A New Method for Localization Studies

A New Method for Localization Studies
A C U S T I C A acta acustica
Vol. 83 (1997) 1 – 2
S. Hirzel Verlag
A New Method for Localization Studies
Bernhard Seeber
AG Technische Akustik, Institute for Human-Machine-Communication, Technische Universität München, Arcisstr. 21, D-80333 München,
Germany, E-mail: [email protected]
Many scientific studies investigate and technical applications use the acoustical localization in the field of vision. Therefore it is suitable to display the perceived auditory direction by a light point. In formerly known methods subjects use a
hand-held light pointer or a pointer mounted on a revolvable axle in front of them. However, the subject’s motor system
or the optical parallax may influence the results of those techniques. The calibration of the system and data logging
also turn out to be difficult. The proposed new method utilizes a laser pointer with a deflection unit instead, which is
controlled by a computer. Subjects enter the perceived sound direction with a trackball. The laser spot moves according
to the rotation of the ball smoothly on a defined track. A complicated mechanical calibration can be avoided by calibrating the deflection unit by a computer. The intuitive experimental operation and the high resolution of the system
make this method particularly suitable for localization research in audiology, psychoacoustics, and virtual acoustics.
The symmetric, bimodal outlay of the experimental task reduces interaction effects between different modalities. Localization results for variable and fixed initial laser position obtained by this method are presented and compared to
results acquired by other methods.
PACS no. 43.66.Yw, 43.66.Qp, 43.66.Pn
1. Introduction
Acoustical directional displays in real and virtual environments gain in importance by the introduction of multimedia in many fields of everyday life. Applications range
from teleconferencing systems over computer games to
user interfaces in control and surveillance systems. The
multitude of new applications is accompanied by an increased demand of knowledge about auditory localization
in the field of psychoacoustics and audiology or within
the scope of a specific application. All methods for the investigation of auditory localization require the subject to
specify the perceived auditory direction. Known methods
compare two directions, i.e. detect the perceived difference between two directions, or directly point to the direction [1]. Besides the information about the mean apparent
direction the directional scatter is of interest for the acuity
of localization. To provide information on the uncertainty
of localization the testing method must add less variance
to the responses than the sensory task. The response bias
introduced by the method should be small and a quantization of the results should be omitted. The method should
be intuitively to handle and easy to learn for the subject.
A computerized data collection is a prerequisite for a high
data acquisition rate and a fast evaluation of the responses.
Several methods in ego- and exocentric space have been
proposed so far. A simple exocentric method requires the
subject to mark the perceived sound position in a coordinate system on a piece of paper [2]. Using this method
distance information can also be assessed. A projectional
pointer method in an exocentric coordinate system is the
Received 1 January 1995,
accepted 1 January 1995.
GELP-method or “Bochum Sphere”, where subjects indicate the apparent direction on a sphere [3, 4]. This comfortable method covers the whole space and provides fast
responses. However, Djelani et al. [5] point out that the
GELP-technique requires some training and the projection
leads to systematic errors in connection with a reduction
in accuracy. If pointing in a non-body-centered system is
not favoured, pointer methods in the egocentric coordinate
system should be chosen. Hand-, head- and eye-pointing
are extensively studied (hand [5, 6]), (head [5, 7, 8, 9, 10]),
(eye [11, 12, 13, 14]). These natural and intuitive methods achieve a high accuracy in frontal direction despite
the use of the motor system for indication of the auditory direction. At lateral positions, strong systematic errors occur due to limitations of the motor act and intersensory projections. The methods are thus limited mainly
to the frontal sector. Further methods involve the naming
of speaker numbers or the angle. The latter is referred to
as “absolute judgement technique” and covers the whole
space but requires extensive training [15, 16]. The precision of the visual system allows visual pointing to acoustic
targets [17]. In order to minimize errors introduced by the
mapping of auditive coordinates to visual or motorical coordinates acoustic pointers can be introduced [18]. Using
these pointers, care must be taken that subjects use directional instead of timbral cues as a decision variable for the
adjustment of the pointer. However, as the test-stimuli and
pointer directions undergo the same coordinate transformation from acoustic (physical, external) to auditive (perceived) directions, a relative rather than absolute direction
will be displayed in terms of a minimum audible angle
[19]. An unimodal advantage is given if the pointer inputinterface allows no direct relation to visual or motorical
2 Seeber: Localization Method
Vol. 83 (1997)
acta acustica
Equalization of
Digital Soundcard and
for LED’s
I2 Z
I0 Z
Calculation of
2-Channel 16-bit
Figure 1. Apparatus.
