# Improving Low-Frequency Panel Absorbers with Two-Dimensional Acoustic Black Hole
## 6 Conclusions  
In this paper, the improvements of the sound absorption performance of panel absorbers including Acoustic Black Holes were numerically and experimentally investigated. A detailed numerical modeling study was performed and the effects of the Acoustic Black Hole position, residual thickness and size were evaluated.  
Numerical analyses showed that creating asymmetry on the panels by moving the ABH further away from the center does not lead to a direct improvement in the frequency response of the system. It was seen that the positioning according to a denser mode distribution would improve the panel behavior. For the evaluated case, putting the center of the ABH into the panels at 60%, this position leads to the best design for frequency distribution. The thinner residual thickness was found to be better in the numerical  
Figure 18: Measured diffuse sound absorption curves of panels. Reference panel measurements (green lines), ABH panel measurements (blue lines) and target curve (dashed line).  
analysis. However, it was not possible to carve panels down to this thickness; therefore, the experimental studies were conducted with designs with a 0.3 mm residual thickness. Considering the size of the ABH, it can be summarized that the larger the ABH gets, the bigger its impact is.  
The experimental studies showed that the presented numerical approach is accurate for determining the vibro-acoustic behavior of panel absorbers incorporating a single ABH indentation. The sound absorption measurements reveal that it is possible to improve the low-frequency sound absorption performance of the panels by including an ABH structure. The sound absorption levels and the number of absorption peaks have been increased with a proper ABH design.  
The integration of ABH structures into panel absorbers is a promising concept for advancing their low-frequency sound absorption capabilities. The systematic investigation of ABH design parameters together with their experimental validation contributes to the ongoing pursuit of compact low frequency absorbers. It should be also noted that the performance for the selected case was not superior, since it was known that a thinner uniform panel would have been a better choice. It is therefore necessary to carry out further studies that push the ABH concept to the limits of production facilities.  
Conceptualization, M.F. and M.S.O.; methodology, M.F.; software, M.F.; validation, M.F. and M.S.O.; investigation, M.F.; resources, M.F. and M.E.A.; data curation, M.F.; writing--original draft preparation, M.F. and M.S.O.; writing--review and editing, M.F., M.S.O. and M.E.A.; visualization, M.F.; supervision, M.E.A. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding. The APC was funded by the Open Access Publication Fund of TU Dresden.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to technical difficulties.  
The manufacturing of the prototypes was performed by Hommel Kuchen & Mobelmanaufaktur GmbH, Reichenbach, and the ABH carving operation was performed by Rabatz Design, [PERSON], Bautzen, Germany.  
The authors declare no conflicts of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} ABH & Acoustic Black Hole \\ APC & Article Processing Charge \\ CNC & Computerized Numerical Control \\ DMA & Distributed Mode Absorber \\ FEA & Finite Element Analysis \\ HPL & High-Pressure Laminate \\ MDF & Medium-Density Fibreboard \\ VPR & Verbundplattenresonator, german for Composite Panel Resonator \\ BC & Back Cavity \\ \end{tabular}