Investigation On Robustness Of Acoustic Focusing System Using Compact Loudspeaker Array

With the rapid development of portable communication devices, acoustic focusing characteristics of audio system have attracted increasingly more attention. There have been large amount of research on the realization of sound focusing system using loudspeaker array. The sound field reproduction system on portable communication devices can only be realized using compact loudspeaker array due to the limited size of such devices. Therefore, it is of great significance to investigate the acoustic focusing system using compact loudspeaker array. Acoustic Contrast Control (ACC) algorithm can focus the sound energy in the bright zone by maximizing the ratio between the mean sound energy in the bright zone and that in the dark zone, which makes it a competitive method to realize sound focusing control using a small number of loudspeakers. The robustness of the acoustic focusing algorithm has attracted many researchers’attention. Most of them concentrated on the influence of the imperfection of loudspeakers and the variation of acoustic transfer functions. Considering that the user of the compact array usually stands near the array, the scattering effects of the users are inevitable in a practical acoustic focusing system, therefore it is meaningful to investigate the robustness to head scattering. In this paper, a compact linear loudspeaker array model is established. The DAS (Delay And Sum) algorithm, ACC (Acoustic Contrast Control) algorithm and RACC (Robust Acoustic Contrast Control) algorithm are employed to concentrate the sound energy around listeners in the bright zone in the endfire direction of the linear loudspeaker array using the ideal freefield point source model. The robustness of the sound focusing system is analyzed in detail when there are listeners around the loudspeaker array.In this paper, a compact sound focusing system using five loudspeakers is established and investigated in simulations. The influence of random variation of loudspeaker transfer functions and the scattering of listeners on sound focusing performance are analyzed. The scattering of listeners is analyzed using one rigid sphere model (effect of one listener) and two rigid spheres model (effect of two listeners). It can be seen from the simulation results that high-frequency and wide-band sound focusing performance can be realized using DAS algorithm, but the low-frequency performance is poor. The system using DAS algorithm is highly robust to the transfer function variations and the scattering effects. For ACC algorithm, good single-frequency and wide-band performance is realized and the system is robust to one rigid sphere scattering and two rigid spheres scattering. However, the performance of the system using ACC algorithm deteriorates significantly with the random variation of loudspeaker transfer functions. When using RACC algorithm, not only good single-frequency and wide-band performance is realized, but also the system is robust to both the transfer function variations and the scattering effects, so that good sound focusing performance can be expected when there are listeners around the compact array.The experiments based on a compact array using 5 miniature loudspeakers are carried out to verify the system performance. Similar to the simulation results, DAS algorithm is robust but has poor performance in low-frequency directivity. Due to the bad robustness of ACC algorithm, neither single-frequency nor wide-band sound focusing can be obtained in experiment system. The focusing performance of RACC algorithm at both single-frequency and wide-band experiments is promising, and the system is robust to both one rigid sphere and two rigid spheres scattering effects.