Figure 2. Block diagram of the experimental setup.
In this paper a new method for the investigation of localization in the field of vision is proposed which utilizes a
laser spot for displaying the perceived auditory direction.
The position of the laser spot is adjusted by the subject
with a trackball. The laser spot moves according to the rotation of the ball smoothly on a defined track utilizing a
laser scanner. As the trackball permits positioning in two
dimensions, the method can be adopted for investigations
in the horizontal and vertical plane. This fast and accurate
method can be intuitively operated, making a wide range
of applications from audiology to virtual acoustics possible. Moreover, by pointing using the trackball from an
egocentric frame of reference, a decoupling from the motorical system is achieved resulting in a bimodal pointer
method. The initial position of the laser spot can be set
arbitrarily by the experimenter trial by trial. This way subjects loose the perception of their position in the completely darkened surrounding space and thus need to point
relatively to the perceived direction. The system can be
calibrated easily by fixed position points as a computer
does the coordinate transformation between the laser and
the subject’s coordinate system. This relative calibration
against the speaker positions assures a remarkably high
accuracy of the visual directional display. The computer
control permits fully automated experimental runs and an
instant evaluation of the results.
diodes (LEDs) are mounted in a distance of 10 cm concentric in front of each speaker. The speakers are switched
by a custom-made relay-unit utilizing high-speed relays
with a switching time smaller than 2 ms (NAIS Matsushita
TK 1). A driver-unit for the LEDs, a power-supply for the
laser and the relay-unit are controlled by a TTL-MultiI/O-Card (Decision-Computer 82192V) installed in a PCtype computer. Figure 2 shows a block diagram of the experimental setup. Additionally, a 2-channel 16-bit digitalto-analog-converter-card is used (Kolter Electronic DAC
16 dual) which controls two laser-scanner galvanometers
(Cambridge Technology CT 6800 HP, closed-loop) by a
driver unit. The position-repeatability of the galvanometers is better than 0.0011 deg (manufacturer information).
The laser beam is deflected by the galvanometers in xand y-direction and projects a laser spot onto a curtain in
front of the loudspeakers. The curtain is opaque for the
subject’s gaze but acoustically transparent and translucent
for the light of the LEDs. The laser and the LED spot
are of equal color (635 nm), brightness (1 mW laser) and
width (5 mm). The calibration of the x/y-deflection of the
laser beam is done by adjusting the laser spot to the position of the LED in front of each speaker. This way the
pointer coordinate system is aligned to all speaker positions, nonlinear effects of the galvanometer or its drivers
are cancelled out and a maximum pointing accuracy relative to the speaker coordinate system is achieved. The
repetition accuracy at the speaker positions is 0.03 deg, reflecting the deviation between pointing and presentation
coordinate system. The precision of the absolute positions
of the speakers determines the precision and linearity of
the system to about 0.2 deg.
The projected laser spot can be moved smoothly according to the rotation of the trackball on a horizontal track
within 70 deg in front of the speakers. The computer
performs the coordinate transformation between the angle of the laser spot seen by the subject and the deflection
of the laser beam using the calibration data. This coordinate transformation also allows for the position of the laser
scanner being out of the center of the projection cylinder.
As the system utilizes a deflection unit in two dimensions it could be extended for investigations in the vertical
2. Apparatus and Experimental Setup
The apparatus is installed in an anechoic chamber (dimensions L * W * H = 7.5 m * 4.2 m * 2.8 m), which is completely darkened during the experiments. Eleven identical loudspeakers (chassis Nokia 49102 10121/2 in a closed
cabinet) are mounted on a circular tube in a distance of
1.95 m from the center of the head of the subject at ear
level. The speakers span an angle of 50 deg left to 50 deg
right with a spacing of 10 deg. Figure 1 shows the apparatus. The frequency response of each speaker is individually equalized by a 128-point linear-phase FIR-filter
(sampling frequency = 44100 Hz) to 125 Hz – 20 kHz
in 2.5 dB at the subject’s head position. Light emitting
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plane by exchanging the projection cylinder with a sphere
and introducing calibration points at different elevations.
