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

Evaluation of independent sound zones in a car
Ji-Ho CHANG1; Wan-Ho CHO1
1

Korea Research Institute of Standards and Science, Republic of Korea

ABSTRACT
Recently sound field control techniques have been applied to vehicles that generate different sound zones in
the inner space with loudspeaker arrays. A typical example in a car is to provide a driver and a passenger withdifferent sound such as voice-guided navigation, music, or telephone voice. Acoustic contrast has been
widely used as a performance index in sound field control that shows the difference between sound levels in
the acoustically bright zone and the dark zone. However, these sound levels are usually measured withmicrophone arrays that are placed in the sound zones without the listeners. Thus these values can be different
from what the listeners would have at their ears. This study compares the acoustic contrasts derived using
microphone arrays with those using dummy head microphones at several positions and those using binauralmicrophones mounted in several subjects’ ears. From this comparison, this study attempts to propose a
simple and effective evaluation method of the independent sound zones.
Keywords: Sound field control, Independent sound zones, Acoustic contrast

1. INTRODUCTION
Sound zone control has been studied for recent decades that generates multiple independent
listening zones with multiple loudspeakers. This technique focuses sound in a designated zone, and at
the same time minimizes sound energy in other zones. The zone where sound energy is focused iscalled acoustically bright zone, and the zone where sound energy is minimized is called acousticallydark zone [1]. This control requires multiple loudspeakers, and thus car cabins are good places for thesound zone control because loudspeakers can be easily mounted on the headrest and on the surface ofthe car cabins. In addition, there is a desire for the driver and the passengers to have different audio
contents. There have been a few studies that attempt the sound zone control in a car cabin [2-5]. In
these studies, acoustic contrast has been used as a performance measure. This measure is defined as
the ratio between acoustic potential energy in the bright zone and the dark zone [1].
However, this measure depends on the measurement setup such as the number and the positionsof microphones. In fact different measurement setups were used in previous studies, and thus thecontrast values in these studies cannot be directly compared. Moreover, these measurement setups did
not take the effect of scattering by the users’ head and torso into consideration, which has been
known to be critical in the sound zone control at high frequencies [6]. Due to this effect, contrast that
listeners would have at their ear positions could be significantly different from those values. The
present study aims at investigating this difference. The ultimate objective of this project is to comeup with an evaluation method that allows to compare the performances of sound zone control
systems, and to provide a practical value for the acoustic contrast.
In the literature, Cheer and Elliott obtained 15-25 dB of acoustic contrast between the front seatsand the rear seats. This contrast was measured with a linear array of 4 microphones for each seat [2,3].
Liao et. al. reported that 15 dB contrast was achieved, which was measured with a planar array of 15
microphones for each seat [4]. Choi showed that 30-40 dB or 20-30 dB of contrast was obtained
depending on the setup of the bright and the dark zones below 1 kHz with 16 loudspeakers. A planar
array of 30 microphones was used for each seat to measure the contrast [5]. However, there have been
no studies to attempt to evaluate the acoustic contrast in a unified way to the best of the authors’knowledge. As a relevant measurement standard, ISO 5128 states measurement of noise inside a car,but this method is not appropriate for evaluating sound zone control systems as it uses only one
microphone for each seat without considering the effect of the scattering [7].
15174jiho.chang@kriss.re.kr

The present study compares three measurement setups. The first one is to measure sound pressure
with a planar array of microphones. Most of previous studies employed a planar array, and thus this canbe considered a conventional method. The second one is to measure sound pressure with the dummy
head microphones at several positions. The third one is to measure at ear positions of several subjects
with in-ear binaural microphones. The details of the measurement setup are shown in Section 2.
Section 3 shows the measurement results, and Sections 4 and 5 discuss the results and conclude thestudy, respectively.

2. PROBLEM STATEMENT
2.1 Experimental setup: optimization filters and bright/dark zones
The experiments have been conducted in a smart sound laboratory at KAIST i n South Korea. A
midsize sedan (Hyundai Genesis EQ900) was used as shown in Fig. 1. In total 18 loudspeakers were
installed in the car cabin including headrest loudspeakers. The driver seat was defined as theacoustically bright zone, and the rear right seat was defined as the dark zone.

