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

Design of acoustic vehicle alerting system sound assuming
listening situation of pedestrians
Hiroshi MATSUDA1; Masato SUZUKI2; Nobuo MACHIDA3
1
2

College of science and technology, Nihon University, Japan

Graduate school of science and technology, Nihon University, Japan
3

College of science and technology, Nihon University, JapanABSTRACT
The role of acoustic vehicle alerting system (AVAS) sound is to tell pedestrians the approaching of quietvehicles such as electric vehicles (EV) or hybrid-electric vehicles (HEV). We propose the AVAS sound thatsatisfies both of comfort and perception. The comfort of AVAS sound in this study means pedestrians do notfeel noisy. The perception of AVAS sound means pedestrians to notice of the approaching quiet vehicles atsafely avoidable distance. The AVAS sound model produced in this study has two peak frequencies. Thesemodels are two kinds of the steady sound with constant sound level and the fluctuating sound formed bymodulating amplitude of steady sound. We measured impression of AVAS sound and perceptual distance.
The impression of AVAS sound was measured by the psychological questionnaire using SD method. The
perceptual distance: the distance between subject and HEV when the subject notices the approaching of HEV
emitting the AVAS sound was measured in a test road. We also measured the perceptual distance of different
listening situation such as standing or smartphone operation during walking. We show the soundcharacteristic to improve the comfort and the perception of AVAS sound, and the effect of the listening
situation on the perceptual distance.
Keywords: Acoustic vehicle alerting system sound, Listening situation, Perceptual distance

1. INTRODUCTION
Acoustic vehicle alerting system (AVAS) sound is a sound to tell pedestrians the approaching of theEV/HEV. Ministry of Land, Infrastructure, Transport and Tourism (MLIT) published the guideline for themeasure against the quietness problem of HEV/EV in 2010 (1). The UN Regulation No.138 on Quiet Road
Transport Vehicles was published in 2016 (2, 3). And then, MLIT announced that the mandatory ofinstalling AVAS in HEV/EV will be enforced from March 2018. Pedestrian’s perception for theapproaching HEV/EV is improved by increasing the sound volume of AVAS but it feels discomfort when
the sound volume is large. On the other hand, pedestrians become difficult to hear because AVAS is masked
by the road environment sound when the sound volume of AVAS is small.
The purpose of this study is to clarify the AVAS sound model that has clarity and comfort, and makes it
possible for pedestrians to recognize the approach of vehicles when the listening situations are different.
The AVAS model prototyped in this study is noise with two peak frequencies. These AVAS models are twokinds of the steady sound with constant sound pressure level and the fluctuating sound formed bymodulating amplitude of steady sound.
We focused on the road environment sound and the discomfort of sound, and performed an impression
evaluation experiment and a perceptual distance experiment. In the impression evaluation experiment, asubjects sat on a chair in the soundproof room were exposed to the AVAS, the impression of AVAS wasevaluated by the psychological questionnaire by SD method, and the common factor was estimated byfactor analysis. In the perceptual distance experiment, HEV mounted with AVAS model was approachedfrom the back of subject, and the distance between the subject and HEV when the subject perceived HEV
was measured. In this study, the minimum avoidance time was estimated from whole-body reaction time
1
2
3

matsuda.hiroshi@nihon-u.ac.jp
csms17023@g.nihon-u.ac.jp

machida.nobuo@nihon-u.ac.jp7169

and average walking speed of an elderly person. Then, we proposed the criteria of the minimum distancenecessary to safely avoid the HEV where the pedestrian approaches by multiplying the minimum avoidancetime by the vehicle speed. From the experimental results, we found that effects of the AVAS sound modelon perceptual distance and comfort sensation and effects of the difference of the listening situation on theperceptual distance.

