PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

The EMPAC high-resolution modular loudspeaker array for wave
field synthesis
Johannes GOEBEL1
Curtis R. Priem Experimental Media and Performing Arts Center (EMPAC)
Rensselaer Polytechnic Institute, USA

ABSTRACT
In order to explore the perceptual potential and limitations of Wave Field Synthesis (WFS) for artistic
productions, a WFS array was built with loudspeakers placed as closely together as possible. Knowing fromexperience, that the high-frequency noisy components found in consonants of speech and transients of
instrumental sounds are important for sound localization, the question arose if their coherent presentation
through WFS with a higher spatial aliasing frequency affects the localization. The system was built with 5.8
cm between each loudspeaker resulting in an aliasing frequency of 2.9 kHz. Experimental configurations
allow us to evaluate aliasing frequencies from below 700 Hz up to 5.8 kHz. Critical listening shows that
increasing the number of speakers while decreasing their distance improves the perceived overall quality,
including timbre. New modules with loudspeaker spaced 2.9 cm apart are under construction. The current
system’s 558 individually addressable loudspeakers are housed in 18 modules, each with 31 channels and an
external subwoofer. The modular design allows for flexibility in deployment, including vertical and overhead
configurations, enabling the simulation of complex acoustic environments and a diversity of artistic andresearch-driven projects. This paper is an invitation to use the system for research.
Keywords: Wave Field Synthesis, Spatial Aliasing Frequency, Focused Sound Sources

1. INTRODUCTION
The framework for Wave Field Synthesis (WFS) was developed in the 1980s by Berkhout, de Vries,
and Vogel (1). A fundamental design criterion is that the distance between the system’s loudspeakers
should be as small as possible to synthesize a wave field that is acoustically accurate and true to anaturally occurring wave field. Spatial aliasing, a function of the distance between the centers of each
loudspeaker in use, results in distortion imposed on the synthesized wave field in the higher
frequencies of the virtual sound sources. By reducing the distance between the speakers, one extends
the boundary of frequencies that are not impacted by spatial aliasing. This increased range results in agreater coherency of projected sound components. All frequencies above this spatial aliasing
frequency are reproduced and audible to the listener; however, they inflict audible artifacts thatcounteract the construction of a coherent wave field.
With a few exceptions, the majority of WFS systems – especially portable systems and those usedin performance environments – space their drivers between 10 and 15 cm apart, with each speaker in
the array assigned a specific signal from a WFS rendering application. Systems with smaller distances
are usually part of permanent installations. A system with, for instance, 4-inch drivers spaced at 12 cm
has a spatial aliasing frequency around 1400 Hz. In a system with 8-inch drivers spaced at 24 cm, thespatial aliasing frequency is half that, at around 700 Hz.
The motivation for the EMPAC high-resolution modular loudspeaker array for wave field synthesis
stems from artistic practice and creating new musical works that can only be realized with thistechnology. The discrepancy between what many existing WFS systems perceptually promise and theactual aural experience of an acute listener encouraged us to design a system that could verify thepotential of WFS based on human perception as a point of departure. We believe scientific research
could also benefit from this system. Its modularity, physical flexibility, and high spatial aliasing
1

1148

jeg@rpi.edu

frequency can allow for a real-world listening situation in a spatio-physical environment, where
different spatial aliasing frequencies can be compared in situ on the same system.

