## 1 Introduction  
The car became the most used audio listening environment for high-quality audio output [1]. On the one hand, the in-car audio experience is getting even more important considering increasing leisure time coming, with progressing autonomous driving [2]. On the other hand, the persisting trend towards lightweight designs due to energy saving causes a decrease of the system's damping and hence increased cabin noise level [3]. Using conventional loudspeaker design, a sufficient acoustical quality can be reached. However, the increasing number of speakers and peripherals significantly increases the overall car mass [4]. A promising alternative is posed by a flat panel loudspeaker design, due to its lower mass and invisible integration [5]. Such loudspeakers use vibrations of existing components with sufficient surface, which are excited by an electro-mechanical transducer [6]. The usage of piezoceramics as an alternative to an electro-mechanical transducer has been known for a considerable time [7; 8]. In particular, the small installation thickness and the low weight are outstanding and allow new interior design concepts. However, the interaction between the used material, geometry of the structure, type and integration position of the piezoelectric based flat panel loudspeaker has to be well understood to use such a design in everyday applications. This knowledge can be gained either by an extensive experimental study or by a numerical design in simulated environments, to ensure satisfactory acoustic characteristics [9; 10; 11]. A challenging aspect results from the fact that the exciter position determines the dynamic response of the structure [12] and hence, efficient vibration excitation in the given frequency range. In the current paper, an adaptronic, piezoceramic-driven loudspeaker for an automotive door side panel is presented, 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--a fully automated polyurethane spraying process with fiber reinforcement--which has been developed for the production of large-scale components [13; 14; 15; 16]. Moreover, this process enables the possibility of manufacturing lightweight components with integrated sensors and actuators [17]. This fact results from the moderate process-intrinsic loads (in particular low reaction temperatures and cavity pressures) [18]. Additionally, the large variability of material- and process-specific parameters results in a broad spectrum of mechanical properties. Thus, for example, by the adaptation of fiber length, fiber mass content and composite density as well as the integration of continuous fiber reinforcements, tailor-made composite properties can be achieved [19; 20].  
The presented study is structured as follows:  
* Numerical pre-design to ensure efficient radiation, where the position of the piezoceramics is determined based on a finite element model;
* Validation of the simulation data by measuring the surface velocity with laser vibrometry on a prototype with externally bonded piezoceramic exciters;
* Selection of the most promising integration positions;
* Manufacturing of the door side panel with structure-integrated piezoceramics;
* Assessment of the acoustic performance of all manufactured structures.  
Based on the results of the above described studies, guidelines for the design of structure-integrated loudspeakers using fiber-reinforced polyurethane and piezoelectric transducers were derived.