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

Investigation of effect on the acoustic transfer function in a vehicle
cabin according to change of configuration
Wan-Ho Cho; Ji-Ho Chang
Korea Research Institute of Standards and Science, Republic of Korea

ABSTRACT
Recently, the various audio information are presented in a vehicle cabin not only for a driver but also otherpassengers and various sound field control methods had been applied for various purpose such as personalaudio. Basically, the measurement of acoustic transfer function in a vehicle cabin is relatively easier thanother living spaces because of its small volume and movement of objects. However the effect ofconfiguration cannot be ignored due to the relatively large volume change, especially related to the change of
number of passengers. Here, the effect on the acoustic transfer function in a vehicle cabin according tochange of configuration is observed and method to estimating robust solution is investigated.
Keywords: Acoustic transfer function, sound field control, vehicle interior

1. INTRODUCTION
Nowadays, various audio information is presented in a vehicle cabin throughout its car audio
system. In the early stage of car, e.g. 19 C, the vehicle do not had a space can called as ‘interior
space’ because its cabin was very small and not enclosed. In the process of advances in 20 C, theaudio system was installed for every car. In the several decades ago, the car audio was relatively
simple configuration consists of couple of loudspeakers and it has been improved by applying
various control with multi-channel audio [1]. Recently, various sound field control method has beenapplied to implement the multi-zone control in a vehicle cabin [2-5]. In order to implement thesevarious sound field control in interior space, it is essential to apply the acoustic transfer matrix forthe process of estimating the control filter. Because the estimated solution largely depends on thistransfer function, the precise information of transfer function is important. However, it is not always
possible to obtain and apply the exact acoustic transfer function and the problem of sound field
control in vehicle interior may not be exception. In this study, the deviation of TF was investigatedbased on the actual measurement in a vehicle cabin.

2. PROBLEM DESCRIPTION
2.1 Acoustic transfer function in a vehicle cabin
The acoustical transfer function (TF) is the path of sound transmission from sources to receivers.
In case of interior space, the TF includes the direct and reverberant components induced by various
boundary condition of space. In case of vehicle cabin, the volume of space is relatively small incomparison with other type of living space. Also, the movement of objects is not frequent and its
range is also small. These characteristics are suitable condition to apply the sound field control
method because the TF would be stable as time passed. On the other hands, the space is relatively
occupied because of its small volume and it can induce relatively large amount of change according
different layouts including the number of passengers. This means that the different transfer matrix
for corresponded layout should be applied for exact solution for precise control. However thisapproach is not practical because the real-time adaptation of control filter should be applied
according to the layout change. Therefore, the application of robust solution to the layout change
would be more practical approach. To this ends, it is necessary to investigate the amount of change
induced by layout change in quantified manner as a baseline.
The measurement of spatial distribution of sound field is conducted by the discrete spatial

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positions with microphone and dummy head system. The measured result will be obtained by thematrix form. For the problem of sound field control in a vehicle cabin, the area to be controlled is theear position of driver and passengers. Here, not only ear position but also the surface around thehead were measured by using the arc-shaped microphone array.
Four loudspeakers were originally installed in the test vehicle and 10 additional 2 inch sourceswere installed at the headrest positions as shown in Fig. 1(a). Figure 1(b) shows the positions of thesource system.
(a)

(b)

Figure 1 – (a) Photo of additional sources at headrest position, (b) tested source positions,
2.2 Test layouts
To evaluate the deviation of TF induced by the layout of vehicle inside, an actual measurement oftransfer matrix was conducted with various layouts. Here, the change of number of passenger andeffect of passenger’s posture were mainly investigated. The case that no person seated at driver’s
seat was not considered because this is still not usual condition for conventional vehicle driving
scenario. Table 1 shows the list the tested layouts. Instead of real passengers, the dummy heads wereplace on the seat.

