Multi-Exciter Panel Compensation for Wave Field Synthesis

Multi-Exciter Panel Compensation for Wave Field Synthesis
DAGA '05 - München
Multi-Exciter Panel Compensation for Wave Field Synthesis
S.Spors, D.Seuberth, and R.Rabenstein
Multimedia Communications and Signal Processing, University of Erlangen-Nuremberg, Germany.
Email: {spors,seuberth,rabe}@LNT.de
Introduction
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New multi-channel audio reproduction techniques alleviate the sweet-spot problem at the price of an everincreasing number of loudspeakers. This development
calls for new multi-channel transducer systems. The
problems and solutions with the use of multi-exciter panels for wave field synthesis are discussed in this contribution.
The theory of wave field synthesis (WFS) has been initially developed at the Technical University of Delft over
the past decade [1]. In contrast to other multi-channel approaches, it is based on fundamental acoustic principles.
The foundation of WFS is given by the principle of Huygens and its mathematical formulation by the KirchhoffHelmholtz integral. The latter states that the acoustic
wave field inside a bounded area can be controlled by
surrounding this area with loudspeakers leveled with the
listeners ears. WFS is capable of reproducing the sound
of virtual sources outside and inside the listening area.
The fact that loudspeakers can only be mounted at discrete positions results in spatial aliasing due to spatial
sampling. Fortunately, the human auditory system seems
not to be very sensitive to these aliasing artifacts when
the loudspeaker distance remains within specified limits. It has been shown that a loudspeaker distance of
∆x = 10 . . . 30 cm is suitable in practice.
A WFS system requires a relatively high number of loudspeakers due to the dense loudspeaker spacing. The loudspeakers themself should cover the entire frequency range
of typical audio systems and at the same time have rather
small dimensions. On the other hand, they should be
simple to build to keep the costs of a WFS system bearable and unobtrusive to allow seamless integration into
the listening room. The next section will introduce the
concept of multi-exciter panels as potential solution.
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Figure 1: On axis frequency responses (magnitude) of four
exciters of the first MEP.
their characteristics are not comparable to high-quality
loudspeakers.
We built four MEPs with eight exciters each. The dimensions of the panels are 1.38×0.75×0.10 m(W×H×D), the
spacing in between the exciters ∆x = 17.1 cm. The next
section will discuss the properties of the panels derived
from acoustic measurements.
Characterization of Multi-Exciter Panels
We measured the impulse responses (IRs) from individual exciters to multiple microphone positions in an anechoic chamber. First each panel was measured individually. For this purpose the microphone positions where
chosen on a parallel axis to the panel with a distance of
d1 = 1.20 m. In total 16 microphone positions with a
spacing of ∆x = 8.55 cm where measured. The entire
measurement procedure for one panel resulted in 8 × 16
IRs. In a second step the panels where set-up in the
configuration when used as WFS system. Two panels
where placed side-by-side, the two other where placed at
their sides with an tilt angle of 60o . The panels formed
approximately the shape of a wide “U”. Again the IRs
from individual exciters to multiple microphone positions
where measured. In this case 32 microphone positions in
a distance of d2 = 2.54 m to the center elements of the
loudspeaker array. This procedure resulted in 32 × 32
IRs.
Figure 1 shows the magnitude of the measured on-axis
frequency responses of the first four exciter positions
(from the left) of the first panel. As can be seen the fre-
Multi-Exciter Panels
Multi-Exciter panels (MEPs) are a relatively new type
of multichannel loudspeakers that have been developed
in the course of the European project CARROUSO [2].
MEPs have a relatively simple mechanical construction.