Figure 1. Measurement with a planar array of microphones in a car cabin (by courtesy of Jung-Woo Choi)
Transfer functions between the loudspeaker input and the sound pressure at a planar array of
microphones were obtained. The planar array is composed of 30 microphones (6 by 5). This array
was placed in front of the headrest at the ear height in the driver seat (the bright zone) and the rearright seat (the dark zone). Figure 1 shows the measurement setup. If the sound pressure at the m-th
microphone in the bright zone is denoted as P rb,mpl ;  where rb,mpl is the position of the

microphone and ω is the angular frequency, the transfer function between the l-th loudspeaker andthe m-th microphone in the bright zone can be denoted as H l  rb,mpl ;  . Then P rb,mpl ;  andH l  rb,mpl ;  have the relation:





18





P rb,mpl ;    H l  rb,mpl ;  q l    ,
l 1







(1)

where ql    is the filter for the l-th loudspeaker. For simplicity, ω is omitted in what follows.
This equation can be expressed in a matrix form:

Pb , pl  H b , pl  q .

In the same way, the pressure in the dark zone is expressed as
Pd , pl  H d , pl  q .

(2)
(3)

Three optimization filter sets were obtained by the acoustic contrast control and provided by thesmart sound laboratory at KAIST. Since the pure acoustic contrast maximization tends not to berobust, a regularization method with brightness constraints were applied [8]. This constraint prevents
the acoustic brightness from decreasing below a certain threshold compared with the maximum
brightness. In this work, the threshold values was set to be -1, -3, and -12 dB, respectively, and thesefilters are denoted as q1 , q3 , and q12 . Three filter sets were used to investigate the effect of the
optimization filter on the results. A filter with the lower threshold tends to provide the larger

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acoustic contrast, but it can be more sensitive to the effect of the scattering and experimental noise.
2.2 Measurement of acoustic contrast
To measure the acoustic contrast, white noise was filtered by the optimization filter sets, and sent
to loudspeakers. Sound pressure levels were measured at microphones, and averaged in time domain.
Then, these values were also spatially averaged.
2.2.1 Measurement with a planar array of microphones
The planar array that was used to obtain the transfer functions was employed again to obtain the
acoustic contrast. Thus, the acoustic contrast values should be equal to what were predicted in the
optimization. The microphone spacing was 4 cm. The acoustic contrast is obtained as

 pl 
2

Pb2,plPd2, pl,(4)2

where Pb , pl and Pd , pl are the spatially averaged sound pressure

Pb2,pl  

1 M
P rb,mpl

M m 1

2, Pd , pl 
2

2

 .

1 M
P rd,mpl

M m 1(5)

2.2.2 Measurement with dummy head microphones
Measurement with binaural microphones for many listeners might be the ideal way of evaluating
the sound zoning system. However, this method is not feasible in practice as it takes too much time andeffort. The measurement with dummy head microphones can be an alternative method because it
reflects the effect of scattering by the head and torso of a listener.
In this experiment, a dummy head microphone set (B&K HATS type 4128) was employed. The
measurement was conducted at 12 positions in the bright zone, and 6 positions in the dark zone. In thebright zone, 12 positions are combination of front/center, high/low, and leaning 1/2/3. For front/center
positions, the seat itself was moved by the automatic positioning. The center position is stored in thesystem, so that the seat can be placed in that position by pressing a button. The front position was 5 cm
in front of the center position. For high/low and leaning 1/2/3, a zig that can fix the dummy head in adesignated position was designed and constructed. In the low position, the ears of the dummy head islocated about 2 cm lower than the headrest loudspeakers. The high position is 5 cm higher than the low
one. The leaning 1 is defined as the position that the dummy head is contacted to the headrest. The
inclination angle from the vertical axis was approximately 20 degrees. Leaning 2 and 3 are more
upright, and the inclination angle was 18 degrees and 13.5 degrees, respectively. Figure 2 shows theleaning 1/2/3. During the measurement, another dummy head microphones were placed in the darkzone, but the signals measured with this microphones were not used in this study.
In the dark zone, sound pressure was measured at 6 positions of the dummy head microphones:
high/low and leaning 1/2/3. The inclination angles were 25, 22.5, and 20 degrees, respectively. During
the measurement, another dummy head was placed in the bright zone.
The sound pressure in the bright zone is denoted as P rb,mdh , and that in the dark zone is P rd,mdh .

