The role of median plane reﬂectionsin the perception of vertical auditory movement
Florian WENDT, Matthias FRANK, Robert HÖLDRICH
Institute of Electronic Music and Acoustics
University of Music and Performing Arts Graz, Austria
Email: {wendt, frank, hoeldrich}@iem.at

Abstract
This contribution considers how delay alterations of a median plane reﬂection inﬂuence the vertical auditory
movement. Depending on the excitation signal we could show two partly contrasting effects dominating ourperception: For broadband noise signals a delay modiﬁcation between the leading direct sound and the lagging
reﬂection yields spectral cues and the auditory movement is explained by the pitch-height effect. In contrast, results obtained for speech signals indicate that other localization cues induced by the interference pattern of direct
sound and reﬂection inﬂuence our perception. Respective movement directions are deduced from the movementof a physical sound source, suggesting that the auditory system learns to associate interference patterns to soundsource movements.
Keywords: localization, movement perception, median sagittal plane, ﬂoor reﬂection;

1

INTRODUCTION

Changes in level, time of arrival, and spectrum of the ear signals are cues for the perception of sound movement [1]. The relative importance of these time-variant cues is different for the perception of vertical andhorizontal movement. In the horizontal plane, the use of inter-aural level and time differences (ILDs and ITDs)
as localization cues is proven. In the median plane, binaural cues are essentially zero and spectral properties play
an important role in contrast to inter-aural differences.
Sound rendering systems typically use level panning techniques to move virtual sound sources along the horizontal plane. At low frequencies these level differences yield inter-aural time differences resembling those of areal source [2]. Regarding vertical localization, studies could show a similar relation between inter-channel level
differences and perceived elevation [3, 4]. Thus, variations in amplitude between vertical loudspeakers are usedin the same manner to create respective auditory movements, e.g. in Vector-Base Amplitude Panning (VBAP) [3]or Ambisonics [5]. Studies on the use of time differences on the other hand did not show stable localization
curves [6] and they are mostly disregarded in common panning techniques.
The direct sound of an emitting sound source is typically followed by attenuated reﬂections. Lateral reﬂections
contribute mainly to qualitative attributes of the perceived sound [7] without affecting the localization [8]. In
contrast, studies reported vertical shifts in localization for conditions with an additional attenuated sound instancein the median plane [9, 10, 11].
This contribution studies how median plane reﬂections can inﬂuence the perception of vertical movement. Twolistening experiments are conducted with an experimental setup simulating direct sound and median planereﬂections. Experiment 1 considers the inﬂuence of continuous delay alterations of reﬂections from ﬂoor andceiling. Based on the results two signal-dependent perceptual effects are deduced. The movement perceptionof noise signals can be explained by spectral cues eliciting the pitch-height effect. For speech signals there
are indications that binaural cues, caused by the fusion of direct sound and reﬂection, inﬂuence the movement
perception. Experiment 2 examines this latter effect in greater detail. The impact of loudspeaker setup and signal
character is studied and results are compared to vertical amplitude panning.

7911

LS45

h = 1.25 m

LS10

y

θ = 10◦LS0xr = 2.5 m

LS−10
θ = −45◦
LS−30

LS−45

h = 1.25 m

Figure 1. Sketch of the experimental setup in the anechoic laboratory. Loudspeakers LS(10,0,−10,−30) are used toplay back the direct sound and loudspeakers LS(45,−45) simulate an attenuated specular reﬂection.

2

EXPERIMENTAL SETUP

All experiments took place in the anechoic laboratory at our institute. The layout consisted of six verticallyarranged Genelec 8020A loudspeakers in the median plane at elevation angles θ = (45◦ , 10◦ , 0◦ , −10◦ , −30◦ , −45◦ )denoted as LSθ , cf. Figure 1. Loudspeakers LS(10,0,−10,−30) were placed at a distance of r = 2.5 m and loudspeakers LS(45,−45) at 1.8 m to the listening position and each was level- and delay-compensated. The listener’sears were adjusted to h = 1.25 m above ﬂoor loudspeakers LS(−30,−45) . During the experiments, listeners wererequested to face the 0◦ direction and to minimize body movement, but small head movements were tolerated.
In this way binaural cues were mostly lacking, but not completely absent.
Loudspeakers LS(10,0,−10) were used to supply the leading direct sound at angle θdir and loudspeakers LS(45,−30,−45)were used for simulating a lagging specular reﬂection from ceiling or ﬂoor at θreﬂ . The level of reﬂections
was attenuated by ΔL = 4.4 dB compared to the direct sound, simulating a θdir -dependent absorption coefﬁcient.
Hence, the presented studies focus on the inﬂuence of temporal properties of the reﬂection.

