PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Comfort Evaluation on the Drivers using Transfer Path Analysis
Nihlatul FALASIFAH1; Dhany ARIFIANTO 2
1

Institut Teknologi Sepuluh Nopember, Indonesia

2

Institut Teknologi Sepuluh Nopember, Indonesia

ABSTRACTThe vibration transmissibility to the seated drivers has important influences on the human body comfort,
health, and safety. To establish the dynamic equation for precondition of modal analysis, Newton-Euler
method is usually used as the common study. However, this method is difficult to determine generalized
coordinates belong to which natural frequency for linear system. On the other hand, by using transfer pathanalysis (TPA) as a new technique, it is possible to determine generalized coordinates of the vibration for the
car machine aﬀected the vibration of the car. The purpose of this research is to quantify the comfort level for
driver’s whole body vibration in passenger car based on ISO 2631-1 and to know the level of vibration on
the car using transfer path analysis. This research tries to characterize the vibration effect of different sources(variation of the car machine) at idle condition. Human as the subjects are involved in this test. The resultsof this study show that low frequency vibration of the seat drivers aﬀected to the ride comfort. Through TPA,
the human body comfort can be evaluated using operational force by multiplying the natural frequencies of
the car with the operational acceleration.
Keywords: Comfort Evaluation on the Drivers, ISO 2631-1, Transfer Path Analysis, Whole Body Vibration.

1. INTRODUCTION
Car becomes one of the transportation that widely used by people especially in Indonesia. Basedon data from the National Statistics Agency of Indonesia, the number of cars has owned by Indonesian
people in 2017 reached 15.493.068 units (1). Along with the increasing use of cars as a public
transportation, the factors of comfort on car drivers are being considered and become an important
thing in automobile system.
Several investigations about the effect of the vibration on the driver seated position have beenreported. Van Niekerk et al estimated dynamic seat comfort prediction using the Seat Effective
Amplitude Transmissibility (SEAT) for 16 different automobile seats (2). Rakheja et al investigated
the interactions of the seated occupants with an inclined backrest at the two driving -points using
apparent mass (3). Investigation by using apparent mass was also done by Subashi et al for nonlinear
subjective and dynamic responses of seated subjects exposed to horizontal whole-body vibration (4).
Coyte et al presented a holistic literature survey about seated whole body vibration analysis,
technology, and modelling (5). The transmissibility measures, impedance, estimation of frequency
response function, and model development were surveyed as the data processing techniques for the
motion of the seated body.
Transfer path analysis (TPA) is a method to solve the vibration problems by identifying theirvibration sources and the contributions of each transfer path to the receiver inside the vehicle (6). TPA
is commonly used in the automotive industry (7,8,9) especially for passenger cars (10,11). This
research tries to implement TPA as a method to investigate the vibration behaviour on the car
especially for the driver’s seat comfort in automobile aspect based on ISO 2631-1. The effect of car
engine as the source of vibration on seat comfort were investigated with variation of the car engine(car engine from classic car and new car) at idle condition. Experimental modal analysis by impact
test is used to determine the natural frequencies as the dynamic characteristics of the car. The natural
frequencies from impact test is multiplied with operational acceleration which is equal to operational
force as transfer path analysis. Transfer path analysis to evaluate the vibration characteristics of the
1
2

nihlatulfs@gmail.com
arifianto@gmail.com

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car related to the driver seat comfort will be discussed in section 4.1.

2. THEORETICAL ASPECTS
2.1 Whole Body Vibration According to ISO 2631-1
The ISO 2631-1 (1997) standard is usually used as the reference for vibration effects to the humanbody. It defines the quantified methods for whole -body vibration (WBV) with respect to vibration
comfort, health, and motion sickness. The ISO 2631-1 focus on vibration comfort measured overlonger time since the seat supports the driver in sitting posture related to whole body vibration. The
human body is complex mechanical system and more sensitive to certain frequencies so that thefrequency weighting is needed due to this fact and also according to the ISO 2631-1 standard (12).
Frequency weighting of signal needs to be divided into its frequency content. Once the frequency
content is calculated and the amplitude is multiplied with desirable weighting so it will change theamplitude depending on the frequency (13). The frequency weighting for human body comfort
reactions to vibration environments can be seen in table 1.
Table 1 – Comfort reactions to vibration environments (12)Weighted rms acceleration (m/s2)