 

 

The number of the measurement is 24 in the bright zone (12 positions x left/right ear), and 12 in thedark zone (6 positions x left/right ears). The odd indices indicate the left ear signals, and the evenindices are the right ear signals.
The acoustic contrast with the dummy head microphones is expressed as
2
1 24
 m
P
r
 b,dhP2
24 m 1.
(6)
 dh  b2,dh 212
Pd ,dh1
 P rd,mdh
12 m 1 

 

2.2.3 Measurement with binaural microphones
In order to obtain the sound pressure at ear positions of users, in-ear binaural microphones (B&K
type 4101-B) were worn on the ears of 13 subjects. The subjects were asked to sit still during themeasurements. They had two postures: standard posture and free posture. In the standard posture, theseat position was fixed, and they were asked to sit up straight and grab the handle. In the free posture,they were asked to choose the seat position and have comfortable postures for them. The ages of the5176

Figure 2. Positioning of the dummy head microphone (leaning 1/2/3)
subjects vary from 30 to 40. All subjects were male. The distance from the seat and the ears varies from
64.8 cm to 72.1 cm.
The sound pressure in the standard posture is denoted as Ps rb,bim  , and that in the free posture is

 

 

Pf rb,bim . For each posture, the number of the measurement points is 26 (13 subjects x left/rightears). The odd index corresponds to the left ear, and the even index to the right ear. For the dark zone,
the sound pressure values measured with the dummy head microp hones were used to obtain the
acoustic contrast:

 

 

2
2
1 26 

Ps rb,bim   Pf rb,bim  2Pb ,bi 52 m 1 .
 bi  2 212
Pd ,dh1
P rd,mdh
12 m 1

 (7)

3. Experimental results
3.1 Measured values of acoustic contrast
Figure 3 shows the acoustic contrast measured by the planar array, dummy head microphones,
and binaural microphones (  pl ,  dh , and  bi ) with three filter sets, q1 (top), q3 (middle), andq12 (bottom). In the top figure, all the contrast values are not as high as those with q3 and q12 . The
maximum value is lower than 30 dB, and most values are lower than 20 dB.
In the middle figure, mostly the acoustic contrast with the planar array is greater than those withother microphones. Especially, from 400 Hz to 1 kHz and above 10 kHz, the difference was larger,
having a value of more than 20 dB at some frequencies. This shows that the acoustic contrast thelisteners perceive can be significantly different from what is measured with a planar array. The bottom
figure shows similar tendency to the middle figure except that the acoustic contrast with the planar
array increases around 2 kHz. However those with the dummy head microphones and the binauralmicrophones do not have a big difference.

3.2 Sound levels in the bright zone
Since the acoustic contrast is the ratio between the sound levels in the bright and the dark zones,
it does not show what happens in each zone. In order to observe more details, sound levels in thebright and the dark zone need to be investigated separately. In this section, sound levels in the brightzone are shown. Sound field in the dark zone is considered to be relatively robust to the scatteringand the experimental noise because the sound level is low. Moreover, the same spatially averaged
sound level measured with the dummy head microphones in the dark zone was used to obtain both
 dh and  bi .

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Figure 3. Acoustic contrast measured by the planar array, dummy head microphones, and binauralmicrophones with three filter sets (top: threshold 1 dB, middle: 3 dB, bottom: 12 dB).
3.2.1 Overall level
Figure 4 shows the overall level from 100 Hz to 10 kHz with the dummy head microphones at
each position. Positions 1 to 6 are high positions in the height, and 7 to 12 are low positions.
Positions 1, 2, 7, and 8 are leaning 1, positions 3, 4, 9, and 10 are leaning 2, and positions 5, 6, 11,
and 12 are leaning 3. Odd numbers are center positions, and even numbers are front positions.
Leaning 1 leads to the highest value. This can be due to either the distance between the ears and theheadrest loudspeakers or the area measured by the planar array.
Although the contrast values with q3 and q12 do not have a big difference (Fig. 2), the soundlevels in the bright zone have a considerable difference. Sound levels with q3 are greater than thosewith q12 at all positions.
Figure 5 shows the overall level from 100 Hz to 10 kHz with binaural microphones for all subjects.
The level difference across the subjects is smaller than 3 dB with q1 and q3 , whilst that with q12 isrelatively large. The maximum difference is approximately 5 dB (between subject 3 and 6).
3.2.2 Levels in one third octave bands
Figure 6 (top) shows the sound levels in 1/3 octave bands with the dummy head microphones. It
is clearly seen that the higher threshold leads to the lower levels in the bright zone. In addition, thehigher threshold leads to the larger standard deviation as shown in Fig. 5 (bottom). This means thatthe sound field generated by the higher threshold is more sensitive to th e scattering and theexperimental noise.
The similar tendency can be observed in Fig. 6, which shows the sound levels and the standard
deviations in 1/3 octave bands with the binaural microphones. The sound level with q12 has thelowest values and the largest standard deviation. At 300 Hz, the standard deviation has a large value
in both Figs. 6 and 7. The wavelength at 300 Hz is about 1.1 m, so this can be due to the effect of thehead and the torso.