2. EXPERIMENTAL METHODSSound pressure level [dB]

2.1 AVAS sound model and sound conditions
Figure 1 shows that the example frequency spectrums of AVAS sound model prototyped in thisstudy. The AVAS sound model is made from white noise and has two peak frequencies. Table 1 shows
the sound stimuli conditions of AVAS sound model in this study. The peak frequency I selected 630
Hz from range below 1000 Hz small in hearing ability difference. The peak frequency II was selectedfrom the frequency band 2500 Hz to 4000 Hz of high auditory sensitivity (4). The sound pressure level
distance was added to the sound conditions for the purpose of reducing the discomfort sensation withlistening of AVAS sound assuming the AVAS sound feels noisy if the sound pressure level of the peakfrequency II is large. These AVAS sound models are two kinds of the steady sound with constant soundpressure level and the fluctuating sound formed by modulating amplitude of steady sound. We added
the fluctuating sound to the sound stimuli conditions of AVAS sound model because the fluctuating
sound because the fluctuating sound can be noticed by small sound level to the approaching of HEV.
The modulation cycle and the amplitude modulation factor were added to the sound conditions offluctuating sound of AVAS sound model. We created a total of 66 AVAS sound models combining thesound stimuli conditions of Table 1.
60

Sound pressurelevel distance

504030Peak frequency Ⅰ(Fixed frequency)

2010

Peak frequency Ⅱ

0100

1000
Frequency [Hz]10000

Figure 1 – Example frequency spectrum of AVAS sound model
Table 1 – Sound stimuli conditions of AVAS sound model
Peak frequency I,

Peak frequency II,

Sound pressure level

Modulation

Amplitude

HzHz

distance, dB

cycle, smodulation factor

0 (Steady sound)
2500
630

3150
4000

0.25
0

0.5

0.45

10

1

0.9

1.5
22.2 Impression evaluation experiment
We conducted the impression evaluation experiment in the soundproof room (floor area 2.7 m 2,
floor height 2.3 m, background noise level L A = 30 ± 2 dB) shown in Figure 2. The subject sat thechair that set in the center of soundproof room, and the subjects was exposed the AVAS sound from7170

the back. The environmental sound recorded in a quiet residential area was emitted from thesub-speaker, and after 20 seconds, the AVAS sound model was emitted for 20 seconds from the mainspeaker. After that, the subject filled out the psychology questionnaire, and after one minute ofresting time, one experimental condition finished. To measure the impressions of the AVAS soundmodel, we used the SD method composed of 13 adjective pairs, such as "discomfort - comfort". The
rating scale for the SD method consisted of seven grades. The volume of the AVAS sound model wasset to 55 dB equivalent sound level (L Aeq,20s ). The volume of the environmental sound was set to 45
dB equivalent sound level (L Aeq,40s ). Thirteen healthy university students with an average age of 22.3
years took part in this experiment.
Main speaker

Sub speaker

1650 mm

Subject

Sound level meter
1650 mmFigure 2 – Experimental environment of impression evaluation experiment
2.3 Perceptual distance experiment
We performed the perceptual distance experiment in the multipurpose test track for automotive
vehicles of College of science and technology, Nihon University. The multipurpose test track is astraight road with a total length of 618 m and a width of 30 m, and an asphalt concrete pavement.
A-weighted sound pressure level (L A) of the environmental sound measured during this experiment
was 45.6 ± 2.2 dB (mean ± standard deviation). In the perceptual distance experiment, HEV mountedwith AVAS model approached from the back of subject at the veichle speed of 10 km/h, and thedistance between the subject and HEV when the subject perceived HEV was measured. We examined
the influence of differences in pedestrian listening situation on perceptual distance. The listening
situation was set four kinds of combination of standing/walking state and with/without smartphone
operation. Subjects were instructed to browse news sites (Google News or Yahoo! News) with one
hand when operating smartphone. Figure 3(a), (b) show the measuring method of perceptual distance.
The measurement method of the perceptual distance is different in standing state and walking state.
The experimental course set up in the multipurpose test track assumes a residential road where thepedestrian walkway and roadway are not separated. The perceptual distance P S in the standing state
shown in Figure 3(a) is obtained from Equation (1).
(1)
PS ൌ 100 - Vt
Where, V is the vehicle speed, t is time from vehicle start to subject's vehicle perception. The
perceptual distance PW in the walking state shown in Figure 3(b) is obtained from Eauation (2).
(2)
PW ൌ ሺ50 + H) - Vt
Where, H is the walking distance, V is the vehicle speed, t is walking time. Twenty four healthy
university students with an average age of 22.3 years took part in this experiment.

