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Title

Effects from Spatial Cues on Detectability of
Alarm Signals in Car Environments

Author(s)Kuroda, Naoki; Li, Junfeng; Iwaya, Yukio; Unoki,
Masashi; Akagi, Masato

Citation

2009 International Workshop on Nonlinear Circuits
and Signal Processing (NCSP'09): 45‑48

Issue Date

2009‑03‑01

Type

Conference Paper

Text version

publisher

URL

http://hdl.handle.net/10119/9967

RightsThis material is posted here with permission of
the Research Institute of Signal Processing
Japan. Naoki Kuroda, Junfeng Li, Yukio Iwaya,
Masashi Unoki, and Masato Akagi, 2009
International Workshop on Nonlinear Circuits andSignal Processing (NCSP'09), 2009, pp.45‑48.

Description

Japan Advanced Institute of Science and Technology

2009 International Workshop on Nonlinear Circuits and Signal Processing
NCSP'09, Waikiki, Hawaii, March 1-3, 2009

Effects from spatial cues on detectability of alarm signals in car environments
Naoki Kuroda1 , Junfeng Li1 , Yukio Iwaya2 , Masashi Unoki1 , and Masato Akagi1
1) School of Information Science,
Japan Advanced Institute of Science and Technology
1-1 Asahidai, Nomi, Ishikawa 923–1292 Japan
Phone/FAX:+81-761-51-1699(Ex.1391)/+81-761-51-1149
Email: {n-kuroda, junfeng, unoki, akagi}@jaist.ac.jp

Abstract
It is crucial to correctly detect alarm signals in noise conditions, for instance, especially in car environments. However, alarm signals are possibly masked by noises from engine, friction between tires and road, etc., in car environments. To design the alarm signals that can be easily detected,
it is necessary to first understand the perceptual characteristics of alarm signals in noisy environments. In this paper,the masked thresholds of alarm signals in the presence ofcar noise were measured in the virtual acoustic environments
generated by using head-related transfer functions (HRTFs).
The main conclusions are: a) when the frequency component of alarm signals is 1.0 kHz, interaural time difference(ITD) and interaural phase difference (IPD) have great effecton the detectability of alarm signals in car noise; b) when thefrequency component of alarm signals is 2.5 kHz, interaurallevel difference (ILD) except ITD and IPD also plays an important role in alarm signal detection when the signal is fixedin the front of listener.

2) Research Institute of Electrical Communication,
Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai 980-8577 Japan
Phone/FAX:+81-22-217-5461 / +81-22-217-5535
Email: iwaya@riec.tohoku.ac.jp

ITD and ILD are regarded as the significant spatial cues. On
the other hand, according to varying the frequency components which signals compose, binaural masked level difference (BMLD) occurs at the same time. For the occurrence of
BMLD, IPD is regarded as the significant spatial cue. It has
been confirmed to use both ITD and IPD as the spatial cues
for perception of alarm signals in car noise[3][4]. The effectiveness of ILD has not yet considered in these conditions,although ILD has been known as one of the significant spatial
cues for signal detection in free sound field.
In this study, the listening experiments were conducted toclarify the effectiveness of the spatial cues for perception ofalarm signals in car environments. The experiments werecarried out in the virtual anechoic environments by using
HRTFs, which can simulated presentation of loud speakers
in an anechoic room. As the advantages of using HRTFs, it ispossible to extract ITD, IPD, and ILD respectively and easily
control presentation directions of a signal.
2. Individualization of HRTF

1. Introduction
Alarm signals are sounds to provide informative notice tousers. These signals convey a variety of information such as
starting, ending, and alarm information to users. These signals are appropriately used for many purposes. It is important
to detect them correctly in many sceneries. There are, however, cases where alarm signals cannot be correctly perceived
in real environments due to noises. In car environments, forexample, alarm signals are possibly masked by noises, andresulting in potential critical accidents in some cases. Therefore, the method for presenting alarm signals to be perceived
accurately for drivers must be demanded. For this purpose,
we must obtain fundamental data about perception of alarm
signals in real environments, in particular, car environments.
For signal detection in the presence of noise, Ebata et
al. reported that the detectability of signals can be improved by using directional information[1]. Moreover, in
free sound field, Saberi et al. showed the detectability of signals was improved when the signal and masker were spatially separated[2]. This phenomenon is referred as to spatial
release from masking (SRM). For the occurrence of SRM,

