Specific properties of phase characteristics of Distributed Mode Loudspeakers

Distributed Mode Loudspeakers (DMLs) are characterized by a specific principle of operation, based on bending oscillations of a plate with a certain stiffness and dimensions. Because of this property DML produce a diffuse sound field in the proximity of the loudspeaker. In this work a series of phase characteristics measurements of a DML was carried out, which were compared with analogous ones, carried out for a conventional electrodynamic loudspeaker with a pistonic diaphragm. The elevations of measurement points were chosen in order to coincide with the most likely positions of a listener relative to the DML. A measurement on the axis of the transducer was also conducted. The results have demonstrated that the variability of phase value resulting as a function of frequency and localization of a measurement point is high. It may be concluded, that the DML is a non-minimum phase system, with some frequency regions of minimum phase behaviour. These regions are strongly dependent on the point of measurement.