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Jansen (1997) derived mathematical formulae for
synthesising moving sound sources. He took into
account the Doppler effect and showed that for its
application one would need to have continuously
time-varying delays. He also showed that for slow
moving sources the Doppler effect is negligible and
one can resort to updating locations and calculating
filters for each location and changing those in time.
This approach was chosen in this project.
Additionally, in order to avoid clicks in playback, an
option was built in to crossfade between two
locations to make the movement sound smoother.
3 System setup at the TU Berlin
The prototype system in Berlin was created with
the specific aim to make a system for the use in
electronic music (Weske 2001). The system consists
of a LINUX PC, driving 24 loudspeakers with an
RME Hammerfall Soundcard.
allows composers to define the movements of their
sounds, independent of the system on which it
eventually will be played. That is, the composer
should be bothered as less as possible with the actual
calculations for each loudspeaker, but instead be
able to focus on defining paths through space for his
sounds.
The current version of the program, WONDER
(Wave field synthesis Of New Dimensions of
Electronic music in Realtime), allows the composer
to do so. It allows the composer to work in two ways
with the program: either he creates a composition of
all movements of all the sound sources with
WONDER, using the composition tool, or he defines
a grid of points that he wants to use in his piece and
controls the movement from another program using
the OpenSoundControl protocol. The main part of
the program is the play engine which can play the
composition created or move the sources in realtime.
Each part of the program will be discussed
separately in the next subparagraphs.
The array configuration can be set in the
program. It is possible to define the position of
various array segments through a dialog.
WONDER includes a simple room model for
calculation of reflections. The user can define the
position of four walls of a rectangular room, an
absorption factor and the order of calculation. The
calculations are done with the mirror image source
model (see also Berkhout 1988).
4.1 Composition tool
The user can define various sources, each with
their own characteristics. A source in this context is
the virtual source from which sound emanates in
space and whose spatial parameters can be given by
the user.
For each source the user can set the type of
source (a point source having a specific location or a
plane wave having only a direction), whether it is
moving or stationary, its location or angle, the sound
input channel at which the sound will be supplied
and in the case of a point source, whether high
frequency damping of the air has to be taken into
account and whether reflections have to be
calculated or not. If reflections have to be calculated,
room characteristics can be defined (these can be
different for each source and even vary in time). In
the case of a moving source, one can define a path
through space and choose to let the movement loop
along the path. All input can be either typed in or
drawn with the mouse on a graphical overview. In
figure 4 a screenshot of the source definition dialog
is given.
After supplying all information and storing it,
WONDER checks the input for any problems and
after that the user can make the calculations for the
filters. He can then store the filters for all positions
calculated and a score for playback.
Figure 3. The loudspeaker array in the studio at the
TU Berlin.
For the calculation (in real time) of the
loudspeaker signals the program BruteFIR by
Torger2 is used. This program is capable of making
many convolutions with long filters in realtime. The
filter coefficients can be calculated with the interface
software described in this paper.
With the current prototype system it is possible
to play a maximum of 9 sound sources with different
locations in realtime, even when the sources are
moving. This is the maximum amount of sources;
the exact amount of sources that can be used in a
piece depend on the maximum filter length used. A
detailed overview of the capacity was given in last
year's paper (Baalman 2003).
4 Interface software
In order to work with the system, interface
software was needed to calculate the necessary filter
coefficients. The aim was to create an interface that
2 Torger, A., BruteFIR,
http://www. ludd.luth.se/-torger/brutefir.html
Proceedings ICMC 2004
