=Paper=
{{Paper
|id=Vol-2299/paper3
|storemode=property
|title=Promenade: A Minimalistic Sculptural Interface for Immersive Soundscape Presentation
|pdfUrl=https://ceur-ws.org/Vol-2299/paper3.pdf
|volume=Vol-2299
|authors=Martin Rumori
|dblpUrl=https://dblp.org/rec/conf/fmt/Rumori18
}}
==Promenade: A Minimalistic Sculptural Interface for Immersive Soundscape Presentation==
https://ceur-ws.org/Vol-2299/paper3.pdf
Promenade
A Minimalistic Sculptural Interface for Immersive Soundscape Presentation
Martin Rumori
Institute of Electronic Music and Acoustics
University of Music and Performing Arts
Graz, Austria
Email: rumori@iem.at
Abstract—Promenade proposes an interface for the semi-
personal, interactive presentation of soundscapes that allows
for the seamless integration of visual material. It combines the
excitation of a flat surface using transducers with transaural
cross-talk cancelled stereophonic projection. The unfolding of
a wide spatial image within the sweet spot is contrasted by a
collapsed, localised impression outside. This inherent interactive
behaviour may be further extended by means of a distance sensor.
Promenade emerged from an experimental, arts-based research
project. It therefore targets contexts that feature custom audio
content rather than aiming at general-purpose applications
requiring optimal acoustic properties.
I. I NTRODUCTION
Exhibitions or other presentations of distinct objects often
call for an unobtrusive sound projection means that affords the
visitors an immersive, mostly individual listening experience.
Achieving such an experience requires spatial capabilities of
the chosen projection technique.
In many such cases, headphones are used. Spatially rich
sound material may be projected in particular with binaural
signals. A major side effect is the inherent reconfiguration
of our multi-modal perception, as the surrounding auditory
domain is largely replaced by the projected one. Additionally,
headphones may be considered as cumbersome or unhygienic
in public exhibitions. Multichannel loudspeaker setups, in con- Figure 1. Promenade sound projection device.
trast, do not require physical engagement with the projection
medium. However, they do not facilitate individual listening
situations and they have to be mounted in an often complex This paper describes the basic idea of Promenade and its
constellation around the target listening area. originating context (section II), followed by introducing the
Promenade is a minimal auditory display prototype that two main techniques involved, transaural crosstalk-cancelled
relates to such issues. It is important to notice that Promenade sound projection (section III) and the principle of flat panel
has not been developed to overcome the limitations of other loudspeakers (section IV). Finally, some findings that arose
projection means. Instead, it emerged from a series of ex- from practical experience are discussed, along with possible
periments in an arts-based research context as part of a sound modifications and extensions (section V).
installation with a focus on aesthetic listening experience. Due
to its simplicity, its purpose and the idiosyncratic technology II. P ROMENADE
it incorporates, Promenade is not a universal, general-purpose The Promenade sound projection device consists of a flat
spatial sound projection system but it manifests certain lim- surface of variable size with two electrodynamic exciters
itations. However, it might provide an appropriate and eleg- mounted on the back (Figure 1). Audio signals are fed to
ant solution in some of the circumstances described above. the exciters such that the surface acts as a loudspeaker mem-
Promenade’s qualities with respect to those circumstances brane. The physical properties of the surface material lead
are regarded retrospectively here in order to explore further to a certain radiation characteristics. The directional effect is
possible fields of application and obvious modifications for further emphasised by crosstalk-cancelled stereophony, which
different forms of appearance. creates a large auditory canvas that is much wider than the
25
Promenade: A Minimalistic Sculptural Interface for Immersive Soundscape Presentation
projection panel (see section III). The effect occurs at a central
listening position, facing the device from a distance of a few
ten centimetres up to about one and a half metres. As soon as
the sweet area is left, the spatial auditory image collapses and
sound seems to be restricted to the panel surface. Although
both exciters are mounted to the same surface, their signal
differences remain intact to a sufficient extent for providing
the effect of crosstalk-cancelled projection.