Subjective response

Less than 0.315 m/s2

Comfortable

0.315 m/s2 - 0.63 m/s2

a little uncomfortable

0.5 m/s2 - 1 m/s2

fairly uncomfortable

0.8 m/s2 – 1.6 m/s2

Uncomfortable1.25 m/s2 – 2.5 m/s2

very uncomfortable

Greater than 2 m/s2

extremely uncomfortable

When investigating the whole body vibration excitation, the ISO 2631-1 standard recommends several
measurements which are appropriate to use. The measurable quantities use weighted root mean squareacceleration (a w) and vibration dose value. The weighted rms acceleration is the most common used
to describe the acceleration time signal with a single value (14) and it is expressed in metres per
second squared (m/s 2). The calculation of the weighted rms acceleration and the vibration total value
are defined in equation 1 and 2.
1ª1 T 2º2
a w « ³ a w (t )dt »
T
¬ 0
¼
(1)
where a w is the weighted acceleration as the function of time (in m/s 2), and T is the duration of
measurement (in second). ISO 2631-1 states that the vibration total value of weighted rms acceleration
should be applied if the vibration comfort become the aim that shown in equation 2
2
2
2
2
2
2
aw(total) (k x awx  k y awy  k z awz )1 / 2(2)
where a w (total) is the vibration total value, a wx is the weighted rms acceleration in x-direction, a wy is
the weighted rms acceleration in y-direction, a wz is the weighted rms acceleration in z-direction, kx ,ky, and kz are multiplying factors (multiplying factors are equal to 1 for comfort evaluation).

2.2 Transfer Path Analysis
Transfer Path Analysis (TPA) is used to estimate the vibration source in the dynamic system andto identify the vibration path for complex mechanical system (15). TPA defines the vibration responseat selected target as a superpostion of vectorial contributions from a set of force inputs that excite thestructure (16,17,18). TPA can be obtained by calculating a set of the operational force where theoperational force (F) is calculated by multipliying the natural frequency of the structure borne path(H) and operational acceleration (a) in frequency domain (15,19). The natural frequency of thestructure borne path is found by using impact hammer, shaker, or impact test. There are several ways
for impact test such as dropping a steel sphere in the structure according to (20). While, the operational
acceleration can be measured when the vibration source is turning on (for automobile system, it isusually from car engine). Calculation of the operational force of the system based on TPA technique4296

is defined in equation 3.

op

op

(3)
where F is operational force, H is the natural frequency of the structure, and aop is operational
acceleration.
F

Ha

op

3. METHOD
This study uses ISO 2631-1 standard as the guidelines for the measurement and evaluation ofhuman exposure to WBV. The frequency range that used in this study is 0.8 Hz to 100 Hz related towhole body vibration. The car that used in the data collection are Toyota Kijang and Toyota Avanzawith specification shown in table 2. The car engine condition and suspension both of these cars are
assumed still good and in accordance with the initial specifications. The car suspension, seatsuspension, and ambient temperature are neglected and not being a focus for this study. One seat pad
accelerometer from Svantek type SV-38V and vibrometer Svantek SV-106 are used for themeasurement of whole body vibration. Four accelerometers from PCB piezotronics are used withmodel 352C33 and they are connected to the data acquisition from National Instrument with model
NI cDAQ-9178.
Table 2 – Specifiation of the car engine
Toyota Kijang