3. RESULTS AND DISCUSSION
3.1 Results of factor analysis and impression evaluation of AVAS
Table 2 lists the factor loading obtained from a factor analysis based on the SD method
concerning the impression evaluation experiment of AVAS sound model. The first factor was7171

100 [m]Subject

50 [m]

Start

H

Subject

2 [m]

V ×t

2 [m]

PS

V ×t

PW(a) Standing state(b) Walking stateFigure 3 – Outline of perceptual distance experimentinterpreted as a clarity factor indicating the clarity of sounds such as "sharp" and "clarity". The
second factor was interpreted as a comfort factor indicating the comfort sounds such as "like" and"comfort". Figure 4 shows the relationship between the average factor loadings of first factor andsecond factor. These AVAS sound models satisfy clarity and comfort if the average score of each of
the first factor (clarity factor) and the second factor (comfort fact or) are both positive. Table 3 lists
AVAS sound models that satisfy both the clarity and the comfort shown in Figure 4.
Table 2 – Factor loadings of impression evaluation experiment
Adjectives
Factor 1
Factor 2
Factor 3
Sharp - Blunt
0.820
0.006
0.000
Clear – Blurry
0.761
0.078
0.155
Light – Heavy
0.701
0.058
-0.308
New – Old
0.622
0.061
0.029
Flashy – Plain
0.554-0.073
0.311
Early – Slow
0.412
-0.2190.054
Like – Dislike-0.0490.903
0.114
Comfort - Discomfort-0.1120.875
0.107
Clean – Dirty
0.476
0.515
-0.044
Smooth㸫Rough
0.148
0.492
-0.071
Quiet – Noisy-0.1570.438-0.452
Large – Small-0.1520.120
0.679
Powerful㸫Unsatisfactory
0.238
0.013
0.577
Contribution ratio
27.26%
18.47%
5.91%
Cumulative contribution ratio
27.26%
45.74%
51.65%<---

Second factor -->(Comfort factor)

㸩1

Steady soundAmplitude modulation factor 0.45
Amplitude modulation factor 0.9

0−1
0−1

<---

㸩1

First factor --->
(Clarity factor)

Figure 4 – Relationship between first factor and second factor

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Table 3 – AVAS sound models that satisfy both the clarity and the comfort
Sound pressure
Peak frequency II,
Amplitude modulation
Modulation cycle,
level distance,Hzfactor
sdB
0
1
2500
00.45
1.5
0.9
100.5
100.45
1.5
3150
20.9
10.25
100.45
12
00.25
00.45
0.5
10.9
1
4000
0
0.5
1100.45
1.5
20.9
13.2 Results of perceptual distance measurement
Figure 5 shows the average values of perceptual distance of the standing and walking stateobtained from the perceptual distance measurement experiment. According to Figure 5, theperceptual distance tends to be longer as the peak frequency II is higher in many conditions.
Statistical significance was examined by one-way analysis of variance and Tukey-Kramer's multiple
comparison test to clarify differences in peak frequency II at the same sound pressure level distance.
As a result, significant differences (*:P<0.05, **:P<0.01) were found among the peak frequencies II
except for walking state with smartphone operation with a sound pressure level distance of 10 dB.
Moreover, when the same test as described above was performed on the condition of the modulatio ncycle at the same amplitude modulation factor, no significant difference was found due to thedifference in the listening situation.

4. DESIGN INDEX OF AVAS SOUND
4.1 Suggestion of minimum avoidance time for AVAS design
In this study, as a criterion to evaluate the measured perceptual distance, we set the minimum
distance that is reference distance necessary for the pedestrian to recognize the approaching vehicle
and safely taking evasive action. The minimum avoidance time was referred to the whole bodyreaction time and the average walking time database. The minimum avoidance time T proposed in thisresearch is calculated from Equation (3).