A. Physical construction
Promenade has been prototyped with two materials and in
two sizes so far. The first series uses finnboard, a lightweight
type of cardboard, with the dimensions 70 by 100 centimetres
(portrait) and 3 millimetres thickness. The second, more
Figure 2. Promenade installation, SMC 2018, Limassol, Cyprus.
compact series is made of screenboard, a more dense type of
cardboard with white cover layers, 43,5 by 64 centimetres and
2 millimetres thick. Other promising materials that have been structures, processes, and realities around us, with the singular-
explored include ply wood of a few millimetres thickness, and ity of particular spots in space that, when located, allow for an
KAPA, a compound material combining a polyurethane foam exclusive access to individual layers of this environment. The
core with cardboard cover layers. crosstalk-cancelled projection technique restricts these spots
The smaller prototype uses Visaton EX-60 R exciters to clear-cut boundaries but on the other hand it potentially
(60 millimetres in diameter), while the larger is equipped with ‘elevates’ each layer’s spatiality as a sublime, temporary
the more powerful Visaton EX-80 S (square shape, 80 milli- overlay to the global environment.
metres edge length). White flat cable serves for feeding the The installation Promenade serves as an experiential starting
exciters and for suspending the device. Both the exciters and point for further investigations into the fields of anecdotal
the cables are glued to the panels. The projection devices are music, narrative identity construction and strollology. All of
fed by small standard amplifier boards installed separately. these areas may be reflected in conjunction with aesthetic
In the installation, the projected signals were generated from experience in our everyday life auditory surrounding and its
a BeagleBone Black computer board equipped with the Bela technical mediation. Departing from a workshop within the
audio cape.1 Sound programming and convolution for crosstalk project Promenade, a publication is currently prepared that
cancellation was performed in SuperCollider.2 As cancellation summarises the described approach and the ongoing process.
filters, the so-called BACCH filters by Edgar Choueiri [1] were
taken from the jconvolver convolution engine bundle by Fons III. T RANSAURAL CROSSTALK - CANCELLED SOUND
Adriaensen.3 PROJECTION
B. Sound installation and artistic context A. Working principle
The sound projection device has been conceived as part Transaural crosstalk-cancelled sound projection, often ab-
of a sound installation within the arts-based research project breviated XTC, has been developed for playing back binaural
Promenade. Anekdote, Alltag, Abbild (Promenade. Anecdote, signals, which normally require headphones, on a pair of
everyday life, image), hence its name.4 loudspeakers (see, e. g., [2, pp. 283–326]). While headphones
Seven playback canvases (five in a later version, Figure 2) isolate the sound propagation paths from each of the earphones
were distributed in the exhibition space, each suspended from to the corresponding ear, respectively, loudspeaker projection
the ceiling. The projection surface was left blank, without any involves transaural crosstalk: the signal radiated from the left
further visual additions. Each of the devices played back a ear also reaches the right one and vice versa. However, the
soundscape composition based on field recordings. Listeners playback of binaural signals requires the separate presentation
were invited to explore and ‘visit’ the different soundscapes of each channel to the corresponding ear.
by moving in space, approaching the projection devices one Transaural crosstalk cancellation is based on time-shifted
by one and ‘entering’ the recorded space by positioning anti-phase signals emitted from the respective opposite loud-
themselves in the sweet area where the crosstalk-cancelled speaker that shall arrive at the near-side ear at the same time
projection effect unfolds. as the unwanted crosstalk signal from the far side speaker
The installation seeks to combine a global sonic envir- in order to cancel it out. Obviously, the cancellation signal
onment that reflects the simultaneity of seemingly unrelated causes a crosstalk at the far side ear, too, which in turn
1 http://bela.io
is eliminated by another correction signal emitted from the
2 http://supercollider.github.io opposite speaker. Several iterations of this process lead to the
3 http://kokkinizita.linuxaudio.org/linuxaudio/ design of a recursive filter network that ultimately seeks to
4 https://esc.mur.at/de/projekt/promenade-anekdote-alltag-abbild ensure an isolated projection of the ear signal pair [3].
