PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Study for Whoop Noise Reduction in passenger vehicle with ClutchHydraulic System
Devendra Kumar KHARE1; Anant Saran PANDEY2; Abhishek Lakhanlal VAISHYA3;
1

Maruti Suzuki India LimitedABSTRACT
The noise, vibration and harshness (NVH) is an important aspect in improving driving comfort. Low noiseand vibration is one of the primary requirements of passenger cars and plays an important role in theperception of customers. It includes a silent cabin with minimum vibrations and smooth and quite operationof the parts.
In Diesel vehicle, Engine vibrations are transferred to clutch pedal through fluid pulsation inside the clutch
operating system. In some cases, while operating clutch, vibration gets amplified and a noise is generated
which is called a whoop noise. Level of whoop noise generated during clutch operation should be as low as
possible to avoid uneasiness to the customers. To strike a balance between cost and good NVH is the biggest
challenge for NVH engineers in present scenario.
This paper elaborates the study for root cause analysis and reduction in whoop noise generated during clutch
pedal operation

INTRODUCTION
In recent years, customers’ sensitiveness is growing for quiet and smooth operating vehicles.
Customers look for the product from the aspects of comfort and effortless driving experience. Noises
inside the passenger cabin and vibrations perceived while operating the clutch pedal gives an insightabout vehicle quality.
The excitations to clutch system come due to deformation of crank shaft generated because ofcombustion. Deformations of crankshaft generate the axial displacement and vibrations of flywheel
over its equatorial line. These vibrations are transmitted to the clutch and from clutch to clutch pedal
through fluid pulsation, causing clutch pedal vibration. Sometimes vibrations are amplified by clutch
pedal housing and low frequency noise is generated during clutch operation in idle or during driving,is called whoop noise. fig no.1 shows when clutch is operated between 9 to 9.5 sec, 11 to 11.5 sec, 13
to 13.5 sec then low frequency (200-400 Hz) noise is generated, it is whoop noise.

Figure 1 – Noise near clutch pedal during clutch operation
1
2
37208

devendrakumar.khare@maruti.co.inanant.mech89@gmail.com
abvauto@gmail.com

The Whoop noise level is affected by several parameters like; Engine vibrations, Clutch
dimensions, flywheel concavity, flywheel run out fluid Pipe material. Figure 2 shows the hydraulicclutch operating System layout

Figure 2 – Clutch Operating System
The paper describes a research on root cause of Whoop noise and possible methods to reduce
Whoop noise in a vehicle.

1. WHOOP NOISE PHENOMENON DURING CLUTCH OPERATION
1.1 Vehicle Parameters
Name

Table 1 – Testing vehicle detailParameter or Type

Vehicle type

Hatchback

Vehicle Kerb weight963 kgDrive modeFWDTransmission

5-Speed Manual Transmission

Fuel type

DieselEngine4-Cylinder , 1300cc

1.2 Problem Statement
Engine Vibration due to combustion are Transferred to Clutch Pedal through Clutch Release
System. Transfer path of vibration and noise is shown in in Figure 3. When Pressing Clutch Pedal,
fluid under pressure causes the piston of slave cylinder to move Release bearing, thus disengaging theClutch. When the pedal is released, the diaphragm spring pushes the slave cylinder back, which forces
the hydraulic fluid back into the master cylinder and in the process clutch engages.
Most NVH related problems are reported during the process of clutch engagement, disengagement
or while holding the clutch pedal. Whoop noise is a phenomenon, caused due to transfer of Engine
vibration to clutch pedal through fluid pulsation inside clutch pipe. The noise can be heard duri ng
clutch engagement and disengagement.
In this particular case, clutch fluid pipe was replaced from metallic and rubber to plastic pipe forcost reduction. One vehicle was prepared with rubber and metal fluid pipe and other vehicle wasprepared with plastic pipe. Whoop noise was observed in vehicle having plastic pipe in idle condition
as well as driving condition during clutch operation.

7209

Figure 3 – Transfer Path of Vibration and Noise

2. Objective data measurement in Vehicle
Objective data measurement was done in vehicle at different locations. Sensors type and locations
has been described in table no.2.
Table 2 – Sensors Positions
NO.

Sensors typeSensors locations

1Microphone

Near CSC and CMC

2Microphone

Near clutch pedal (passenger cabin)

3

MicrophoneVehicle center position (passenger cabin)

4

Accelerometer

Fluid pipe

5Accelerometer

Clutch pedal

6Accelerometer

E/G cylinder head

During clutch operation whoop noise was not observed in vehicle with metal and rubber pipe, noise
near clutch pedal (inside passenger cabin) is shown in figure 4. X axis of graphing is showing time insecond, Y axis is showing frequency while color is showing magnitude of noise in dBA.
During clutch operation whoop noise was observed in vehicle with plastic pipe, noise near clutch
pedal (inside passenger cabin) is shown in figure 5.

Figure 4 – Noise near clutch pedal with rubber and metal7210

Figure 5 –Noise near clutch pedal with plastic pipe
Whoop noise was not observed in both vehicles near clutch master cylinder (CMC) and near clutch
salve cylinder (CSC). Engine vibration level was also similar in both vehicles. Cylinder head vibration(Peak to peak) of both vehicles has been shown in figure 6.

Figure 6 – Cylinder head vibration (Peak to peak 0-500 Hz)
Vibration data was also measured on plastic fluid pipe and on rubber portion of metal and rubberpipe. Vibration data was measured in all 3 directions but difference was observed in fluid flow
direction. High Vibrations is observed on plastic pipe in comparison to rubber pipe. Vibration data ofplastic pipe has been shown in figure 7 and vibration data of rubber pipe has been shown in figure 8.