2. CONTEXT OF THE EMPAC MODULAR WAVE FIELD SYNTHESIS ARRAY
Having experienced WFS speaker arrays that did not perceptually align with the effect described by
the presenter or artist, and noting that critical frequencies for localization cues and timbral integrity
were well above the aliasing points these systems could deliver, we decided to build a system thatwould allow us to find answers. The goal was to verify how the perception of a wave field was affected
through a closer spacing of drivers and the resulting higher spatial aliasing frequency. For this, weused a technical framework that could provide a high channel count while allowing, at the same time,
flexibility in the physical placement of the speaker arrays.
From artistic works using acoustic and electro-acoustic sounds in spaces of different sizes andqualities, we know through critical listening experiences that consonants in human speech, transientsat the onset of instrumental sounds, and the carefully shaped attacks in electronic music provide
localization cues. The timbral quality of sustained sounds is also greatly shaped by these higher
frequencies. In order to determine how sound sources projected through wave field synthesis relate tolisteners in complex environments, generation of a wave field that coherently integrates frequencies ina system suitable for performances and installations, with an aliasing frequency as high as possible
was deemed essential.
WFS was initially developed for the reproduction of instrumental recordings to create an auditory
field in which the localization of individual instruments is stable independent of the listeners’ position.
This included the auditory envelopment as if players and listeners were in a performance space. We seesuch “natural” reproduction, or rather re-synthesis, as a subset of any auditory environment that can becreated with WFS, where the localization of static or moving sounds can be perceived independent of
the listener’s position within the physical space of the loudspeaker array. The understanding of thespatial and timbral conditions established by the physical parameters of a loudspeaker array isespecially important when using sounds that are not imitating, modeling, or modifying acoustic
instruments or timbres known to a listener, but which are “unheard of.” With familiar vocal or
instrumental sounds, the knowledge and memory of such sounds may make up for “non-fidelity”
introduced by a wave field synthesis loudspeaker array. However, when creating sounds without apoint of reference to generally known sounds, a musician may want to know what the resulting
auditory quality will be if the music is played on arrays with wider or closer spacing of loudspeakers
than the one she created the sounds on. This enables the composer to shape the details of sounds insuch a way that their core characteristics can be either perceived through systems with different
spacing or restrict the piece to a defined minimum loudspeaker spacing.
It was not our intention to limit ourselves to the creation of continuous wave fields in front of or
around listeners, ones that simulate enveloping natural acoustic environments. Instead, we sought amethod that allows the user, artist, or scientist to configure physically the arrays in many different
ways. For example, by creating non-contiguous areas of speakers anywhere in a venue; combining ahorizontally oriented array with another arranged vertically; placing arrays above an audience
projecting downwards; or even positioning several “windows of sound” in different locations, on thewalls and ceiling of a space.
Working with a variety of artists and researchers, we have been able to test different configurations
using the EMPAC WFS array. Composers and researchers with extensive WFS programming andcontent creation on other systems, have noted that the linear array set-up of the EMPAC systemprovides a perceivable gain in quality over systems with larger distances between the drivers. Figure 1
shows a section of a linear configuration.
We also noted that halving the distance between the drivers and pushing the spatial aliasing
frequency to 5.9 kHz results in a clearly perceivable increase of sonic quality and engagement versus
the 5.9 cm distance at half the spatial aliasing frequency. We achieved 5.9 kHz by positioning twoarrays on top of each other, offsetting half the on-center distance between drivers as shown in Figure 2,
resulting in 2.9 cm between speakers. Whether this perceived increase of quality is due to the raisedaliasing frequency, or by adding speakers in general (while maintaining the same level of loudness),still needs to be investigated.

1149

Linear configuration with 5.8 cm distance between drivers.

Experimental set-up, halving the horizontal distance between drivers.

3. TECHNICAL SPECIFICATIONS
The array is comprised of 3 primary systems:• 18 speaker modules with 31 drivers for WFS, and two 16-channel integrated DANTE
power amplifiers (Attero Tech) that provide a 31.1 amplification platform, adding up to a
total of 576 individually addressable channels, 558 for WFS plus 18 for the subwoofers.
• An uncompressed, low-latency, multi-channel audio network built on the DANTE
protocol.
• IRCAM SPAT software [2] suite running on 2 Apple Mac Pros.
A small ancillary subwoofer array (Figure 3) is used to fill out the lower octaves below 140Hz,
since this range is not realistically attainable with the WFS array, given the size of the driver and thegeometry of its enclosure. There is one subwoofer associated with each WFS module. The signals
for the subwoofers are sourced from the 32nd channel of the DANTE amplifiers residing on the backof the WFS enclosures (Figure 4).