Toyota AvanzaYear production

1991

2018

Type of car

Super Kijang KF 40 short

Grand New Avanza 1.3L

Type of engine

Carburetor 4 cylinders in line, SOHC

Volume of cylinder

1800 cc

1300 cc

Maximum power

73.028 PS/ 4600 rpm

95.5 PS/ 6000 rpm

Maximum torque14.2 kgm/ 4600 rpm

12.3 kgm/ 4200 rpm

4 cylinders, 16 valves, DOHC, dual
VVT-i EFI

3.1 Subjects
Seven people (men) as the test subjects took part in this study with average age 25 years ± 6 .5 (21to 39 years old), the average height 173 cm ± 6.7 (163 cm to 180 cm), and the average weight 79 kg± 20.6 (65 kg to 125 kg). The subjects were in good physical health and did not have any history ofneurological, musculoskeletal (back pain and spinal pain), cardiovascular, and respiratory diseases.
The subjects also did not used any controlled medicines and have not been submitted to recent surgery
according to recommendations of (21).
3.2 Procedure
This study used one seat pad accelerometer for the driver’s seat car for whole body vibration
analysis. This seat pad accelerometer was connected to vibro-analyzer Svantek SV-106 and placed at
the seat cushion as shown in figure 1(a). Three axes indicated on the seat pad should be coincidedwith the specified axes of vibration shown in figure 1(b). The measurement used translational
movement as the vibration direction where the fore and aft direction is defined as the x -axis, lateralas the y-axis, and vertical direction as the z-axis (22). Four shear accelerometers were also used andconnected to portable DAQ. We conducted the measurement on zero condition (car engine off) andidle speed condition (car engine on). These shear accelerometers were attached on the some selected
node positions : near car engine, steering wheel, driver seat cushion, and foot floor (near driver
position). The comfort reactions on the drivers are evaluated using the weighted rms acceleration and
1/3 octave band frequency analysis using equation 1 and 2. Transfer Path Analysis is used to analyze
the vibration characteristics on the car. The natural frequencies were determined using modal analysis
by dropping a steel sphere based on the experiment by Qi and Yin (20) and the value of operational
accelerations were determined by measuring the acceleration when the car engine is turning on at idle
condition and car placed on concrete floor. TPA method were used to identify and trace the flow ofvibration on the car using operational force as in equation 3.

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(a)
(b)
Figure 1 – (a) Installation seat pad accelerometer on the driver’s seat cushion, (b) specified axes ofvibration (23)

4. RESULTS
4.1 Vibration Characteristics of the Car using Transfer Path Analysis
The operational acceleration of the car engine was measured at low frequency engine idle speed
vibrations between 0 Hz to 100 Hz. The value of operational accelerations when the car engine runningat idle speed as the vibration source has frequency of 49.61 Hz for Toyota Kijang and 59.57 Hz for
Toyota Avanza as shown in figure 2. Table 3 shows the weighted rms displacement from car engine
as the vibration source at idle speed to some receivers : steering wheel, driver seat cushion, and footfloor. We can see in table 3 that the highest rms displacement value both Toyota Kijang and Toyota
Avanza were on the position of near car engine, followed by foot floor, steering wheels, and dri verseat cushion. The energy of vibrations that transmitted from the car engine to some selected positions
decreased their magnitudes. On the other words, the vibration that received by the driver that seat on
the driver seat cushion has smaller amplitude compare to vibration amplitude near the car engine. The
car engine rms displacement of Toyota Kijang was higher than Toyota Avanza. It was caused by the
amplitude of operational vibration for Toyota Kijang has higher value 0.015 m/s 2 compared to Toyota
Avanza which has amplitude 0.002 m/s 2 (see figure 2). It means the vibration amplitude of Kijang was
7.5 times higher than the vibration amplitude of Avanza.
Vibration characteristics of the Toyota Kijang and Toyota Avanza decreased their amplitudes fromcar engine position to some selected positions. This was caused by the car structure which has thedamping coefficient of the material to reduce and absorb the vibration. Beside that, the system bothof those cars have no resonance effect as proven in transfer path analysis method where resonance isa condition which occurs when a natural frequency is excited by an e xternal forcing frequency. The
natural frequencies both Toyota Kijang and Toyota Avanza which obtained through impact test by
dropping a steel sphere are shown in figure 3. Equation 3 was used to determine transfer path analysis
by multiplying the natural frequencies of the car with operational acceleration at idle condition. F igure
4 and 5 show the transfer path analysis where the forcing frequency of operational condition at idle
condition did not coincide the natural frequencies both Kijang and Avanza in the frequency range from
0 Hz to 100 Hz. Therefore, the operational frequency at idle condition would not be amplified by thenatural frequencies which can cause the frequency at which a machine will naturally amplify vibration.
Table 3 – RMS displacement values with car engine as the source of vibration at idle condition

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Selected node positions

RMS displacement for Kijang

RMS displacement for Avanza

Near car engine

6.0 mm

5.6 mmSteering wheel

2.9 mm

2.3 mmDriver seat cushion

1.8 mm

1.1 mmFoot floor

4.1 mm

5.5 mmFigure 2 – Operational acceleration of the car engine as the vibration source at idle condition for
Toyota Kijang and Toyota Avanza

