## 6 Discussion and Conclusions  
This paper presents a piezoceramic driven loudspeaker for an automotive door side panel, which is characterized by low weight, low installation space, and a good possibility to integrate during the manufacturing process. The door side panel itself is manufactured using the long fiber injection (LFI) process, which gives possibilities to obtain various mechanical properties in the same component within the same production step. Despite their favorable characteristics, the application of structure-integrated flat panel loudspeakers is still connected, with many challenges. The lowest frequency limit and the non-flat frequency spectrum are especially noticeable. It could be shown that an actuator at one position can lead to significantly different acoustical properties depending on the type of integration. Depending on manufacturing properties, the flatness of the response could be achieved or the low-frequency performance and sensitivity can be improved. Additional reinforcement is not suggested at this stage. Generally, the usable range of structure-integrated flat panel loudspeakers analyzed within this paper starts at 300 Hz. Consequently, this design is predestined for a midrange/tweeter driver, which can be supported by a common woofer to cover the low frequencies. Linearizing flat panel loudspeakers with a DSP to increase the acoustical quality is recommended. Furthermore, it is useful to improve the linearity of the speaker in its original configuration by using bracing, new reinforcement combinations, and changes in the form factor. Besides, the radiation should be extended to lower frequencies, which is possible by increasing the vibrating membrane surface.  
Potential applications of the proposed integrated, lightweight and space-saving loudspeaker would be, for example, the reproduction of warning signals or navigation instructions in vehicles without high-quality hi-fi equipment. In mechanical and plant engineering, for example, enclosures or  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Properties** & **Area1—A** & **Area1—B** & **Area1—C** \\ \hline \(SPL_{m,AVG}\) (500 Hz–5 kHz) [dB] & 74.51 & 76.20 & 69.68 \\ \(f_{flow}\) (\(-\)10 db) [Hz] & 330.6 & 303.1 & 393.1 \\ \(\alpha_{log}\) (300 Hz–10 kHz) [dB] & 4.95 & 9.16 & 9.16 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Evaluation of the following properties mean SPL, lower frequency limit and the logarithmic standard deviation of all three specifications Area1—A–C.  
plane panels could be functionalized, in order to make acoustic signals usable for warning or error messages in addition to the visual signals that are usually present. In order to achieve the desired effects, the boundary conditions resulting from the used material, the desired functionality and the manufacturing process must be taken into account during conceptual design, dimensioning and manufacturing, according to the presented approach.  
Conceptualization, M.D., B.Z. and S.G.; methodology, M.D. and B.Z.; validation, B.Z.; formal analysis, B.Z. and K.H.; investigation, B.Z. and O.W.; resources, N.M., S.G., M.E.A. and M.D.; data curation, B.Z.; writing--original draft preparation, K.H., M.D., S.G., and B.Z.; writing--review and editing, K.H., M.D., O.W. and N.M.; visualization, B.Z.; supervision, N.M., M.D., M.E.A. and S.G. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.