Active Noise Control in Open Spaces Using Directional Parametric Loudspeakers

This study describes an active noise control (ANC) solution to open environments based on directional parametric array loudspeakers (PALs). The conventional ANC systems using voice coil loudspeakers (VCLs) have the problem of acoustic feedback, low directivity, and low effective range, especially in outdoor environments where the reverberation is minimal. To eliminate these drawbacks, a feedforward ANC system was invented, which uses PALs (which can produce focused anti-noise through ultrasonic carrier demodulation). The system utilized an improved Filtered-x LMS algorithm incorporating online secondary path modelling and adaptive auxiliary noise power scheduling that would converge in real-time. The experiments were conducted at different distances (2 m, 4 m, and 6 m) and environmental conditions. PALs achieved significantly greater attenuation compared to VCLs, with up to 13.2 dB reduction at 2 m, reduced acoustic feedback and improved confinement of quite zones. The polar plots and the SPL heatmaps verified the higher spatial selectivity of PALs. Signal-to-noise ratios were found, even though there was slight harmonic distortion. These findings imply that PAL-based ANC can potentially target outdoor noise reduction in industrial and urban environments.