Four modules of the EMPAC array with associated subwoofer per module.

1150

Backside of array modules, with two
16-channel DANTE amplifiers and power supplies.

Speaker installation, with openings
for port tubes and internal subdivisions.

The array is comprised of eighteen 187 cm-long units, totaling around 33 meters in linear length.
Each module is 22.8 cm high with a depth of 15.7 cm, not including the rear-mounted amplifiers andpower supplies. Each unit is internally subdivided into 31 loudspeaker enclosures housing a ported 5.5
cm “full range” electrically and mechanically decoupled driver, 5.8 cm on center from its neighbor(Figure 5). These loudspeakers (Tang Band W2-2136S) act as individual 20-watt cells. Mounting andrigging assemblies at the ends of each module allow for a high degree of flexibility in theirconfiguration and placement (Figure 6 and 7).
The modular approach allows for arbitrary placement of modules. So far, each array set-up
contiguously along a straight line has been calculated independent of array sections positioned at adiffering angle. Synthesizing virtual focused sound sources or a unified reverberation environment
within a space circumscribed by rectangular or circular array lines, where all jointly contribute to thesynthesis of a unified sound field, will require further development.

Configuration as linear array withadded inverted array in center portion to reduce
distance between speakers to 2.9 cm.

Circular configuration,suspended from trusses (viewed from above).

Along with generous support in the development, adaptation and testing from IRCAM, Paris,
IRCAM's SPAT real-time processor toolset computes EMPAC's WFS render within Cycling 74’s
MAX/MSP on several Apple MacPros. The number of computers used expands and contracts with thenumber of discrete arrays rendered. Currently, DANTE's PCIe driver architecture imposes a 256
channel hardware limit per machine. There is a dedicated OSC network that provides external accessand control of SPAT. MAX/MSP, IRCAM's TOSCA and Hexlers's Touch OSC are the dominant
external controllers used at this time.

1151

4. EVALUATION THROUGH LISTENING IN MUSICAL CONTEXTEMPAC built the WFS array as an instrument for artistic, acoustic and perceptional inquiries andprojects aiming for the highest possible quality hearing can detect. A scientific validation withappropriate tests and statistical evaluation is welcome and we invite researchers to use this physicalsystem to also verify experimental results they have deduced from binaural renderings ofcomputational models of loudspeaker arrays for wave field synthesis.
The EMPAC system was built to understand and potentially prove how the closer spacing of
speakers and resulting higher spatial aliasing frequency would affect the perception and use of such
systems. In comparing different sounds and music with different array configurations, two aspects
could be experienced and confirmed in dramatic ways.
The long linear array with speakers spaced at 5.8 cm on-center, as shown in Figure 1, provides
superbly stable sound localization without any strong spatial aliasing. But once two arrays were set on
top of each other (Figure 2 and 6) and the distance on-center was halved to 2.9 cm, the quality
increased in immediately perceivable ways. By this comparison, it became clearly apparent how muchthe extension of the spatial aliasing frequency beyond 1.5 kHz enhanced the quality of the projectedwave field and that it perceivably improved the system’s fidelity.
The most radical results emanate when localizing sounds in front of the array. It has been
demonstrated that WFS is the only system through which sounds can be placed and heard in an area infront of the speakers, as long as listeners do not move into the area between the array and the sound’s
projected placement. With closer-spaced speakers at 5.8 cm, and even more so at 2.9 cm, front-focusedsound tests of prerecorded instrumental music projected at distances up to 8 m off the front of the array
rendered a “stunning” experience.
4.1 Recording and Projection of Instrumental Music
W.A. Mozart’s String Quartet No.3 in G major, KV156, as well as his Serenade in E-flat major, KV
375, for eight wind instruments were recorded in EMPAC’s concert hall. Each piece was recorded as
an ensemble, with each instrument closely miked, and with additional room microphones capturing theambience of the hall. After the initial recording, each instrument was recorded solo, with the fullrecording serving the instrumentalist as a synchronization reference over headphones. A video
recording of the full wind ensemble was played back for the individual instrumentalists so they could
pick up the visual cues from the other players. In contrast to playing with a click-track, this allowed forfreely flowing agogical deviations from a strict meter, important for music of this genre and period.
The wind octet was rendered through the WFS array on the EMPAC theater stage, with 8 modules(248 channels) creating a linear array of almost 15 m in length along the upstage wall. The virtual
sound sources for the instruments were projected out onto the stage, with each instrument placed in thearea in front of the array (the area was 8 meters in depth and not quite as wide as the array itself).
Figure 8 shows where their locations were marked on the floor, so one could move between those spots
and experience being right next to, or even “in” the specific instrument, while still hearing all other
instruments in their respective locations under the restriction that precise localization of focusedsound sources is not possible when one is closer to the array than the spot of the sound source. For
example, in Figure 8, standing in the location of Clarinet 1 will result in the diffuse localization of
Oboe 1 and 2.