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Figure 4. Overall level (100 Hz to 10 kHz) measured with the dummy head microphones in the bright zone

Figure 5. Overall level (100 Hz to 10 kHz) measured with the binaural microphones in the bright zone
In contrast, there are some differences between these figures. At 6300 Hz, the standard deviation
has the largest value only in Fig. 7. This difference can be attributed to individual differences ofhead and torso that the dummy head microphones does not have. From 1 kHz to 4 kHz, the soundlevel in Fig.6 (top) has a fluctuation along the frequency, whilst that in Fig. 7 (top) has a relatively
smooth curve. This might be related to the difference in the absorption of sound on the skin and thecloth of the subjects.

4. DISCUSSIONS
4.1 Estimation of ‘listener acoustic contrast’
One of the ultimate goals of this work is to propose an evaluation method of the independent soundzone system that provides the similar results to the measurement with binaural microphones by using
measurements with dummy head microphones. For example, the averaged squared sound pressure with2
binaural microphones Pb ,bi could be expressed with the squared sound pressure with dummy head

 .
 m

microphones P rb , dh

2

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Figure 6. Sound levels (bottom) and standard deviation (bottom) in 1/3 octave bands with the dummy headmicrophones

Figure 7. Sound levels (bottom) and standard deviation (bottom) in 1/3 octave bands with the binauralmicrophones

M

  ,

Pb2,bi     m   P rb,mdh
m 1

m

2

(8)

where is the weight. To make this weight non-negative, non-negative least squares method canbe applied [9].

51804.2 Factors affecting the acoustic contrast
The sound pressure was measured at 12 positions with the dummy head microphones. Among theposition changes, the inclination angle of the head and the torso was the most critical factor that
affects the acoustic contrast. As shown in Fig. 3, the shorter is the distance between the ears and theheadrest, the larger is the contrast value. On the contrary, the height of the ears and the position of
the seat itself were not as critical as the inclination angle.

5. CONCLUSIONSThis study compares acoustic contrast values measured in different ways. The first method was tomeasure with a planar array of microphone, and take average of the sound pressure values. The
second method was to measure with a dummy head microphone, and the other one was to measurewith binaural microphones for several subjects.
Results show that the acoustic contrast with a planar array is considerably different from thosewith the other microphones. This implies that the acoustic contrast with a planar array is not enoughto reflect the performance of the system. The acoustic contrast that listeners perceive can besignificantly different.
Another finding is that the distance between the ears and the headrest is more critical than theother factors. Thus, in the measurement of the acoustic contrast, the inclination angle should beconsidered.
In addition, the noticeable differences between measurements with the binaural microphones andthe dummy head microphones was found such as the fluctuation with the dummy head microphones
from 1 kHz to 4 kHz and the high standard deviation with the binaural microphones at 6300 Hz. The
reasons for these differences should be further investigated.

ACKNOWLEDGEMENTS
The authors would like to thank Jung-Woo Choi and Hyung-Jun So at KAIST for providing thefilter sets and the equipment for the experiments. The authors also thank the subj ects for their
participation. This research was financially supported by Korea Evaluation institute of industrial
technology (KEIT, Korea) and the Ministry of Trade, Industry & Energy (MOTIE, Korea) through thecore technology development program of Automotive Industry (10082561, Development of Sound
Field Control System for Realizing Personal Sound Zone in Vehicle Interior).

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