HRTF is the acoustic transfer function from a sound source
to an ear drum of a listener. Virtual sound presented to the listener from any point in three-dimentional space can be simulated by convoluting stimuli with impulse response of HRTFs.
However, suitable HRTF is needed for each listener because
HRTFs depend on physical characteristics of each listener.
Otherwise, the ability in localizing sound images definitely
decreases. Therefore, we must individualize HRTF for eachlistener to carry out listening experiments.
In this study, we find out the individualizing HRTF for eachlistener using a determination method of optimum impulseresponse by sound orientation (DOMISO), which based ona tournament-style listening experiment[5]. DOMISO is theindividualized method of HRTF which based on subjective
evaluations by using tournament among HRTF data for listeners. 114 HRTF data which was obtained in the anechoic
chamber in Tohoku university were used to individualize
HRTFs for each listener.
Localization experiments of sound sources were carriedout, in order to objectively evaluate whether the individualized HRTFs obtained by using DOMISO were suitable. The
virtual sound sources were set 0◦ to 350◦ at the step size of- 45 -

10◦ , and stimuli were randomly presented to listeners.
The evaluation criteria were set over 70% when the rangeof correct answers was ±10◦ , over 90% when the range ofcorrect answers was ±20◦ , and under 10% in front-back confusion error. Only the listeners who satisfied the evaluation
criteria participated in the listening experiments.
3. Listening experiments
It is important to clarify how spatial cues effect perceptionof alarm signals in noisy environments. As first step, we carried out the listening experiments under the same condition
as that of Saberi et al.[2] (Experiment I). As second step, we
carried out the listening experiments for perception of alarm
signals in car noise environments (Experiment II).
Figure 1: Illustration of difference in arrival time between
both ears of a sound wave from a sound source in a directionof the angle θ radians for the observer.

3.1. Experiment I
3.1.1. Purpose
ITD and ILD have great effect on SRM. Saberi et al. carried out listening experiments by using loud speakers in an
anechoic chamber. As their results, in free sound field, as
the difference of presentation directions between the signal
and noise increased, the amount of SRM was bigger and bigger. The maximal degree of masking release was about 14
dB[2]. For the occurrence of SRM, Nakanishi et al. showedthat about 7 dB of masking release was caused by ITD in asound-proof room[3]. Therefore, ITD is a significant spatial
cue for perception of the pulse signals in white noise. However, the effectiveness of ILD has not yet considered. In orderto clarify the effectiveness of ILD for the detectability of signals in noise, we carry out the experiments by using individualized HRTF. From the above-mentioned, the purposes of thisexperiment are :Figure 2: Environment for the masking experiments.
1. To confirm whether the virtual acoustic environment
generated by using individualized HRTF to the particular participant can simulate the presentation using loud[m] is the path difference from sound source to both ears. In
speakers in an anechoic chamber.
this paper, r was set to 0.09 m and c was set to 343.5 m/s. Pre2. To confirm how large degree ILD effects SRM in the sentation directions of the target signal (or the masker) were
simulated environment.
varied from 0◦ to 90◦ at the step of 15◦ , the direction in frontof the listeners was 0◦ . In this paper, the configurations of thetarget signal and the masker are assumed to be S m Nn (m=0,3.1.2. Methods
15, …, 90, n=0, 15, …, 90). For example, S 60 N0 represents
A pulse train signal, which was composed of 62.5-µs 100 that the arrival directions of the target signal and masker arerectangular pulses per 1 s, was used as the target signal. White 60◦ and 0◦ , respectively.
noise was used as the masker. Stimuli consisted of a 1-s target
The arrival directions of the sound sources were varied
signal and a 2-s masker. Here, the sampling frequency was 48
byconvoluting the stimuli with the corresponding HRTF for
kHz.
each
direction when ITD and ILD were used as the spatial
Figure 1 shows the control method of arrival directions ofcues.
the sounds by varying ITD which was calculated by
Figure 2 shows the diagram of the apparatus used in thed r(θ + sin θ)
experiment.
The experiment was carried out in a sound-proof
IT D = =(1)ccroom by using Tucker-Davis Technologies (TDT) SystemIII.
where r in [m] is the radius of head, θ in [rad] is the direction Stimuli were presented to each listener through ear phonesof the sound source, c in [m/s] is the sound velocity and d in (MDR-EX90SL).