Index

Driverseat

Set 1
Set 2
Set 3
Set 4
Set 5
Set 6
Set 7

Dummy 1
Dummy 1
Dummy 1
Dummy 1
Dummy 1
Dummy 1
Dummy 1

Table 1 – List of the test layouts
Rear passengerFront passenger
seatseat(Driver side)
Empty
Empty
Dummy 2Empty
Dummy 2Empty
Dummy 2
Dummy 3
Empty
Empty
Empty
Dummy 2Empty
Dummy 2

Rear passenger
seat(Driver side)
Empty
Empty
Dummy 3
Empty
Dummy 3
Dummy 3Empty

3. MEASUREMENT OF TF
3.1 Measurement setup
The measurement was conducted in an SUV type vehicle for 5 passengers and the test vehicle wasplaced in a semi-anechoic chamber. Figure 2 shows the configuration of the measurement system.
The array microphones (B&K Type 4958) were applied to the array system and the microphone outsignals were recorded by the multi-channel DAQ system (B&K LAN-XI modules + B&K PULSE)
with sampling rate of 32 kHz. As the test signal, the band limited white noise to up 12.8 kHz is
supplied by the generator module (B&K LAN-XI Type 3060) to the multichannel amplifier system(minDSP 16CH).

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Figure 2 – Configuration of the measurement system.
To measure the surface around the head, the arc-shaped array shown in Fig. 3 were designed andattached on the neck of dummy head (B&K HATS). The spacing between microphones is 10 o for thevertical direction and it was rotated 20 o step to horizontal direction. Also, this dummy head set on
the zig which is possible to change of inclination and two different case, upright and lean to back ofseat were observed.
(a)

(b)

Figure 3 – Microphone array to measure the acoustical TF on the surface around a dummy head: (a) Arraydesign, (b) example of installation to the dummy head.

3.2 Result of TF measurement
Figure 4 shows an example of the measured binaural response and the example of measured
response distribution around the head is shown in Fig. 5. These data can be applied to design thefilter to control sound field.
To observe the deviation induced by the change of number of passengers, the measurements wereconducted with the layouts listed in Table 1. Figure 6 shows the example of the change of thefrequency response induced by the number of passengers and the difference from the layout (driveronly) are shown in Fig. 7. Most of frequency range, the deviation is bounded ±2 dB however thefrequency range near of the trough in the response, shows much higher deviation. More drastic
deviation was observed if the posture of passenger is changed as shown in Fig. 8. The tendency of
deviation is also changed according to the relative position between sources and receiver.

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(a)

(b)

Figure 4 – Example of the measured response at ear position: (a) loudspeaker at near of driver seat is ON,(b) loudspeaker at near of front passenger seat is ON (blue line: left ear, red line: right ear).

Figure 5 – Example of the measured response distribution around the head (Driver seat, Loudspeaker atnear of driver seat is ON).

Figure 6 – Example of the change of the frequency response induced by the number of passengers (Driverseat, Loudspeaker at near of driver seat is ON).

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Figure 7 – Example of the difference of frequency response according to the change of number ofpassenger, reference to the case of driver only (LEFT: left ear, RIGHT: right; Driver seat, Loudspeaker atnear of driver seat is ON).

Figure 8 – Example of the difference of frequency response according to passenger’s posture (Upright andlean to back of seat; Driver seat, Loudspeaker at near of driver seat is ON).

4. CONCLUSIONS
The deviations of acoustical transfer matrix induced by the change of number of passengers areinvestigated in a real vehicle condition. To evaluate the deviation of TF induced by the layout ofvehicle inside, an actual measurement of transfer matrix was conducted with various layouts. Here,the change of number of passenger and effect of passenger’s posture were mainly investigated. The
measurement was conducted in an SUV type vehicle for 5 passengers and the test vehicle was placedin a semi-anechoic chamber.
In the example measured at the driver position, the deviation is bounded ±2 dB however thefrequency range near of the trough in the response, shows much higher deviation. More drastic
deviation was observed if the posture of passenger is changed. The tendency of deviation is alsochanged according to the relative position between sources and receiver.

ACKNOWLEDGEMENTS
The authors would like to thank Prof. Sung-Hwan Shin and the students of A3VC Lab. at Kookmin

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University for providing the semi-anechoic chamber for real vehicle measurement. This research wasfinancially supported by Korea Evaluation institute of industrial technology (KEIT, Korea) and theMinistry of Trade, Industry & Energy (MOTIE, Korea) through the core technology development
program of Automotive Industry (10082561, Development of Sound Field Control System forRealizing Personal Sound Zone in Vehicle Interior).

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