 609 Hz fixed frequency Sine wave, listen to the generated sound/pitch and compare to the whine you can hear with your ears in the cabin; are they similar in pitch? They will not be a 100% match however as our offending whine comprises of other frequencies to ultimately derive the offending sound we can hear. This technique however will potentially link the suspect peak to our offending noise (See below)
Step 6. Apply filtering as we describe here post49441.html#p49441 . Given channel B has captured a suspect peak that appears during light acceleration between 65 & 70 mph, hide channel A and use a Band pass filter to remove lower and higher frequencies either side of our suspect peak at 609 Hz (See below) This may or may not improve playback as the energy within the peak of interest is only just above the ambient noise level! In addition, applying the Band pass filter may have removed other frequencies which ultimately combine to derive the offending sound we can hear.
At this stage, what could cause this suspect frequency of 609 Hz (which increases in proportion to road speed) during our light acceleration window between 65 and 70 mph?

Possible causes:

• Transmission
• Bearing failure
• Differential
• Engine
• Engine auxiliaries
• AC / Climate control components

It is at this stage a combination of experience, product knowledge and drivetrain configuration come into play. The mere fact that drive, cruise and over-run can affect the noise level suggest this is a gear meshing noise and so AC, Bearing failure and Engine/Auxiliaries can be discarded for now.

Transmission is a possible concern as this seems to be an issue in 8th gear only and detectable inside the cabin by the driver who sits adjacent to the transmission/tunnel

We can put the transmission to bed quickly by calculating the gear mesh frequencies of the planetary gear set/sets responsible for 8th gear

With reference to the ZF Transmission workshop manual, 8th gear is achieved as follows:

Sun gear (48 teeth) 1 & 2 is held fixed

The input shaft is connected to the planet spider 2 (Carrier 2) whose planetary gears roll off and rotate about the fixed sun gear, so driving the ring gear 2 (96 teeth) at a higher speed.

The configuration of transmission clutches C & D ensures that planetary gear sets 3 and 4 rotate in block operation and serve only as a rigid transfer element. The gear ratio and therefore output shaft speed for 8th gear is generated in planetary gear set 2 only

Before we move on, there is a great video here https://www.youtube.com/watch?v=bWtK5mzuddo by Justin Miller looking at gear ratio calculations of planetary gears sets using a physical gear set

Given the Carrier is our Drive element and the Ring gear our driven element we need to derive the number of teeth of the planetary carrier as a whole? This is achieved by adding together the teeth of the Sun gear to the teeth of the Ring gear (48 + 96) = 144

The calculation is therefore as follows;

Ring gear 2 tooth count / (Sun gear 2 tooth count + Ring gear 2 tooth count)

96/ (48+96) = 96/144 = 0.667:1

With the above in mind, we can calculate the tooth contact frequencies of the entire planetary gear set using the formula

Tooth contact frequency = Gear tooth count X Frequency of Gear

Ring gear 2 tooth contact frequency

Carrier2 (input) at 24.78 Hz / 0.667 = 37.15 Hz Output (Ring gear 2 frequency)
Tooth contact frequency of Ring gear 2, 96 teeth X 37.15 Hz = 3,566.4 Hz or 3.566 kHz

Carrier 2 tooth contact frequency

Carrier 2 teeth 144 X 24.78 Hz = 3,568.32 Hz

Tooth contact frequency of planetary pinions

Ring gear 2 at 37.15 Hz with 96 teeth
Planetary carrier 2 includes 4 x pinion gears
Each pinion gear tooth count 144 /4 = 36 teeth
Ratio between Ring gear 2 and Planetary pinions 36 / 96 = 0.375:1
Each pinion gear is rotating at 37.15 / 0.375 = 99.07 Hz
Tooth contact frequency of pinion gears = 36 teeth * 99.07 Hz = 3,566.52 Hz

None of the above frequencies are our suspect offending frequency as we are focused on approx. 609 Hz and therefore, we can eliminate the transmission

So where to now?

Step 7. Add more sensors to our NVH set up and place them at static locations relative to the rear differential based on our list of possible causes (we have not eliminated the rear differential)
Below we have added a mic on channel C (green) aimed at the rear differential. Note the overall higher noise level (30.3 dB) compared to the cabin mic (channel B red). Also note the missing "suspect" offending frequency peak at 609 Hz, why?
This all comes down to ambient/environmental airborne noise about the rear differential when driving at 65 to 70 mph. This can be likened to opening the door window/drop glass at such speeds where we experience high levels of wind noise and buffeting. (The diff whine cannot be detected by the mic connected to channel C)

Step 8. Utilize a contact microphone (Accelerometer) attached to the rear differential. Bingo!
Note below how the offending 609 Hz detected by the cabin mic is also captured by the contact mic during our period of light acceleration
So how can we attribute the offending 609 Hz frequency to a differential component

Once again, knowledge of the differential tooth count would help here, however at this stage I have not been able to confirm the tooth count of the ring gear or pinion. If anyone can confirm these tooth counts, it would be much appreciated.

With that said, we can estimate the tooth count based on the offending frequency (approx. 609 Hz) divided by the RT1 (approx. 13.7 Hz)

We know that RT1 = 13.7 Hz, therefore the ring gear is also at 13.7 Hz

Estimated ring gear tooth count 609 Hz / 13.7 Hz = 44.45

We know we cannot have 44.45 teeth but we have to allow for the constantly changing road and engine speed during the light acceleration period

Ring gear tooth contact frequency: 44 Estimated ring gear teeth x 13.7 Hz = 602.8 Hz

To conclude:

• Keeping the cabin mics in place provides our customer reference by which to compare and correlate additional sensors attached to suspect components such as the rear differential
• Cabin microphone position is fundamental to diagnosis and revealing potential offending frequencies
• Try not to use mic’s outside the vehicle in direct airflow streams
• Make use of contact mics in the form of accelerometers in order to measure structure borne noise (immune to ambient and environmental noise)

Please find the pddata file used during the above analysis here https://drive.google.com/file/d/1XJVDT9 ... sp=sharing

Please find the analysis video here

I hope this helps, take care……Steve

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