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3.1.3. Procedure

controlling ITD (Signal move)controlling ITD (Noise move)in environment by using HRTF (Signal move)in environment by using HRTF (Noise move)2

3.1.4. Results and Discussion
Figure 3 shows the results of the mean masked thresholds
at each direction for perception of the pulse train signals inwhite noise. The vertical axis shows the relative masked
thresholds which are normalized by the masked thresholds
at S 0 N0 . The horizontal axis shows azimuth of the pulse train
signal or the white noise. The blue lines show the results under the conditions that ITD was used as the spatial cue, andthe red lines show the results under the environment by using individualized HRTF. In addition, the solid lines show theresults of the S m N0 (m=0, 15, …, 90), and the dotted-line
show the results of the S 0 Nm (m=0, 15, …, 90). The error
bars show standard deviation of relative masked thresholds
for each presentation direction. These results showed that almost the same tendency observed as the previous research[3],when the listeners used ITD as the significant spatial cue. In
the S m N0 , ITD is used as the significant spatial cue because
the blue and red lines are the same tendency. There is therange that the masked thresholds in the red line are higher
than that in the blue line. This means that the effectivenessof ITD decreases by adding the effectiveness of ILD, resulting in proceeding the interaural differences becomes difficult,and the masked thresholds slightly increase. In the S 0 Nm , themaximum degree of masking release is about 16 dB in the virtual acoustic environment generated by using individualized
HRTF, and this almost equals to that of Saberi et al.[2]. From
the above-mentioned results, we found that the experiments
by using individualized HRTF can simulate the experiments
by using loud speakers in an anechoic chamber and ILD has

Relative masked threshold (dB)In this experiment, the masked thresholds of the target signals by the masker were measured by using the method oflimits. There are descending and ascending series in themethod of limits. In descending series, at the beginning of
the experiment, the sound pressure level of the target signal in
the stimuli was chosen randomly from the range that the listener can distinctly detect the target signal. Then, the soundpressure level of the target signal was varied from high to low
at the step of 1 dB. In ascending series, at the beginning of
the experiment, the sound pressure level of the target signal
in the stimuli was chosen randomly from the range that thelistener cannot distinctly detect the target signal. Then, thesound pressure level of the target signal was varied from low
to high at the step of 1 dB. In addition, the starting positionof the target signal in the stimuli was chosen randomly. The
sound pressure level of the masker was fixed to 65 dB at the
beginning of the experiments. Both descending series and ascending series were carried out 10-trials respectively. Whenthe difference in the mean of each series was 2 dB or less, themasked threshold was determined as the mean of all measurements.
Six graduate students aged from 23 to 26, five males andone female, participated in this experiment. All had normal
hearing (15 dB HL from 0.125 to 8 kHz) and experiences inattending the other psychoacoustical experiments.

0
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−8
−10
−12
−14
−16
−18
−20

0

20

40

60

80

100

Azimuth of the click sound or the white noise (degree)

Figure 3: The mean masked thresholds as function of the presentation direction of the signal or masker for perception of
the pulse signal in white noise.

great effects.
3.2. Experiment II
3.2.1. Purpose
In the previous papers, the effectiveness of ITD and IPD forperception of alarm signals in car noise, was investigated[4].
As the results, when IPD of the signal or noise was homophasic, the detected limit of the alarm signals increased, in contrast, when IPD of the signal or noise was antiphasic, the detected limit of the alarm signals decreased. From these results, it was important to use both ITD and IPD as the spatial
cues for perception of the alarm signals in car environments.
However, the effectiveness of ILD has not yet been considered. Therefore, the purpose of this experiment is to investigate how ILD effects the detectability of the alarm signals in
car noise.
3.2.2. Method
In this experiment, the target signal and masker were thealarm signals and car noise respectively. Alarm signals were
the signal conveyed most warning provided JIS S 0013[6].
These signals had the repeated patterns of ON and OFF (ON= 0.1 s, OFF = 0.05 s) for 1 s. The frequency components of
the alarm signals were 1.0 and 2.5 kHz. The car noises wererecorded in a car interior with the window open while the car
was traveling at 60 km/h. The sampling frequency was 48
kHz.
The setting method of ITD and the environment of this experiment are the same as those in Experiment I.
3.2.3. Procedure- 47 -

Procedure was the same as in Experiment I.

