Michael Heilemann

Driver placement for selective modal excitation in flat-panel loudspeakers

The audio quality of flat panel loudspeakers may be improved dramatically by selectively driving specific panel modes using an array of force actuators distributed on the panel surface. An optimal actuator array layout will maximize the energy coupling between the actuators and selected panel modes, while simultaneously eliminating modal crosstalk within the operating frequency band. We present results for optimal force actuator array layouts for both dynamic force actuators and piezoelectric patch actuators. Experiments on panel loudspeakers using optimized force actuator array layouts show that each of the panel modes within the tuning bandwidth may be independently addressed, and that the optimized array can limit the inter-modal crosstalk between −17 dB and −22 dB within the bandwidth using non-ideal force transducers. The methods described may also be useful in structural vibration control and noise reduction.

Measures of vibrational localization on point-driven flat-panel loudspeakers

Previous publications on flat-panel, or distributed-mode, loudspeakers generally assume that a localized driving force is able to spread energy evenly across the surface of a panel. However, investigations have shown that panel vibrations remain localized around the driving point at high frequencies, and this paper presents a deeper investigation into this phenomenon. Energy spreading will only occur when the panel is actuated in a frequency region with a low density of modes, as many modes actuated together will combine to form a band-limited delta function at the location of the driving force. A quantitative measure of localization is introduced, based on the ratio of the energy contained in a small region around the driving point to the energy contained in the entire panel. Experiments demonstrate that the localization rises uniformly above a critical frequency, and this critical frequency correlates well with predictions of the high-frequency plate region using standard modal overlap metrics.

Modal response of a simply supported plate driven by a single, asymmetric piezoelectric actuator for use as a flat-panel loudspeaker

An approximate analytical solution for the mechanical response of a thin plate to a single, asymmetrically bonded piezoelectric driver has been found. Previous work has focused only on the response of plates driven by symmetrically aligned actuators or for beams with drivers bonded to one side. The relative magnitudes of extensional and flexural waves produced by the single-sided driver configuration were determined as a function of the physical parameters of the plate and driver. The modal response of flexural waves for a simply supported thin plate also was determined. Interactions at the plate boundaries with the support structure may convert extensional waves into sound-producing flexural waves, which can lead to audio distortion. A non-moment inducing support scheme with high damping is proposed to minimize this effect.