3

EXPERIMENT 1: Delay-based perceptual phenomena for vertical sound movement

The spectral coloration evaluated by the auditory system for vertical sound localization is mainly caused by the
directional ﬁltering of pinna, head, and torso [12].
Consider a source and a receiver at a certain distance and at equal height above the ﬂoor. The vertical movementof the source along the median plane changes delay and level of the ﬂoor reﬂection. Studies have shown aninﬂuence of the reﬂection pattern on timbre and thus on the spectrum of the perceived sound [13] providing
additional information on the height of the source.
The kind of localization cues that are introduced by median plane reﬂections is examined by means of a listening
experiment. Experiment 1 studies how the time delay of a median plane reﬂection can inﬂuence the perceived
height of a sound source.
3.1 Listening test procedure
Starting from the delay T of the lagging sound simulating the specular reﬂection, the listeners task was to adjust
both an upper and lower limit of the delay, denoted as T+ = T + Δt+ and T− = T − Δt− , yielding the most shifted
virtual sound source evoked by a continuous vertical movement.
The upper and lower limits of the delay Δt± were determined separately, using the same task. Using a fader, thedelay initiated with Δt± = 0 ms was increased as long as a monotone vertical source movement was perceived.

7912

T

T + Δt+fader movement

Figure 2. Exemplary illustration of the sequence repetition for a condition with increasing delay T+ and noise
bursts. At the beginning of direct sound and ﬂoor reﬂection are separated by the specular delay T . By moving
the fader, the listener increases the delay of the lagging reﬂection until reaches T+ = T + Δt+ after 4 noise bursts.
Once the listener determined the fader position yielding the most vertically deﬂected source, they responded as
to the direction in which the sound source was perceived to move, i.e. up or down, and the adjusted delay Δt±
max = 2 ms, which
was stored by pressing a button. The maximum allowable delay alteration was ﬁxed with Δt±
was determined by informal listening of the authors. If no or only unstable movements were perceived, listeners
could also answer with no. After logging in the answer, the next sample was loaded and the motorized fader
jumped back to the zero position Δt± = 0 ms and the next condition started.
Three conditions −10/− 45, 0/− 45, and 10/− 45 were tested consisting of direct sound and reﬂection playedback at θdir /θreﬂ with corresponding specular delays T = (2.1, 3.0, 3.9) ms. Additionally, a control condition
with θdir = θreﬂ = 0◦ and T = 3 ms, denoted as 0/0, was included in the test in order to evaluate the inﬂuenceof pure comb ﬁltering. Tested signals were chosen to investigate the inﬂuence of spectro-temporal properties(envelope, bandwidth, etc.) and familiarity to the effect: female speech and two 100 ms long pink noise burstsof which noise1 had onset and release times of 10 ms and noise2 of 2 ms and 98 ms, respectively. The noise
samples were followed by a pause time of 60 ms, whereas for the 7 s long speech sample no pause was added.
For each condition, the samples were played back in a loop at 70 dB(A) until the listeners logged in theiranswers. Figure 2 shows an exemplary illustration of the sequence repetition for a condition with increasing
delay T → T + Δt+ .
The test sequence for every listener was an individual random permutation of the entire set yielding 3 sounds ×
4 conditions × 2 repetitions × 2 delay alterations = 48 adjusted delays and 48 movement directions per listener.
Twelve experienced listeners participated in the experiment (all male; age 24 to 54) and all of them reported
normal hearing acuity. Before conducting the experiment listeners familiarize with the signals in a training.
Therefore, single loudspeakers were used for playback and listeners were aware which loudspeaker is active.
3.2 Results
The listeners’ consistency is obtained by comparing their answers concerning the direction of perceived movementof both repetitions. Overall the consistency is relatively poor and after excluding one listener (consistency 40%),
it accounts 56% on average (min. 50%, max. 67%).
A pairwise statistical analysis of noise1 and noise2 reveals no signiﬁcant difference of the signals for any
condition (p > 0.10). Therefore further analysis is done by pooling the data of both noise signals.
Figure 3 shows histograms of obtained answers concerning the direction of perceived movement. Since thedistance between each item category (up-no, no-down) on the ordinal scale is equivalent, mean values are obtainedby assigning a value of +1 for up responses, −1 for down responses, and a 0 for no responses. The standardized
difference between computed mean values of the incrementing/decrementing (inc./dec.) delay in each condition
corresponds to the effect size expressed as Cohen’s d. Respective effect sizes for conditions of Experiment 1 arelisted in Table 1.