Figure 4 shows the results of the mean masked thresholds
at each direction for perception of the alarm signals in car
noise. When the alarm signal or car noise was provided thelisteners to ITD as the significant spatial cue, we observedthat ITD and IPD effect the detectability of the alarm signals in car noise. These results showed that almost the sametendency in the previous results[4] was observed. However,there is difference between the amount of masking release at
the S m N0 and S 0 Nm . This means that the extent of effectiveness of ITD and IPD is different between these conditions.
For the virtual acoustic environments generated by using individualized HRTF, the amount of masking release was smaller
than that of the previous studies in the S m N0 condition[4]. We
reconsider that the calculation of interaural correlation becomes difficult due to the effect of ILD. On the other hand, interaural correlation could be calculated in the S 0 Nm because
the sound pressure level of the alarm signals equals in bothears. In the S 0 Nm , the amount of masking release for 1.0
kHz is almost the same tendency as in the virtual acoustic
environment generated by using individualized HRTF. Thismeans that the listeners did not use ILD as the significant
cue. In contrast, amount of masking release for 2.5 kHz signal is greater than that of the previous study[4]. Hence, thedetectability of the alarm signals in car noise is improved byusing ILD as the significant cue, and we can then observe
the occurrence of BMLD by using IPD. In a summary, whenfrequency component of the alarm signal is 2.5 kHz, we clarified that ILD has great effects for the perception of the alarm
signals in car noise at the S 0 Nm .

[2] K., Saberi, Dostal, L., Sadralodabei, T., Bull, V., and Perrot, D., R., “Free-field release from masking,” J. Acoust.
Soc. Am., vol. 90, no. 3, pp. 1355–1370, 1991.
[3] J., Nakanishi, M., Unoki, M., Akagi, “Effect of ITD andComponent Frequencies on Perception of Alarm Signals
in Noisy Environments,” J. Signal Processing, vol. 10,
no. 4, pp. 231–234, 2006.
[4] H., Uchiyama, M., Unoki, and M., Akagi, “Improvement
in detectability of alarm signal in noisy environments by
utilizing spatial cues,” Proc. WASPAA2007, pp. 74–77,
New Paltz, NY, 2007.
[5] Y. Iwaya, “Individualization of head-related transfer
functions with tournament-style listening test: Listening
with other’s ears,” Acoust. Sci. & Tech., vol. 27, no 6, pp.
340–343, 2006.
[6] JIS S 0013, Guidelines for the elderly and people withdisabilities – Auditory signals on consumer products.,
2002.

2

controlling ITD(Signal move)controlling ITD(Noise move)in environment by using HRTF(Signal move)in environment by using HRTF(Noise move)0

Relative masked threshold (dB)

3.2.4. Results and Discussion

4. Conclusions

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(a) 1.0 kHz−12

In this study, we measured the masked thresholds of thealarm signals in the virtual acoustic environment generated byusing individualized HRTF. As a result, when the frequency
component of the alarm signal was 1.0 kHz, we did not observe that ILD effects SRM in both the S m N0 and S 0 Nm .
While, we found that the effectiveness of ITD and IPD was
observed in these conditions. When the frequency component of the alarm signal was 2.5 kHz, we observed that ILD
had great effects SRM in the S 0 Nm . As the summary, in thispaper, we clarified the followings.

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0

20

40

60

80

100

Azimuth of the alarm signal or the car noise (degree)controlling ITD(Signal move)controlling ITD(Noise move)in environment by using HRTF(Signal move)in environment by using HRTF(Noise move)

Relative masked threshold (dB)2

1. When the frequency component of the alarm signals was
1.0 kHz, ITD and IPD were used as the significant spatial cues for perception of the alarm signals in car noise.

0
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−8
−10

(b) 2.5 kHz−12

2. When the frequency component of the alarm signals was
2.5 kHz, ITD and IPD were used in the S m N0 , ITD, IPD,
and ILD are used as the significant spatial cues for perception of the alarm signals in car noise.
References

−14
0

20

40

60

80

100

Azimuth of the alarm signal or the car noise (degree)

Figure 4: The mean masked thresholds as function of presentation directions of the signal or masker for perception of thealarm signal in car noise : (a) 1.0 kHz and (b) 2.5 kHz.

[1] M. Ebata, T. Sone, and T. Nimura, “Improvement of hearing ability by directional information,” J. Acoust. Soc.
Am., vol. 43, no. 2, pp. 289–297, 1968.

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