26
Promenade: A Minimalistic Sculptural Interface for Immersive Soundscape Presentation
B. Binaural vs. stereophonic signals with XTC The experimental artistic context of Promenade did not
As mentioned above, transaural crosstalk cancellation has pursue a scientific approach to such a technique for transaural
been developed for the projection of binaural signals on loud- sound projection. In multiple iterations, the used combination
speakers, that is, ear signals involving interaural time (ITD) of materials, dimensions and mount points of the exciters have
and level differences (ILD) as well as elevation-dependent been adjusted such that the desired effect could be achieved
spectral cues (see [4]). Nevertheless, XTC filters are also used sufficiently well. A validation by acoustic measurements,
for the playback of ordinary loudspeaker stereophonic signals. however, has not yet been carried out.
The latter application is known as Ambiophonics in consumer V. D ISCUSSION
entertainment contexts [5].
Experimental findings of the Promenade project with re- The presented sound projection device shows an appropri-
spect to stereophonic signals are discussed below (Section V). ate performance within its frame of origin, the Promenade
arts-based research project. There, it allows for experiencing
IV. F LAT PANEL LOUDSPEAKER PROJECTION the often surprising, sometimes uncanny effect of crosstalk-
The excitation of flat, solid matter surfaces as an alternative cancelled sound projection and supports the exploration and
loudspeaker working principle has been investigated during reflection of aesthetic experience in multi-layered spatial aud-
the past decades (see, e.g., [6]). Major advantages are the itory environments.
appealing form factor requiring dramatically reduced mounting Beyond the attached context, the principles exploited in
depth, and the potential to create diffuse sound fields due to Promenade may be applied to other use cases. In the following,
the low directivity even at high frequencies. some findings and possible directions are discussed, such as
One important factor for exciting flat panels is the mounting the use of stereophonic signals, the integration with visual
location of the exciter. Depending on the application, the presentations, extensions towards interactive applications, and
physical properties and the size of the panel, a basic question other form factors, materials, or objects.
is whether to exploit or to avoid the direct excitation of pre- A. Crosstalk cancellation of stereophonic signals
dominant modes. Often, an out-of-centre location avoiding low
integer partition of the surface dimensions is recommended When developing the Promenade sound installation with
for a more balanced response. In Promenade, exciters were field recording material and XTC projection, it was found
mounted according to the golden section as an experimentally that coincident stereophonic signals involving only level dif-
found trade-off of acoustic and visual qualities. ferences but none of phase provide a more consistent and
Apart from the construction of high-end loudspeakers, elec- stable spatial image than recordings in spaced omnidirectional
trodynamic excitation of manifold objects for sound projec- stereophony that is based on phase differences. There is no
tion is frequently applied in custom presentation media and well-researched explanation for this observation. It is likely
in sound art (see, e.g., SHHH SHHH SHHH5 by Amanda that both the listening conditions and the projection device are
Dunsmore, 2008, or Touched Echo6 by Markus Kison, 2007). not optimal for propagating phase differences to the listener’s
As described above, the exploration for Promenade departs ears. In particular the fact that both exciters are working on
from the latter. In such realms the presented sound material is the same projection surface may degrade the fidelity of phase
usually created or adapted especially for the specific playback reproduction, although a similar observation was made using
medium. Consequently, the projection device does not need two separate loudspeakers.
to meet the same criteria as general-purpose loudspeakers, Overall, the discovery that intensity stereophony is well
especially with respect to linearity and neutrality, because suited to span the extremes of the projection base – from ear
such deviations can be corrected in the adaption process. This to ear using headphones, nevertheless crossing the listener’s
largely facilitates achieving a beneficial result with relatively head, from speaker to speaker in an ordinary stereophonic
low-cost equipment. setup, and along a well externalised, virtual stereo base of
A special technical issue of the Promenade device is that about 150 degrees opening angle using XTC projection –
only one panel is used for projecting two audio channels at is most valuable for the production of appropriate material.
the same time. The crucial question is whether the crosstalk Following from that, field recordings were carried out in
cancellation cues are propagated to a sufficient extent for mid/side stereophony, which allows for further benefits with
achieving a similar effect as with two small-spaced loudspeak- respect to adaptation and interaction (see subsection V-C).