7913

10/-45

0/45

0/0

0

-10/-45 0/-45

10/-450/45

0/0-10/-45 0/-45

10/-45

0/45

75%

0/0

1

100%
50%
25%

0
T-

T-

-1

-1

T

1

Relative frequency-10/-45 0/-45

TRelative frequency

-1

0

T+

T+

0

T

1

T

1

-1
-10/-45 0/-45(a) Speech

10/-450/450/0(b) Noise

Figure 3. Histogram of obtained answers concerning the direction of perceived movement when increasing (red)and decreasing (blue) the delay T for speech (left) and noise signals (right). The value +1 corresponds to up
responses, −1 to down responses, and 0 to no responses. Corresponding means are indicated as ×.
Table 1. Effect size expressed as Cohen’s d for Experiment 1. Values are color coded according to theclassiﬁcation of [14] with |d| = 1.2 deﬁning a very large , |d| = 0.8 a large , and |d| = 0.5 a medium effect .
cond.
T−10/− 45
2.1 ms0/− 45
3.0 ms10/− 45
3.9 ms0/45
3.0 ms0/0
3.0 msfemale
noise

1.88
0.00

0.43
−0.68

0.42
−1.00

−0.65
−0.84

−0.19
−0.93

A Mann-Whitney U test reveals the signal type to be a (weakly) signiﬁcant factor for most conditions (p ≤ 0.08),
except for incrementing conditions 0/45, 0/− 45 and decrementing condition 0/0. The factor alteration of thedelay (incrementing vs decrementing) is highly signiﬁcant for conditions −10/− 45 and 0/45 (p < 0.02) for
speech, while for noise it is highly signiﬁcant for all other conditions (p ≤ 0.01). This agrees with the effectsize d listed in Table 1. Interestingly, the effect direction of conditions with a ﬂoor reﬂection (θreﬂ < 0◦ ) is
opposite for the two signal types and we deduce two different perceptual phenomena. First for noise, increasing
the delay of the reﬂection yields downwards movements, and vice versa. This result, which is obtained similarly
for the control condition 0/0, is attributed to the pitch-height effect [1, 15], as incrementing (decrementing) the
delay results in a time-variant comb ﬁltering. Several listeners mentioned that they heard comb ﬁltering forbroadband noise signals, perceived as an downward (upward) glissando. Based on respective effect sizes inTable 1, the lower limit of the effect is reached with condition 0/− 45 and T = 3.0 ms. An upper limit was notreached with tested delays.
For the speech signal on the other hand the effect direction depends on the direction of the reﬂection. Increasing
the delay of ﬂoor reﬂections yields upwards movements, and vice versa. For a reﬂection from the ceiling an
inverse relation is found. However, this effect is not active for the control condition. A statistical analysis of test
conditions for both delay alterations separately with speech using the Friedman test reveals the condition to besigniﬁcant when the delay is increased (pinc < 0.01) and weakly signiﬁcant when it is decreased (pdec = 0.08).
From this analysis it is not clear whether the parameter T or θdir is inﬂuencing the effect size as both parameters
are varied across conditions. Though answers for decrementing speech conditions (0/− 45, 10/− 45, 0/45) inFigure 3 are more consistent than respective incrementing conditions, which is why we assume the delay T to
be the effect parameter.

7914

Table 2 lists adjusted delays as mean values and standard deviations for both alterations. Delays are highly
subjective yielding high standard deviations. Nevertheless, mean values of conditions are similar with overallmeans for incrementing and decrementing conditions of Δt¯± = 1 ms. Such a delay alteration of a physical ﬂoor
reﬂection is achieved by shifting the height of an emitting source at θ = 0◦ and r = 2.5 m by approximately
Δθ± = 10◦ .
Table 2. Mean values and standard deviations of adjusted delays Δt± for speech and noise.