Stage lay-out for Mozart Octet with
248 channels, 15m long, and virtual locations of
instruments marked on floor.

1152

Mozart Quartet with virtual locations ofstrings and the two sonic portals.

Sitting in the audience chamber in front of the stage with the instruments placed across the stage, as
if musicians were sitting there, yielded an extraordinary, acoustically transparent experience.
The projection of the string quartet recording used the same set-up of the WFS array on the theater
stage, but this time in a less reverberant listening space with curtains blocking the audience chamber
and a carpet covering the floor. Figure 9 shows the three areas for focused sound sources. The string
quartet was virtually placed to the far right of the system, the four instruments set in a half circle infront of the array, with the distance between the instruments equivalent to a “real” string quartet. A
recording taken with stereo room microphones in the middle of the concert hall was focused on an areain front of the center of the array, with the two audio channels as individual virtual sound sources. To
the left end of the array, the stereo room microphones from the balcony of the concert hall createdanother superimposed acoustic field.
A listener could go directly to the string quartet’s virtual position, stand in front of the ensemble
and listen to the group, then “stick one’s head” right into the individual positions of the two violins, theviola, and cello, while still hearing the other instruments in their respective location. While the music
played and the focused sound sources of the quartet players stayed in their spots, one could move to thecenter of the array and enter, through a “sonic portal,” the middle of the concert hall. Finally, one could
move to the left and find oneself in the further afield ambient space of the balcony. So listeners could
move between the three different sound fields, entering different acoustic spaces, and experiencing thevery different acoustic environments in relation to the string quartet, which “stayed in its spot.”
4.2 Electronic Music, Stereo Recordings, Music for Theater, and Field Recordings
In a third scenario, a piece of electro-acoustic, acousmatic music produced in stereo was playedover the WFS array, with each track focused in front of the array. Again, lthe istening experience while
sitting in the audience chamber, yielded a spatial, transparent, and stable sound stage, which would
have been impossible to obtain with a traditional left-right speaker configuration. A similar positive
experience was made when playing the stereo recording of a chamber ensemble through the WFS arrayset up on stage of EMPAC’s concert hall. Further listening experiments were conducted placing stereo
channels behind and in front of the WFS array and comparing studio productions of Rock and Blues
music with concert recordings of medieval choir music made in a reverberant church. The approach ofplaying stereo or multichannel recordings through a WFS array needs further investigation into whichkind of stereo productions benefit from being played back through WFS systems.

Two arrays at larger vertical off-set.
Placement of focused sound sources the lower array
were marked by projected circles on the floor, which
also moved as the sound positions were moved.

