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

Effect of individualized near-field head-related transfer functions ondistance perception in dynamic virtual auditory display
Guangzheng YU1; Liliang WANG1; Linda LIANG1, 2; Bosun XIE1, 2; Jianliang JIANG1; Chengyun
ZHANG3, *1

Acoustic Laboratory, School of Physics and Optoelectronics, South China University of Technology, China
2State Key Laboratory of Subtropical Building Science, South China University of Technology, China
3

School of Mechanical & Electrical Engineering, Guangzhou University, China

ABSTRACT
Virtual auditory displays (VAD) aim to create virtual sources at various directions and distances inheadphone reproduction by using binaural synthesis based on head-related transfer functions (HRTFs).
Incorporating the variation of binaural signals caused by head turning in three dimensions of freedom, adynamic VAD reduces front-back and up-down confusions, and improves the externalization of perceived
virtual sources. It is well known that individualized far-field HRTFs can also help to improve the direction
perception. In the present work, a virtual source localization experiment is conducted to further examine theeffect of individualized near-field HRTFs on distance perception in dynamic virtual auditory display. Results
indicate that dynamic binaural synthesis with both individualized and non-individualized near-field HRTFs
can create perceived virtual source at various target distance within 1.0 m deviating from the median plane,
especially at the lateral directions. Individualized HRTFs have little influence on distance perception, but
improve the accuracy of direction perception in terms of elevation errors.
Keywords: Individualized HRTFs, Distance perception; Dynamic virtual auditory display; Binaural hearing

1. INTRODUCTION
Sound source localization ability of human being, including direction and distance localization,
plays an important role in our daily life. For instance, spatial hearing ability helps a listener toimprove speech intelligibility when the noise and signal sound source are separate at distance, and to
avoid obstacles in traffic environment, etc. Generally speaking, the ability of the human auditory
system to estimate sound source distance is poorer than that of direction [1, 2]. Therefore, comparedwith direction perception, the distance perception of human beings has been paid less attention inprevious researches [3].
Our ability to estimate physical distance of a sound source is influenced by a number of factors,
including sound intensity variation with the sound source distance, reflections and binaural cues fornearby sound source, dynamic cue caused by head movement, etc. [4, 5]. Sound intensity is the first cue
taken into account: the weaker the intensity, the farther the source shall be perceived. In an ideal freefield, where sound pressure and sound intensity levels are assumed to be equal, the intensity of anomnidirectional sound source decays at 6 dB for each doubling distance and can thus be predicted bya 1/r pressure attenuation law [6, 7], where r is the distance from the source. Therefore, the soundintensity variation is also called distance cue [5]. Among the above factors of distance perception, theeffect of binaural cues is important especially for a nearby sound source distance perception in an
ideal anechoic chamber and can be assessed via the virtual auditory display (VAD) based on HRTFs [2].
The dynamic cue refers to the dynamic variation of HRTFs caused by relative position variation
between the sound source and the subject during the subject’s head movement. Previous researches
proved that, when the dynamic cue is contained in the VAD system, a listener could perce ive theelevations more easily, and the front-back and up-down confusions will be reduced during perceiving
or localizing a virtual sound source [8, 9]. However, the effect of individualized near-field HRTFs on*

Corresponding author: cyzhang@gzhu.edu.cn5247

distance perception in dynamic VAD has not been reported yet.
HRTF describes the acoustic transmission from a point sound source to a subject’s ear drums in thefree-field. For a sound source distance to head center larger than 1.0 m, HRTFs are approximately
independent of source distance and are called far-field HRTFs. While for a source distance less than
1.0 m, HRTFs are relevant to source distance and are called near-field HRTFs [10, 11]. As defined,
HRTF is an individualized physical quantity, and thus the individual cue of a subject’s HRTFs could
have influence on the distance perception of a sound source. Actually, individualized HRTFs are vital
to virtual reproduction system and sometimes are required in order to create a better virtual soundenvironment [12]. Some researchers have found that, via a statistic VAD system, individualized HRTFs
help a subject to perceive the sound source direction better than non-individualized HRTFs [13], but
this is not the case for distance perception. A recent work is addressed to assess the effect ofindividualized HRTFs on distance perception via a static virtual reproduction platform [14]. However,there are few researches which involve comparison of different effects of individualized andnon-individualized HRTFs on auditory distance perception of a nearby sound source via a dynamicVAD system (namely the dynamic cue and individualized HRTFs have not been concerned
simultaneously).
Therefore, we mainly aim to find out the effect of individual cue on auditory distance perception for
nearby sound sources by means of psychoacoustic experiments via a dynamic VAD platform. In order
to compare the difference, both individualized HRTFs of 4 subjects and non-individualized HRTFs ofa KEMAR (Knowles Electronics Manikin for Acoustic Research) have been adopted.