Additionally, the coordinate transformation would have to
be substituted by two- instead of one-dimensional spline
Sound reproduction is done as follows: After the filtering of the sound for the equalization of the speakers
or for virtual acoustics, the digital sound data are written out through a digital soundcard (Sek’d Prodif 32) to a
D/A-converter (DAT Sony DTC 57ES). The speaker signal is amplified (amplifier Sansui AU-X 201i) and calibrated with a voltmeter (B&K 2409) before reaching the
initially described switching unit. The experimental procedure is controlled by a Matlab routine with the help of
customized interface software for the experimental setup.
3. Method and Subjects
In a localization experiment the subject is placed in the
center of the speaker array on a chair. The head is stabilized by a head rest and subjects are instructed not to move
their head. The subjects’ experimental operation and the
fixation of the head is monitored through an infrared camera. During all localization tasks head movements were
not detected by the experimenter in the present experiments. In the beginning of an experiment a light appears
straight ahead for five seconds in the completely darkened anechoic chamber to align the head to the frontal
direction. After a pause of 500 ms a target sound is presented in a randomly selected angle, i.e. played from a
certain speaker. The angles for presentation are distributed
from 50 deg left to 50 deg right in 10 deg intervals. Gaussian white noise (125 Hz - 20 kHz, continuous sound pressure level 60 dB SPL) serves as a target sound which is
divided into 5 pulses (pulse duration 30 ms, duration of
pauses 70 ms, 3 ms Gaussian shaped slopes). After a pause
of 500 ms a light spot appears under a randomly chosen initial angle in the range of 20 deg around the presented physical angle of the target sound. The subject’s
task is to adjust the light spot to the perceived direction of
sound incidence. By pressing one of the trackball buttons
the subject acknowledges the indicated direction and the
light spot disappears. The three trackball buttons might be
used to code the perceived position of the sound source
as in front, inside the head or behind of the subject. After a pause of 500 ms the next trial starts with the presentation of the target sound under a different angle. After testing each of the eleven directions the fixation LED
straight ahead lights up for five seconds to allow subjects a
short pause. Subjects received no feedback about their responses and conducted no training sessions except seven
single trials without feedback in the beginning of each
session for accommodation. The experiment yields quasicontinuously distributed data of the adjusted light angles.
A localization experiment as described with ten repetitions
of all eleven angles of sound incidence (110 trials) takes
about nine minutes.
Seeber: Localization Method
Exp. 1: Twelve subjects, age 23-28, two female, ten
male, participated voluntarily in the localization experiment with variable initial laser direction and conducted 20
trials per direction of sound incidence in two sessions of
ten trials. The Békésy audiometric method showed normal
threshold in quiet within 20 dB in 20 Hz-16 kHz for all
subjects. Three subjects, including the author, were members of the institute, eight subjects were students, and one
subject was invited to the experiments from outside the
university. Two of the subjects had previous knowledge
of the positions of the speakers, further four subjects had
knowledge of a discrete distribution of the speakers.
Exp. 2: To estimate the influence of the initial laser position a second experiment was conducted. The procedure
and the stimuli of exp. 2 were identical to exp. 1, except
for the fact that the initial laser position in each trial was
not varied within 20 deg around the sound direction but
was fixed at 0 deg in front. Eight of the subjects of exp. 1,
two female, six male, age 23-28, participated in the second
experiment and conducted ten trials per sound direction in
one session.
Exp. 3: A third experiment was conducted with seven
of the subjects of exp. 1, one female, six male, age 24-27
to assess the influence of the visual pointing accuracy on
the localization method. Therefore the procedure of exp. 3
was kept identical to exp. 1, but remembered visual targets
instead of sound directions were indicated. The LED’s in
front of each speaker served as visual targets. Likewise in
exp. 1, in each trial a LED in a randomly selected angle lit
up for 500 ms and after a 500 ms pause the adjustable laser
spot appeared within 20 deg around the previously presented direction. The seven subjects conducted ten trials
for each of the eleven directions in one session.