Experiments were also conducted to gain experience with two arrays positioned at different heights,
as shown in Figure 10. Combining two horizontal arrays at a vertical distance yielded a moreenveloping perception and allowed for virtual sound sources at different elevations. In this case, eacharray was treated separately, with the signal processing for one array not being related to the other.

1153

Additionally, flying WFS arrays overhead of listeners has been explored. In 2017, while inresidency at EMPAC, the sound designer Bobby McElver collaborated with the performer Andrew
Schneider on a new theater piece by using two parallel arrays positioned above the audience to movesounds highly successfully through the audience. He subsequently built a copy of the EMPAC system
for touring the work. – In 2019, two arrays in combination with a 7 th order Ambisonic system werehung in the EMPAC Concert Hall for presentations by Chris Watson and Tony Myatt (Figure 11),
performing underwater field recordings from around the globe and from an elephant habitat (the center
stage subwoofer being constructed for sub-audio elephant communication).

5. FUTURE DEVELOPMENT
As addressed before, our primary goal for using WFS is not to recreate the “as if” experience of aconcert hall or to recreate a “natural” acoustic environment, but to use it as an instrument in artistic
works. The listening tests with instrumental recordings have allowed the evaluation of the wave field’s
precision, as we can compare it to our experience with acoustic instruments in performance spaces
and thus allow veracity assessment of the rendered wave field in relation to perception.
On the application side, a tracking system for persons on stage will be integrated with thehigh-resolution WFS system as stage reinforcement option. A WFS array placed along a stage lip or at
an upstage wall will map sound to a performer’s position. This will allow a horizontal alignment of aperformer and the amplified reinforcement independent of the listeners’ position. For the spatialalignment of the word spoken by a person on stage with its amplification, a higher aliasing frequency
seems to be essential.
An important next step will be to include algorithms to treat distributed sets of arrays as a unified
instrument, so each array, wherever placed and however contiguous or not, takes the other arrays into
account and is used as a contributing factor to the sound sources created by all arrays.
A second generation of the modules with a higher density of speakers and a spatial aliasing
frequency of 5.9 kHz is under development. Figure 12 shows a rendering of the module, without
drivers and ports installed. The two inner rows will hold the 64 speakers and the outer rows will holdthe ports. The offsets at each end allow the interlocking of modules without affecting the distance
between drivers where two modules meet. One module will be 24.7 cm high, 22.8cm deep, and 188.5
cm long, with DANTE integrated power amplifiers on the back.

64-channel module, 2.9 cm distance between drivers.

6. CONCLUSION
The construction of a portable, modular WFS loudspeaker array allows for the deployment ofhighly flexible configurations, for research and for new artistic works, including flying arrays above
listeners. Listening comparisons using complex sounds in musical context made the difference inspatial and timbral fidelity between speakers at 5.8 cm distance and 2.9 cm perceivable; whether this isa result of higher aliasing frequency, needs further investigation. The positioning of focused soundsources was tested with exceptional results up to 8 m and more in front of a linear array. Composers
and researchers who had worked with other WFS systems with lower spatial aliasing frequencies
testified to the new quality reached on account of the closer placement of the drivers. Researchers arewelcome to use the system for further investigations.

1154

ACKNOWLEDGEMENTS
The author would like to thank Markus Noisternig/IRCAM for his continuous illuminations andtheoretical input; Todd Vos/EMPAC, for the array's systems definitions, design, construction, andintegration; Jeff Svatek/EMPAC for networking and applications services. Argeo Ascani/EMPAC
contributed to the programmatic development and presentation of the system.

REFERENCES
1. Berkhout A J, de Vries D, P. Vogel P. Acoustic control by wave field synthesis. The Journal of the
Acoustical Society of America 93, 2764 (1993); doi: 10.1121/1.405852
2. http://forumnet.ircam.fr/product/spat-en/

1155