2. METHODS
2.1 Participants
Four volunteers, all males aged around 25, have completed this experiment as listeners. They all
have normal hearing and all have experiences of psychoacoustic experiments.
2.2 HRTFs of the subjects
HRTFs used in the dynamic VAD system are calculated by using the fast multi-pole accelerated
boundary element method (FMBEM), with a high spatial resolution of 1 both in the azimuths andelevations, and distance sampling at 0.2 m, 0.3 m, 0.5 m and 1.0 m. In order to verify the effect ofindividualized HRTFs on distance perception, individualized HRTFs of the four human subjects andnon-individualized HRTFs of KEMAR have been numerically calculated via the same numericalsimulation method (namely FMBEM). Before numerical simulations of HRTFs, the 3D digitalmodels of the four subjects and KEMAR have been scanned by using a laser scanner whose productname is UniSCAN. The distances of HRTFs include 0.2 m, 0.3 m, 0.5 m, and 1.0 m, respectively.
In the psychoacoustic experiments, non-individualized HRTFs of KEMAR were used for all
listeners as control group. HRTF’s representation in the time domain is the head-related impulse
response (HRIR), which will be directly applied into the signal processing of stimulus generation,
see section 2.4 for more details.
2.3 Environment
All experiments were performed in a listening room of our laboratory which is in compliancewith the international electronic standards (IEC), with background noise not more than 30 dBA andreverberation time less than 0.15 sec.
2.4 Stimulus generation
Five directions including 0, 45, 90, 135 and 180, and four distances including 0.2 m, 0.3 m,0.5m, 1.0 m, have been chosen in these experiments, which makes 20 direction-distance combinations,
namely 20 individualized and 20 non-individualized HRIRs for each listener. A mono signal of white
noise with full frequency was convolved with each participant’s individual ized HRIRs andnon-individualized HRIRs, respectively, to generate the stimuli signals of virtual sound sources atvarious distances and directions. Stimulus convolved by individualized and non-individualized HRIRs
were used as the compare group and control group, respectively. Each stimulus signal has a time lengthof 1.0 sec. It is worthy of attention on the coordinate system used in the psychoacoustic experiment. In
the coordinate system, the head center of a subject is the origin. In the horizontal plane, azimuth 0° is
straight ahead, 90° is direct right and 180° is straight astern, respectively.

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In order to make the individual cue more effective during dynamic virtual reproduction for nearby
sources, all other distance localization cues were excluded, such as the reflections in the room, and thevisual impact, and signal familiarity etc. Therefore, in the experiments, the individual cue, dynamiccue, and the intensity variation of sound pressure with distance changing were included and compared,
which results in two groups of experiments with distance cue (1/r) being included or excluded asfollows,
1) individualized HRTFs and dynamic virtual reproduction;2) non-individualized HRTFs and dynamic virtual reproduction.
If the distance cue needs to be compensated (or equalized), before convolution with the white
noise, the gain coefficients {a i } (i = 1, 2, 3, 4 for four subjects, respectively) should be applied to keep
the sound intensity of subjects’ individualized HRTFs consistent with non -individualized HRTFs ofKEMAR. We set the sound pressure P0 (r0 , 0 ) of KEMAR as the reference sound pressure. The
psychoacoustic experiments have been separated into 5 groups with different azimuths. Therefore, foreach listener, we get gain coefficients ai for each azimuth through calculating the sound energy ratio
between the individualized and non-individualized HRTFs (the reference value) at distance of 0.2 m.
Stimulus signals were presented through plug-in earphones (type Etymotic Research ER2) which
saved the procedure of headphone equalization of binaural virtual reproduction.
2.5 Procedures
As aforementioned in section 2.4, the psychoacoustic experiments were separated into 5 groups
on account of five azimuths of the stimulus signals. For each participant, the sequence of the 5groups was random. There were 4 different auditory distances in each direction. For eachdistance-azimuth combination, both individualized and non-individualized signals were reproduced
via the dynamic VAD system. Each signal was repeated for 5 times. So for each subject, there were
4×2×5 = 40 signals in each direction and they were all presented randomly. During experiments, anelectro-magnetic head tracker (Polhemus FASTRAK) was used to detect the orientation of thesubject’s head, and also record the coordinate of a virtual sound source, where the listener placed theother tracker at the place of the virtual sound source. For each judgment, the participant was allowed
to hear repeatedly not more than 3 times. All signals were presented randomly and the se quenceswere different among the five azimuthal groups.

3. Results and discussions
The results with distance cue included and excluded are shown in Figure 1 and Figure 2,respectively.

Figure 1 – Overall average results and standard deviations of perceptual distance and simulated
distance with distance cue being included.

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Figure 2 –Overall average results and standard deviations of perceptual distance and simulated
distance with distance cue being excluded.
As we can see, the perceptual auditory distance ranges (standard deviations) of Figure 2 are wider
than those of Figure 1, which indicates that the accuracy of auditory distance perception under thecondition of distance cue being excluded is worse. That is to say, by means of virtual soundreproduction, the distance cue plays an important role in distance perception, which is consistent
with results in the references [3-6]. Further, for the situation where the distance was excluded, theeffect of individualized HRTFs on distance perception can be observed. As shown in Figure 2,especially at directions of azimuths 45 and 135, since the individualized HRTFs were adopted, theaverage value is more consistent to the simulated distance and the standard deviation decrease from
20 cm down to 15 cm (even down to 10 cm at specific distances). Some similar results can beobserved in other azimuths. On one hand, the average value of perceptual distance with distance cue
is more consistent with the simulated distance. On the other hand, the standard deviation ofperceptual distance with distance cue is also smaller. However, in other directions, especially infront and back of subjects, the improvement of the perceptual distance is unobvious. The reason
could be the difficulty of distance perception in median plane.

4. CONCLUSIONSIn this work, the effect of individualized HRTFs on distance perception was assessed via thepsychoacoustic experiment that was based on the dynamic VAD system. For comparison, thenon-individualized HRTFs of KEMAR and individualized HRTFs of four subjects have been adopted
to generate stimulus of control group and compare group, respectively. Results show that theauditory distance cue (1/r) mainly affects the distance perception. When the distance cue wasexcluded, the positive effect has been discovered in the dynamic virtual reproductions. In the future,
more volunteers will be recruited and more detailed results will be measured and analyzed.

ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China (Grant No.
11574090) and the Natural Science Foundation of Guangdong Province (Grant No. 2018B030311025).

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