Figure 3 – The natural frequencies for Kijang and Avanza

Figure 4 – Transfer path analysis at idle condition for Kijang

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Figure 5 – Transfer path analysis at idle condition for Avanza
4.2 Evaluating Method of WBV using the Total Weighted RMS Acceleration
As shown in figure 6(b), we can see that the mean of the total weighted rms acceleration of Toyota
Kijang a w (kijang)= 0.1 m/s 2 has greater value compared to Toyota Avanza a w (avanza)= 0.003 m/s 2 atidle speed condition althought both of them are still in the comfortable zone ( < 0.315 m/s 2) based on
comfort zone criteria by ISO 2631-1 stated in table 1. This was caused by engine car vibration of
Toyota Kijang as the source of vibration has an amplitude 7.5 times higher compared to engine
vibration from Toyota Avanza as discussed in chapter 4.1 (see figure 2). The Toyota Kijang has
amplitude of 0.015 m/s 2 and the Toyota Avanza has amplitude of 0.002 m/s 2. Beside the car engine,
the higher value of the total weighted rms acceleration from Toyota Kijang was caused by the vibrationtransmitted to the seat’s driver from the car engine which has greater rms displacement value
compared to the Toyota Avanza as stated in table 3 where Toyota Kijang has displacement of 1.8 mm
and Toyota Avanza has displacement of 1.1 mm on the driver seat cushion position. The driver seatcushion was used to place the seat pad accelerometer sensor for whole body vibration. Therefore, thevibration displacement and vibration amplitude of the Toyota Kijang were gre ater than the Toyota
Avanza at idle condition. We can observe in figure 6(d) that the VDV values at idle condition both
Kijang and Avanza were less than 9.1 m/s1.75 for Exposure Action Value (EAV) and less than 21 m/s1.75
for Exposure Limit Value (ELV) which meant it was safe and healthy from vibration exposure thatcaused potential health risks to the drivers according to the European Directive standard (24).

Figure 6 – Comparison between Kijang and Avanza for the total weighted rms acceleration on :(a) zero condition (b) idle, and vibration dose value on : (c) zero condition (d) idle
4.3 Evaluating Method of WBV using 1/3 Octave Band Frequency Analysis
According to ISO 2631-1, the frequency weighting that used for the measurement of whole body
vibration are principal weightings in one-third octaves (Wd for the x and y directions, and Wk for thez direction). These frequency weightings were applied for seated driver on the effect of vibration on
comfort with multiplying factor k=1 for all of the directions on supporting seat surface vibration. The

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Weighted RMS Acceleration (m/s^2)

dominant frequency was identified as the 1/3 octave band frequency in the range from 0.8 Hz to 100
Hz associated with the highest frequency weighted acceleration value s. The relation between
frequency weighted rms acceleration and 1/3 octave band frequency in the vertical direction (z- axis)
were shown in figure 7.
We can observe in figure 7 that idle condition has higher value compared to zero condition both
Kijang and Avanza. Generally, Toyota Kijang has higher amplitude of the weighted rms acceleration
as the comfort parameter compared to Toyota Avanza for the driver’s whole body vibration. Kijang
has the highest amplitude at frequency 20 Hz with the weighted rms acceleration value 0.155 m/s2 andAvanza has the highest amplitude at frequency 4 Hz with the weighted rms acceleration value 0.041
m/s2 . This was confirmed with the results of the vibration characteristics on the car as discussed in
section 4.1 where Toyota Kijang has higher amplitude and also rms displacement compared to Toyota
Avanza on the driver seat cushion as the position of the driver (see table 3). Therefore, the vibration
amplitudes that received by drivers for Toyota Kijang were greater than Toyota Avanza. However,Toyota Kijang and Toyota Avanza were still in the comfortable zone at idle condition because thevalue of the total weighted rms accelerations were less than 0.315 m/s 2 based on comfort zone criteria
classified by ISO 2631-1.

0.160
0.120
0.080
0.040
0.000
Frequency (Hz)
Zero Condition (Kijang)

Idle Condition (Kijang)

Zero Condition (Avanza)

Idle Condition (Avanza)

Figure 7 – One third octave band frequency analysis for Kijang and Avanza

5. CONCLUSIONS
The vibration level and vibration characteristics that affected by different vibration source(variation of the type of car) were analyzed. The results showed that low frequency vibration of theseated drivers affected to the comfort of ride both Toyota Kijang and Toyota Avanza using weighted
rms acceleration and 1/3 octave band frequency analysis according to ISO 2631-1 standard. At idle
condition, the driver seat comfort were in the comfortable zone both of those cars. Transfer pathanalysis has shown the level of vibration on the car. Through TPA, the human body comfort can beevaluated by multiplying the natural frequencies of the car with the operational acceleration.

ACKNOWLEDGEMENTS
The authors would like to sincerely thank to the member of Vibration and Acoustics Laboratory,
Department of Engineering Physics - Institut Teknologi Sepuluh Nopember (ITS), for the technicalsupport given during the research.

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