They consist of a foam board with both sides covered by
a thin plastic layer. Multiple electro-mechanical exciters
are glued equidistant in a line on the back side of the
board. The board is fixed on the sides and placed in a
damped housing. The benefits of MEPs are their simple
construction and the possibility of seamless integration
into walls. Due to their multiple exciters MEPs are designed for the use with WFS. However, because of their
simple mechanical construction it can be expected that
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DAGA '05 - München
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Figure 2: Impulse responses from the 12th exciter to all microphone positions (U-shaped setup).
joint equalization of all channels of the MIMO system
compensates for the frequency and directional response
of each individual channel. The multiple-input/output
inverse-filtering theorem (MINT) [3] states that suitable
equalization filters can be found in nearly all practical
situations. However, the calculation can become unfeasible for a high number of channels. Using minimumphase IRs derived from the measured ones can minimize
this problem to some extend. However, the calculation
of minimum-phase responses often maintains only the
magnitude of the frequency response. As a result, multichannel minimum-phase equalization [4] cannot cope
with inter-panel reflections. A promising solution to overcome the complexity issue is to apply the concept of
wave domain adaptive filtering (WDAF) for loudspeaker
equalization [5]. Unfortunately, listener position independent equalization can only be performed below the
spatial aliasing frequency. Above individual equalization
of the exciters may be used [4].
Conclusion
We presented the concept of MEP loudspeakers. It has
been shown that they are especially useful in the context
of WFS. However, their relatively simple construction results in poor reproduction quality. Pre-equalization is
therefore mandatory when using MEPs. Different approaches for the calculation of suitable filters where reviewed. The most promising ones are multichannel equalization approaches and wave domain adaptive filtering.
Informal listening tests using the algorithm presented
in [4] proved that MEPs can be used for high-quality
auralization with WFS.
quency responses do not only differ significantly from the
ideal flat frequency response, they also show dependency
on the position of the exciter. The position dependency
is a consequence of the different distances of the exciters
with respect to the support. The remaining four exciters
(not shown) have similar frequency responses due to the
symmetry of the panel.
Figure 2 shows the IRs from the 12th exciter in the Usetup to all 32 microphone positions. As desired, the
shape of the first wavefront visible resembles the shape
of a point source. However, there are also traces visible which originate from reflections at the side panels.
The trace beginning at t ≈ 11 ms is caused by the first
panel, the trace beginning at t ≈ 15 ms is caused by the
fourth panel. Both panels are tilted with respect to the
two center panels. Thus, even in this “open” setup an
considerable amount of inter-panel reflections is present.
Acknowledgment
We would like to thank the “Institut für Rundfunktechnik
(IRT)” in Munich for the use of their anechoic chamber
and Helmut Wittek for his kind assistance.
References
[1] A.J. Berkhout, D. de Vries, and P. Vogel, “Acoustic control by wave field synthesis,” Journal of the Acoustic Society of America, vol. 93, no. 5, pp. 2764–2778, May 1993.
Compensation of Multi-Exciter Panels
The characteristics of MEPs as presented in the previous section indicate that these cannot be used for highquality reproduction of sound, unless proper digital preequalization is applied. The theory of WFS states that
it is sufficient to equalize the MEP responses on a line
between the listener and the panel. The wave field in
front of that line will then be equalized for all listener
positions. However, this is only valid below the spatial
aliasing frequency which is given by the exciter distance.
The methods used for the equalization of the MEPs can
be classified into three classes: (1) individual equalization
of each exciter, (2) multichannel equalization, (3) multichannel minimum-phase equalization.
Individual equalization of each exciter can only cope with
the frequency response, not with the radiation characteristics. We will not consider this approach further. The
IRs from each exciter to each microphone can be regarded
as multiple-input/multiple-output (MIMO) system. A
[2] S. Brix, T. Sporer, and J. Plogsties, “CARROUSO - An
European approach to 3D-audio,” in 110th AES Convention. Audio Engineering Society (AES), May 2001.
[3] M. Miyoshi and Y. Kaneda, “Inverse filtering of room
acoustics,” IEEE Transactions on Acoustics, Speech, and
Signal Processing, vol. 36, no. 2, pp. 145–152, February
1988.
[4] E. Corteel, U. Horbach, and R.S. Pellegrini, “Multichannel inverse filtering of multiexciter distributed mode loudspeakers for wave field sythesis,” in 112th AES Convention, Munich, Germany, May 2002, Audio Engineering Society (AES).
[5] S.Spors, H.Buchner, and R.Rabenstein, “Efficient active
listening room compensation for Wave Field Synthesis,”
in 116th AES Convention, Berlin, Germany, 2004, Audio
Engineering Society (AES).
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