4

condition

Δt+ in ms
femalenoise

Δt− in ms
femalenoise−10/− 450/− 4510/− 450/45
0/0

1.2 ± 0.6
1.1 ± 0.5
1.1 ± 0.6
0.9 ± 0.6
1.2 ± 0.7

1.0 ± 0.5
1.2 ± 0.5
1.1 ± 0.6
0.6 ± 0.4
1.5 ± 0.6

1.0 ± 0.5
1.2 ± 0.6
0.8 ± 0.5
1.1 ± 0.7
1.0 ± 0.7

0.8 ± 0.4
1.1 ± 0.5
1.1 ± 0.4
0.8 ± 0.6
1.2 ± 0.5

EXPERIMENT 2: Temporal cues induced by a ﬂoor reﬂection

In Experiment 1 two different effects were shown to inﬂuence the movement perception, depending on the
excitation signal. The pitch-height effect is well-known in localization studies. Results obtained by speech
indicate the existence of other localization cues induced by a median plane reﬂection. To the authors’ knowledge,
such an effect was not studied in a listening experiment of the literature so far and the few studies investigating
the inﬂuence of temporal cues on vertical localization focus on the precedence effect, e.g. [17, 18].
Experiment 2 studies the effect obtained for speech more closely. Possible inﬂuences of the pitch-height effectare minimized by restricting the study on ﬂoor reﬂections, for which effect directions of obtained effects wereshown to be contrasting. The listening experiment was carried out to assess the inﬂuence of the overall delay T
and loudspeaker angles θdir/reﬂ .
4.1 Listening test procedure
In Experiment 2 listeners were asked to indicate perceived source movements for continuous alterations of thedelay from T to T+ and T to T− . Based on the results from Experiment 1 delay alterations of Δt± = 1 ms wereconstant for all conditions and covered a range of T = 0...6 ms. To keep the testing time decent, a gap between
4 ms and 5 ms was inserted yielding ﬁve delay increments T → T+ = (0 → 1, 1 → 2, 2 → 3, 3 → 4, 5 → 6) ms
and corresponding delay decrements T → T− = (1 → 0, 2 → 1, 3 → 2, 4 → 3, 6 → 5) ms. To study directional
dependence, two different angles of the direct sound were used in combination with two angles of the reﬂection,yielding conditions 0/− 45,−10/− 45, and 0/− 30. Tested sounds were 3 s long samples of male speech andcongas. These non-stationary signals were chosen to prevent the pitch-height effect.
In the experiment, delays of the reﬂections were automatically increased or decreased between 0.5 s to 2.5 s ofthe sample lengths and listeners had to rate the perceived movement of the sound source with up, down, or no
continuous movement. The speech sample was tested with all conditions, whereas the conga sample was tested
with condition 0/− 45 only. Additional control conditions included dynamically panned male speech signals using
VBAP between LS0 and LS−10 and vice versa. Just like the delay alterations, the panning started at 0.5 s andended at 2.5 s of the sample length.
The test sequence consisted of [(3 speech conditions + 1 conga condition) × 5 delays + 1 VBAP] × 2 repetitions
× 2 alterations = 84 samples per listener. Ten experienced listeners with normal hearing acuity participatedin the experiment (all male, age 24 to 54), of which eight already participated in Experiment 1. The training
conducted before evaluation was similar than for Experiment 1 and included additional VBAP conditions.

7915

Relative frequency

25%
VBAP

50%
0/-45

75%
-10/-45

100%
0/-30

1

1

0

0
-1

-1
-10°

0°

0

1

1

2

2

3

3

4

5

6

1

1

0

0

-1

-1
0°

-10°

1

0

2

1

3

T

Panning angle

2

4

3

6

5

0

1

1

2

2

3

3

4

5

6

1

0

2

1

3

2

4

3

6

5

T in msT(a) Speech

T in ms(b) Conga

Figure 4. Histograms of obtained answers concerning the direction of perceived movement when increasing anddecreasing T by 1 ms for tested signals speech (left) and conga (right). Delay conditions are color coded (blue,
red, yellow). Additionally for speech, auditory movement is studied using VBAP (violet).
Table 3. Color-coded effect size expressed as Cohen’s d for Experiment 2: very large , large , and medium .
signalcond.

