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Distributed Mode Loudspeakers


Introduction to Distributed Mode Loudspeakers

A Distributed Mode Loudspeaker (DML) consist of a flat panel of a light and stiff material to which one or more dynamic exciters are attached. The exciter mechanical vibration force the panel material to vibrate, which in turn radiates a sound field. In contrast to the piston-like surface movement of cone loudspeakers, the radiation mechanism of DML panels is produced by means of bending waves that travel across the surface of the panel. The flat membrane of the bending wave transducer principle used in DML technology is excited chaotically. Due to this characteristic, the bending wave membrane shows a wider dispersion angle than a pistonic radiator.

The typical impulse response of an exciter in a DML panel consists of a first direct pulse, just as dynamic loudspeakers, followed by a long tail related to the distributed modes of the panel. In contrast to the random behavior of the panel, this initial part of the impulse response is deterministic and creates an adequate spatial impression.

Multiactuator Panels

Multiactuator panels (MAP) are an extension of the distribute mode technology to be used in WFS applications. An array of small DML panels that substitute dynamic transducers is conceptually valid. However, this solution is not optimal because of the poor response in low frequency and the edge effects that a set of adjacent panels would cause. Then, a larger panel is needed but without increasing the distance between exciters if a certain aliasing frequency has to be maintained. Single panels driven by multiple exciters, each one with different driving signals, is a configuration called multiexciter or multiactuator panel.



If such panels have to be used in WFS, each exciter must only drive a small part around the exciter position, acting as discrete radiating point. However, since exciters are now attached to the same panel, there is a risk of cross-influence between neighboring exciters. Therefore, it is necessary to use a panel material that has sufficient internal damping to ensure that each exciter effectively acts like an individual loudspeaker. Recently, it has been shown that strong point source radiation emanates from a single exciter position when driven individually, and that a certain amount of scattering is observed in neighboring exciters.

There are some benefits for MAPs to be used in WFS reproduction. They can easily be integrated into a living room because of its low visual profile. Furthermore, the vibration of the surface is almost negligible so that it can be used as projection screens. For this purpose, each exciter must act as an individual loudspeaker in a vibrating surface, which is driven by many excitation points with different signals. As a consequence, in terms of sound radiation, MAPs behavior is quite different from that of the dynamic loudspeaker arrays.

References

[1] N. Harris and M. O. Hawksford, “The distributed-mode loudspeaker as a broad-band acoustic radiator” 103rd Convention of the Audio Engineering Society, no. 4526, New York, USA, Sept. 1997

[2] M. Boone and W. de Brujin, “On the applicability of distributed mode loudspeaker panels for Wave Field Synthesis based sound reproduction,” 108th Convention of the Audio Engineering Society, Paris, France, Feb. 2000.

[3] M. Boone, “Multi-actuator panels (MAPs) as loudspeaker arrays for Wave Field Synthesis,” J. Audio Eng. Soc., vol. 52, no. 7–8, pp. 712–723, July 2004.

[4] M. Kuster, D. de Vries, D. Beer, and S. Brix, “Structural and acoustic analysis of multiactuator panels,” J. Audio Eng. Soc., vol. 54, no. 11, pp. 1065–1076, Nov. 2006

[5] Z. Suzhen, S. Yong, S. Xiaoxiang, and D. Jinglei, “Model optimization of distributed-mode loudspeaker using attached masses,” J. Audio Eng. Soc., vol. 54, no. 4, pp. 295–305, Apr. 2006