4. Results and Discussion
Figure 3 displays the deviation of the indicated from the
stimulus direction obtained in two localization experiments with wide-band noise pulses, exp. 1 and 2 ,
and with pointing to remembered visual targets, exp. 3 .
The data are presented as medians of individual medians
and medians of individual quartiles. The results of all experiments show a high agreement between presented and
localized direction and a small variability in the displayed
The standard localization experiment, exp. 1, using a
laser spot which is randomized in initial position trial by
trial, yields localized directions which are indicated about
1.5 deg right of the presented direction and slightly overestimated for lateral angles. The sound source at 50 deg right
was localized at 54.9 deg, whereas the source at -50 deg
left was judged to -51.3 deg. The lateral spread of the indicated direction is reflected in the linear factor of 1.05 of a
least-squares line-fit to the median localization responses.
The median distance of the median indicated from the presented direction is 1.6 deg, which represents the mean absolute error. The rank correlation coefficient according to
Spearman for the signed distance of the median localized
Deviation of indicated from presented direction in deg
4 Seeber: Localization Method
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fixed at 0°
−50 −40 −30 −20 −10
10 20
Stimulus direction in deg
acta acustica
Figure 3. Medians and quartiles of results of localization experiments with wide-band noise pulses and visual stimuli given relative to stimulus position. Exp. 1 Random initial angle of laser
pointer position within 20 deg, Exp. 2 Fixed initial laser position of 0 deg in front of the subjects, Exp. 3 Pointing to remembered visual targets; random initial laser spot position. Angles
50 deg left to 50 deg right were tested in 10 deg increments. The data
of Exp. 1 and 3 are shifted horizontally for better distinction.
and presented directions also displays this relationship by
a highly significant correlation with the presented direction ( = 0.94, p = 0.000022). The median upper (lower)
quartile of the individual localization responses, an indication of the average intra-subject variance and the dispersion of localization responses, is 2.3 deg (-2.4 deg).
Exp. 2 was conducted to assess the influence of the initial laser spot position. The results show an underestimation of the lateral angle and a decrease in variance of the
data compared to exp. 1. The most lateral sound sources
at 50 deg were estimated to -44.9 deg and 49,5 deg. The
underestimation of lateral directions can also be expressed
by a linear fit of the medians to the presented directions:
the linear coefficient is 0.95 whereas the additive constant
is 1.6 deg (right). The correlation of the deviation of the
indicated from the presented direction with the stimulus
direction is highly significant and negative, i.e. a positive
lateral shift in the presented angle results in a negative deviation, an underestimation of the angle (Spearmans rank
correlation coefficient = -0.90, p = 0.00016). The rightshift of all indicated directions against the presented ones
is equal to exp. 1. Similar shifts were also observed by
Blauert, who estimates the right-shift to 1 deg [1, p. 41].
The absolute error is 1.9 deg. Besides the underestimation
effect a decrease in variation of the responses is observed
with fixed laser spot position. The median upper (lower)
quartile of the responses is 1.7 deg (-1.7 deg) and about
0.6 deg smaller than with variable laser spot position.
The localization results using this method coincide
with results previously obtained by different methods or
even surpass previous results. Makous and Middlebrooks
[9] observed azimuthal errors of 2 deg in the front and
9 deg at 60 deg lateral in a 2-dimensional localization task
using head pointing after substantial training sessions.
Bronkhorst [8] observed 3 deg of mean absolute azimuthal
error in a similar task, whereas this study found the absolute error to be 1.6 deg (exp. 1). The errors in eye pointing usually turn out to be greater than with head pointing:
Frens and van Opstal [11] as well as Heuermann and Colonius [14] observed an azimuthal error of about 6-8 deg in
the range of 30 deg. The first study also reported an
underestimation with a linear factor of 0.95. Many other
studies reported only 2-dimensional errors without addressing the azimuthal component [5, 15, 16]. The smaller
errors in the present study might be attributed to the bimodal and symmetrical outlay of the laser-pointing experiment as well as the impact of the high accuracy of the
visual system on the direct comparison task with auditory
directions. The proprioceptive interaction in head pointing leads to a strong increase in error with lateral position
which is not observed to this amount in the laser pointing experiment [9]. The azimuthal standard deviations observed with head pointing range from 2 deg in the front
to 7 deg at 60 deg lateral [9]. Bronkhorst [8] found a mean
azimuthal variability of 5 deg. The better variability results
in the present study might be also attributed to the bimodal
advantage, whereas parts of the improvement might result
from the data-analysis method based on quartiles and from
the one-dimensional experimental task. The superiority of
the fixed laser against the variable laser condition (exp. 1
vs. 2) in terms of variability is due to the supply of information of the subject’s position in the surrounding space.