0  1 ms

1  2 ms

2  3 ms

3  4 ms

5  6 ms

speechspeechspeechconga

0/− 45−10/− 450/− 300/− 45

1.45
1.35
1.23
1.66

1.14
0.90
0.40
0.59

0.05
0.40
−0.40
−0.34

0.30
0.07
0.17
−0.21

0.11
−0.27
−0.16
−0.11

4.2 Results
The listeners consistency is monitored by comparing answers of both repetition and the mean consistency of
58% (min. 50%, max. 65%) is just above the value found for Experiment 1. The data of all listeners is shown in
Figure 4 as histograms and mean values. A statistical analysis of perceived movement direction for both speechand conga with condition 0/− 45 reveals the signal type not to be signiﬁcant for any of the delay alterations(Mann-Whitney U: p ≥ 0.72). For all conditions, movement directions resemble those obtained by speech inExperiment 1 and increasing the delay tends to be perceived as upwards movement, while a decrease is perceived
as a downward-moving source.
While distinct movements are achieved at short delays, the effect reduces with increasing the delay T . The
Friedman test reveals the delay to be a weakly signiﬁcant factor for both incrementing and decrementing delays
with speech (pmale ≤ 0.09) and strongly signiﬁcant with the conga signal (pconga  0.01). For the latter the factor
alteration is highly signiﬁcant only for the shortest delay (0  1 ms, p  0.01). The same statistical signiﬁcanceis obtained for speech with condition 0/− 30, whereas for conditions 0/− 45 and −10/− 45 the signiﬁcant range
extends to the two shortest delays (0  1 ms, 1  2 ms; p  0.01). Based on the effect size of speech conditionin Table 3, the effect seems to be dependent on the aperture angle between direct sound and reﬂection, withwider angles yielding a stronger effect.
To compare the obtained effect with the VBAP condition (θVBAP = −10◦  0◦ ), speech conditions −10/− 45 and
0/− 45 with shortest delays are considered (−10/− 45: 0 → 1 ms; 0/− 45: 1 → 0 ms). The difference of respectivemeans is similar to the difference obtained by VBAP, cf. Figure 4. Consecutively, the effect size expressedas Cohen’s d, describing the standardized difference between means, reveals similar values, i.e. ddelay = 1.75,
dVBAP = 1.86, and according to the classiﬁcation of [14] a very large effect is obtained with both methods.

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5

POSSIBLE EXPLANATION OF EFFECTS

We have shown that by a continuous alteration of the delay between a vertically arranged lead and attenuated lagin the range of ±1 ms the auditory movement can be inﬂuenced. We believe, that depending on the excitation
signal, different cues are evaluated by the auditory system.
5.1 Spectral cues
For stationary broadband noise, incrementing the delay of a lagging sound instance yields the perception of
downward movements. This can be explained by the pitch-height effect as time-variant comb ﬁltering is perceived
as downward glissando. Thus, this effect is independent of the direction of lagging reﬂections and it can beconcluded that reﬂections from the ceiling support the localization of a moving physical sound source, whereas
ﬂoor reﬂections yield localization cues that do not comply with the physical movement.
5.2 Binaural cues
For time-variant signals with multiple narrowband components, e.g. speech, a different relation is obtained. A
continuous increase of the delay T between the direct sound and the ﬂoor reﬂection yields an upward movementin the perception, whereas for the ceiling reﬂection the perceived movement is opposed. Thus, the perception isin compliance with the movement of a physical sound source.
A possible explanation of this effect can be found in [19], which studies the impact of early reﬂections on
binaural cues by means of numerical models. They could show that an early reﬂection produces a discontinuity
in the inter-aural phase difference at the interference frequency f , which can be arbitrarily large and yields an
ITD that can take any value between ±1/(2 f ). Based on this insight, the authors in [19] conclude that ﬂoor
reﬂections with short delays provide reliable cues for elevation estimation by changing ITDs. It is possible thatthey are learned in the same way as pinna cues and small angles of the head are sufﬁcient to give a directional
sense. Although the delay of ﬂoor reﬂections mostly exceed the active range found in Experiment 2 (T ≤ 2 ms),
short delays occur if we consider reﬂections from the torso or other obstacles, or far away sources (> 10 m).

6

CONCLUSIONS

A method is presented to control the perceived vertical movement of a virtual sound source with loudspeakers
resembling a listening situation consisting of direct sound (lead) and a median plane reﬂection (lag). Twolistening experiments were conducted and it is shown that a simple delay modiﬁcation in the range of ±1 ms
between leading and lagging sound is sufﬁcient to evoke an auditory movement along the median plane.
Results of Experiment 1 show a signal dependency of the movement direction and we assume two partly
contrasting perceptual effects dominating the perception of auditory movement. For noise signals, continuous
delay alterations yield time-variant comb ﬁltering and movement perception is governed by the pitch-heighteffect. This spectral effect, obtained when the overall delay exceeds 3 ms, appears independently of directions ofincoming sounds. For speech signals the auditory movement is deduced from the movement of a physical soundsource and the direction of the lag speciﬁes the direction of movement.
In Experiment 2 the latter effect is studied more closely and compared to amplitude panning. We could prove it
for continuous delay alterations with speech and conga samples. It was found to be strongest for short delays(overall delay T ≤ 2 ms) with a wide loudspeaker spacing of lead and lag. For such a condition the effect size
was found to be comparable to vertical VBAP. Agreeing with [19] we believe that median plane reﬂections
produce localization cues which are encoded by the auditory system.

ACKNOWLEDGMENTS
Our research was partly funded by the Austrian Science Fund (FWF) project nr. AR 328-G21, Orchestrating
Space by Icosahedral Loudspeaker (OSIL). We thank all listeners taking part in the experiment.

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