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Perceptual distance [m]

5040

*

302500Hz
3150Hz
4000Hz******WalkingstateStanding
state**:P<0.01* :P<0.05****

WalkingstateStanding
state

*
**

**

20
100StandingstateStanding
state

WalkingstateWithout smartphone
operation

With smartphone
operation

Without smartphone
operation

WalkingstateWith smartphone
operation(a) Sound pressure level distance 0 dB(b) Sound pressure level distance 10 dB
Figure 5 – Measurement results of perceptual distanceT ൌ T1 ൅ T2 ൅ T 3(3)
Where, T1 is the whole-body reaction time, T2 is the avoidance action time (the time required to move 1 mestimated from the average walking time), and T3 is the face around time. In this study, T1 = 0.7 s, T2 = 1.2s, T3 = 1.0 s, and in consideration of the maximum value of the above database, T = 3.0 s was set as theminimum avoidance time. The reference distance PSTD is calculated from Equation (4) by multiplying theminimum avoidance time T obtained from Equation (3) by the vehicle speed V. The reference distance is8.4 m because the vehicle speed V in this experiment is 10 km/h (2.8 m/s). If the measured perceptual
distance exceeds the reference distance, it means that the pedestrian notices the approach of the vehicle andcan safely avoid.
(4)
PSTD ൌ T ൈ V

Perceptual distance [m]4.2 Proposal of AVAS sound in consideration of listening situation
In this study, we considered that the AVAS model is not easily affected by the listening situation when
the perceptual distance does not change even if the listening situation changes. Figure 6 shows thedifference in perceptual distance between the condition of standing state without smartphone operation withthe longest average perceptual distance and the condition of the walking state with smartphone operation of
the shortest average perceptual distance. The sound conditions indicated by mark † in Figure 6 are the top
two of the AVAS sound models in which the difference in perceptual distance is small and which is noteasily influenced by the difference of listening situation. The AVAS sound proposed in this research needs
to have clarity and comfort in addition to the perception exceeding the reference distance. Therefore, the50

Perceptual distance differenceStanding state, Without smartphone operation
Walking state, With smartphone operation

403020††

1002

10.45

0.9315000.5

200.45

0.5

120.9
4000

Modulation cycle [s]Amplitude modulation factor
Peak frequency II [Hz]Figure 6 – Perceptual distance difference at sound pressure level distance 0 dB7174

AVAS sound model satisfying both the sound conditions distributed in the first quadrant of Figure 4
and the sound conditions of the mark † shown in Figure 6 was examined. As a result, it was foundthat the AVAS sound model of the sound conditions of the mark † had clarity and comfort.

5. CONCLUSIONSIn this research, we examined the AVAS sound model that has clarity and comfort, and makes it
possible for pedestrians to recognize the approach of vehicles when the listening situations are different.
The conclusions obtained are shown below.
(1) The factor structure of the AVAS sound model is divided into clarity factor and comfort factor, andthese factors can explain about 45% of whole. The extracted two factors are the same in all the AVAS
sound models.
(2) The clarity and the comfort of AVAS sound are improved when the amplitude modulation factor of thenotification sound is set to 0.45.
(3) The perceptual distance tended to increase as the peak frequency II increased when comparing thesame condition of sound pressure level distance. This tendency is the same in different listening
situation.
(4) We estimated the minimum avoidance time that pedestrians notice the approach of the vehicle and areable to safely avoid based on the human whole-body reaction time and average walking time. Theminimum avoidance time was multiplied by the vehicle speed to calculate the reference distance,
which is the minimum required distance for safe avoidance.
(5) The sound condition of the AVAS sound model that satisfies perception, clarity and comfort when thepeak frequency I is fixed at 630 Hz is the peak frequency II 4000 Hz-sound pressure level distance 0dB-modulation cycle 1 s-amplitude modulation factor 0.9, the peak frequency II 3150 Hz-soundpressure level distance 0 dB-modulation cycle 2 s-amplitude modulation factor 0.45.

ACKNOWLEDGEMENTS
This work was supported by JSPS KAKENHI Grant Number JP16H03021.
REFERENCES
1. Guideline for the Approaching Vehicle Audible System. Ministry of Land, Infrastructure, Transport andTourism (MLIT), 2010. (in Japanese)
2. United Nations: UN Regulation No.138 on Quiet Road Transport Vehicles (QRTV), 2016.
3. Yamauchi K. Questionnaire Survey on the Encounter Experience with Quiet Vehicles. Proc Inter-noise
2017, 2017.
4. Kurakata K, Mizunami T, Matsushita K, Ashihara K. Statistical distribution of normal hearing
thresholds under free-field listening conditions. Acoust. Sci. & Tech., 2005; 26(5): 440-446.

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