7211

.
Figure 7 – Plastic pipe vibration

Figure 8 – Rubber pipe vibration
With rubber pipe below 400 Hz vibration level is less than plastic pipe but between 500 -600 Hz
vibration levels is slightly high. 500-600 Hz vibrations are not responsible for whoop noise.
Plastic pipe vibration is higher in 300-400 Hz band but peak was at 350 Hz in noise data taken nearclutch pedal (passenger cabin), refer figure no.5. Peak observed in sound pressure level near clutch
pedal may be because of amplification of vibration from clutch pedal. To identify the root cause ofamplification from clutch pedal housing, FRF test was done.

3. FRF Test of Clutch Pedal HousingFrequency response function is experimental modal analysis to find out resonance frequency,Damping and mode shapes. In experimental modal analysis many types of excitations can beprovided. In our test, excitation was done by impact hammer. Since frequency of noise was less than
500 Hz. so plastic tip was used in impact hammer. Excitation was provided at point where clutch cable
is attached to clutch pedal housing and response was measured at var ious position of clutch pedal
housing and clutch pedal. test result has been shown in figure 9, in FRF test resonance was observed at350 Hz in clutch pedal arm.

Figure 9 – FRF test result7212

3. Whoop Noise Test with weight on clutch pedal housing
To evaluate effect of clutch pedal resonance on whoop noise, 100 gram weights were added onclutch pedal at position where higher vibration was observed in FRF test and whoop noise test wasdone. Weight position has been shown in figure10.With weight whoop noise has been decreased by 6dB inside passenger cabin near clutch pedal around 350 Hz.Noise level of std condition and withweight is shown in figure 11.

Figure 10 – weight position on clutch pedal

Figure 11 – Effect of weight on (on clutch pedal) whoop noise

4. Evaluation of Clutch pipe length and hardness on whoop noise
Other vehicles of same segment having plastic clutch fluid pipe were checked in idle condition andin driving condition by pressing clutch and it was found that whoop noise level is less than test vehicle.
Main difference was found in clutch pipe length and pipe hardness. To evaluate effect of clutch pipe
length, length of clutch pipe was increased by 23 percentages and hardness was reduced by increasing
inner diameter by 60 percentages and outer diameter by 23 percentages.
Clutch fluid pipe vibration was measured near CMC with long and low hardness pipe and lessvibration was observed in comparison to std condition. Vibration data of clutch pipe near CMC isshown in figure 12 and passenger cabin noise data of driver seat is shown in figure 13. Vibration data
has been shown on dB scale by taking reference 0.000001 m/sec 2 .
Peak vibration near CMC has been decreased by 7-8 dB while passenger cabin noise peak has been
decreased by 2-4 dB in 300 to 500 Hz frequency band.

7213

Figure 12 – Effect of pipe length on pipe vibration near CMC

Figure 13 – Effect of pipe length on Passenger cabin noise

5. Evaluation of rubber mass damper effect on whoop noise
Rubber damper was prepared and it was installed near CMC of clutch pipe. Vehicle was parked
inside semi-anechoic chamber having cut of frequency 100 Hz. E/G rpm was increased to 2000 rpm
and clutch engagement-disengagement was done to generate whoop noise. With rubber damper near
CMC, noise level reduction was very less subjectively.
In 2nd trial, Rubber damper was installed near CSC and clutch engagement-disengagement wasdone at 2000 rpm. With rubber damper near CSC, significant improvement was observed. Stdcondition data was measured again in semi anechoic chamber to ensure similar test condition andbackground level. Std condition data is shown in figure 14.

Figure 14 – STD condition noise data inside passenger cabin7214

With rubber damper near CSC also objective data was measured. Noise data measured near clutch
pedal is shown in figure 15. With rubber damper passenger cabin noise near clutch pedal is reduced by6dBA.

Figure 15 – With Rubber mount noise data inside passenger cabin

6. CONCLUSIONS
Experimental study of root cause analysis of whoop noise has been presented. Some experiments
were carried out on vehicle to check effect of clutch pipe length, pipe hardness, pipe material andrubber damper on whoop noise. Correlations between clutch pedal housing resonance and whoop noise.
has been established and countermeasure effectiveness is also shown.

ACKNOWLEDGEMENTS
The authors would like express their gratitude towards their seniors for their continuous
encouragement support during entire study. Authors are thankful to all members of Transmission NVH
team of Maruti Suzuki India Limited.

REFERENCES
1.
2.

Hasebe, T., Yamamoto, H., Morita, K., Hibi, K. et al., "Experimental Study of Reduction
Methods for Clutch Pedal Vibration and Drive Train Rattling Noise from Clutch System,"SAE Technical Paper 932007, 1993, https://doi.org/10.4271/932007.
De Luca, J. and Girard, D., "Correlation Between Clutch Hydraulic Release Pressure
Pulsation
andPedalVibration,"
SAE
TechnicalPaper
2002-01-1191,
2002, https://doi.org/10.4271/2002-01-1191.

Contact Information
Author Name: Mr. Devendra Kumar Khare
Email ID: Devendrakumar.khare@maruti.co.in
Contact Number: +919999563212
Organization name: Maruti Suzuki India Limited

Definitions/Abbreviations
CSC
CMCdBA
RPM
FRF

Clutch release cylinder
Clutch master cylinder
Decibel (A-weighted)
Rotation per minuteFrequency response function

7215