# Improved Sound Radiation of Flat Panel Loudspeakers Using the Local Air Spring Effect
## 6 Summary  
This paper introduces an alternative constructive approach to improve the frequency response of an exciter-driven flat-panel loudspeaker. The introduced modal-related air compliance is used to suppress modes with a high amount of anti-phase components. This is realized with a separation plate, which is mounted at a certain distance directly behind the panel to create an irregular shaped enclosure. The panel is stiffened due to the additional air spring caused by the paneled volume leading to a flatter transfer function. This design can also be used for existing flat panel loudspeakers with an open backside. This concept is developed based on the modal and sound pressure-related decomposition results. The numerical results were verified with a real adaptable prototype, which shows a sufficient fit. It could be shown that dips in the frequency response are not necessarily caused by a lack of kinetic energy in the panel, rather by high anti-phase components. These anti-phase components minimize the acoustic output, although the panel has considerable movement. Based on these results, a design for the separation plate is introduced, which provides additional stiffening for the anti-phase components caused by Mode 3. The separation plate design smooths the frequency response. The influence is large at lower frequencies, and changes decrease with increasing frequency. Above 500 Hz, nearly no changes are detectable, which is caused by the decreasing displacement and pressure change at higher frequencies. Furthermore, a numerical simulation model is developed to analyze the system's behavior. Modal-dependent air stiffening reduces anti-phase components, which minimizes dips and peaks in the frequency response. In the last section, the cut-out diameter and the thickness of the paneled volume are varied to achieve an optimal frequency response. With the help of simulation tools, an optimum irregular enclosure shaped, based on thickness and cut-out diameter variation, can be presented. The prospective impact on the temporal behavior should also be mentioned. Due to the strong suppression of peaks due to the irregular shaped enclosure, the temporal behavior of the loudspeaker is also improved and the transient response is shorter. As a result, this system can also be used to improve the temporal behavior. This has to be investigated in further experiments and the efficiency for various configurations has to be confirmed. This work is to be seen as the basis for a new design method of an irregular shaped enclosure. This paper presents a significantly smoothed frequency response by a constructive approach that changes the modal structure of the system without modifying the panel. It is advised to investigate this research topic further. Very important points are the limits of this system and the versatility of its applicability. Therefore it is recommended to analyze the system for other flat panel loudspeakers (with different sizes, different panels or ratios) and to analyze the achievable optimizations. However, the geometrical properties need to be adapted according to the present system. Therefore, the design of the separation plate has to be designed for each case individually. The presented workflow can be used to minimize the anti-phase components of individual systems. Figure 18: Comparison of the simulation results of the standard version, the separation (\(r=0.07\) m, \(t=0.01\) m) and the optimized version (\(r=0.12\) m, \(t=0.012\) m). **Author Contributions:** Conceptualization, B.Z. and R.S. ; methodology, B.Z. and R.S. ; software, B.Z. ; validation, B.Z. and R.S. ; formal analysis, B.Z. and S.M. ; investigation, B.Z. and R.S. ; resources, M.E.A. ; data curation, B.Z. ; writing--original draft preparation, B.Z., R.S and S.M. ; writing--review and editing, B.Z., S.M and M.E.A; visualization, B.Z. ; and supervision, S.M. and M.E.A. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding.