Improved Directivity of Flat Panel Loudspeakers by Minimizing the Off-Axis Radiation below Coincidence

Flat panel loudspeakers are a promising alternative to conventional loudspeakers. In particular, quasi-omnidirectional radiation at higher frequencies is stressed as an advantage of these systems compared to conventional speaker systems. However, this advantage can also be considered a disadvantage. Compared to that from conventional speakers with a flat and smooth on-axis and off-axis response, this wide radiation from flat panel loudspeakers occurs with an inconstant directivity factor, which can cause coloration and unusual spatial artifacts. This paper investigated the root causes of inhomogeneous directivity by using numerical methods. Based on these analyses, specific prototypes with various damping layups were built to overcome this problem. The additional damping layer reduces the off-axis radiation without significantly reducing the pressure level in the listening window. This approach is simple, robust, inexpensive and effective for improving the directivity of flat panel loudspeakers.

Window-Based Constant Beamwidth Beamformer

Beamformers have been widely used to enhance signals from a desired direction and suppress noise and interfering signals from other directions. Constant beamwidth beamformers enable a fixed beamwidth over a wide range of frequencies. Most of the existing approaches to design constant beamwidth beamformers are based on optimization algorithms with high computational complexity and are often sensitive to microphone mismatches. Other existing methods are based on adjusting the number of sensors according to the frequency, which simplify the design, but cannot control the sidelobe level. Here, we propose a window-based technique to attain the beamwidth constancy, in which different shapes of standard window functions are applied for different frequency bins as the real weighting coefficients of microphones. Thereby, not only do we keep the beamwidth constant, but we also control the sidelobe level. Simulation results show the advantages of our method compared with existing methods, including lower sidelobe level, higher directivity factor, and higher white noise gain.