This work investigates experimentally the effect a porous layer on the acoustic response of distributed mode loudspeakers (DMLs), which are manufactured under licence from NXT plc. The experiments were carried out in an anechoic chamber. The results suggest that a porous layer between a rigid base and a DML panel can considerably alter the acoustic emission in the near-field and in the far-field. This is typically illustrated by a reduction in the level of fluctuations in the emitted acoustic pressure spectra. These fluctuations are normally associated with interference between the sound emitted by the front surface of the speaker and by that emitted from the back. The results also suggest that the interference pattern in the air gap is altered by the porous layer, so that some individual resonances in the acoustic pressure spectra which inevitably occur between the rigid base and the vibrating plate can appear suppressed. A numerical simulation was carried out to model this effect.
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