ers or whether the inter-channel crosstalk in the panel degrades B. Visual integration
the cues too much. A similar technique has been investigated
for the application of Wave Field Synthesis (WFS) using even In the Promenade sound installation, major emphasis was
more than two exciters on the same panel, coined Multi- put on listening to mediated auditory environments. Therefore,
Actuator Panels [7]. Later, the method has been generalised the metaphor of a canvas was picked up that could have shown
for creating multiple sound sources with a single panel [8]. a landscape but instead projected a soundscape while remain-
ing visually ‘silent’. However, the surface of the projection
5 http://www.lit.ie/Dunsmore/linz/kunstraumwindows.html medium may be easily used for a seamless integration of visual
6 http://www.markuskison.de/touched_echo.html and auditory material. In the simplest case, the cardboard
27
Promenade: A Minimalistic Sculptural Interface for Immersive Soundscape Presentation
material may be printed or laminated. The backside may be Such transitions may be triggered or emphasised by evaluating
used as well, restricted by the mounted exciters. the listener’s distance or overall presence via the sensor.
Alternatively, the panel may serve as a video projection
D. Other form factors, materials, or objects
screen, including three-dimensional technology involving po-
larised or shutter glasses. The projection angle and distance Theoretically, combining two-channel electrodynamic excit-
have to be envisaged such that the spectator is not disturbing ation of solid matter with transaural crosstalk cancellation is
the projection but may still position herself appropriately with possible with various objects and materials of any form or
respect to crosstalk-cancelled sound projection. extension. For example, architectural prototypes, engineering
Using stereophonic signals, auditory events can be evoked models or basic commodity might be enhanced by an integ-
for the listener anywhere in the horizontal dimension within rated auditory layer that invites for spatial immersion.
the borders of the canvas and beyond. Therefore, horizontal The determining factors remain those that have been con-
audio-visual congruence is well achievable, for example, for sulted while conceiving Promenade:
annotating visual items in maps or alike. Psychoacoustic • the physical properties of the excited material with re-
effects may be exploited for suggesting vertical correspond- spect to sound propagation,
ences. Depending on the stiffness of the projection panel, • appropriate mounting locations for the exciters and fitting
the location of the exciters on the panel is audible in certain form factors of the object and the exciters, and
frequency bands. In this case, a second pair of exciters may • sufficiently isolated radiation of both channels for achiev-
serve as a panning means in the vertical dimension, at least to ing the desired effect of crosstalk cancellation.
a certain extent. VI. C ONCLUSION
Generally, any visual presentation that favours a fixed,
frontal and relatively close viewpoint can be expected to work In this paper, the Promenade spatial sound projection device
well in combination with a Promenade like sound projection has been presented, along with its originating context, a sound
device. installation as part of the arts-based research project of the
same name. The device has been presented as combining the
two techniques of transaural crosstalk cancellation for inviting
C. Interactivity, tracking, sensors
a rich spatial auditory experience with the excitation of solid
From the beginning, the Promenade device has been con- objects for an unobtrusive appearance, differing from that of
ceived with an additional optical distance sensor such that ordinary loudspeakers. The presentation form of a canvas,
the playback can react interactively, depending on whether, though not visually occupied in the original installation, invites
how close and for how long a listener is present in front for manifold applications that involve audio-visual correspond-
of the canvas. For the installation versions involving several ence. Further extensions include the use of distance sensors
projection panels, actual use of the sensors has been dismissed for interactive functionality, both with respect to the presented
so far. It turned out to be more desirable in the first place to narrative and to the evoked spatial impression.
explore the field of sweet spots distributed in space which,
ACKNOWLEDGEMENTS
when being entered, change the listening experience dramat-
ically. This effect may be regarded as an inherent ‘interactive’ The research project Promenade is supported by the Vice
behaviour of the projection dispositif, although no technical Rector’s office of research at University of Music and Per-
reactivity is involved. forming Arts Graz, the Cultural Office of the City of Graz,
Nevertheless, involving the sensor bears several interesting and ESC Media Arts Laboratory, Graz.
use cases. If multiple projection panels are present, the sensors R EFERENCES
may serve as a simple tracking device in order to activate or
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