A variation of the initial laser spot position suppresses the
buildup of usable information for this case. The auditory
direction can be evaluated in the fixed condition relative
to the frontal direction which reduces the uncertainty in
the display. This was also reported by the subjects. One
further methodical difference between the fixed and variable method should be noted. The mainly symmetric influence of the laser position on the indicated sound direction
in the variable case (exp. 1) is changed in the fixed case
to a one-sided influence from the frontal, inner position.
Thus, the indication of different lateral positions is not methodically equal as the mean traversed angle differs with
position and memory effects might influence the results.
However, other localization methods, e.g. head pointing,
cannot control the initial position or reduce the described
methodical influence.
The data on auditory localization obtained by a visual
pointer can only be correctly interpreted if the effect of
the visual pointer is discussed. Exp. 3 was designed in the
same way as the standard localization experiment, exp. 1,
but with the difference of pointing to remembered visual
instead of auditory targets. As in exp. 1, the initial laser
spot position was varied to yield comparative data. Figure 3 displays the results in comparison to the auditory
localization experiments. It can be seen that the quartiles are substantially smaller for visual targets and that
no deviation from the presented direction occurs. The indicated directions were tested against the presented directions for equality and no significant deviation was observed (Wilcoxon signed rank test ! -corrected for 11 di-
Vol. 83 (1997)
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Seeber: Localization Method
not significant at 1%). This is also reflected in
the median absolute error of 0.2 deg. Further, the median upper (lower) quartile of the indicated directions is
0.5 deg (-0.6 deg) and thus about 4.6 (3.4) times, or 1.8 deg
(1.2 deg), smaller than in auditory experiment 1 (exp. 2).
When comparing this accuracy with the minimum observable auditory angle of about 1 deg [1, 19] it becomes evident that the visual pointing accuracy itself is adequate
for pointing to auditory targets. The exact reproduction of
visual positions with a movable pointer of variable initial
position requires a mechanism of exact memorization of
the perceived positions as the medians are not shifted significantly. Existing interference effects between the memorized and the pointer position are reduced by the statistically symmetrical, two-sided scattering of the initial
laser positions. Thus, these interference effects rather result in an slight increase in variance than in a shift of the
indicated position. The comparison of the perceived visual pointer position with the auditory coordinates, however, introduces some variance which is difficult to measure. This coordinate mapping from auditory coordinates
to directions in other modalities is inherent to all pointer
methods as they require the subject to compare both directions. The main benefit in this regard of the new method
in contrast to many other formerly known methods is that
it involves only two modalities, auditory and visual. As
no motorical interactions occur and visual interactions are
kept at a minimum, it can be assumed that the interference
from other than auditory modalities is relatively low with
the new method.
Although this method can be used only in the field of vision, the high accuracy of the method and the possibility of
evaluating the variation in the responses give this method
a wide range of applications. Besides applications in virtual acoustics and psychoacoustics, the intuitive and fast
handling of the apparatus and method make this method
particularly suitable for localization research in audiology [20]. In localization studies with hearing impaired
subjects, the initial direction of the laser spot should not
vary around the presented sound direction but should be
straight ahead. Thus it can be assured that the variability in the adjusted direction is a measure for the accuracy
in localization. As hearing impaired subjects gain more
information from monaural level differences between the
trials as normal hearing subjects, the level should be randomized in each trial. Besides that the mean level can be
increased to 70 dB SPL.
The author is indebted to Prof. H. Fastl for his support.
This work was supported by the Deutsche Forschungsgemeinschaft (DFG GRK 267).
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