## 5 Validating Tests of the Final Design  
This section analyzes the range of properties achieved in the LFI-manufacturing process and their influence on the acoustic behavior. Furthermore, recommendations for specific material properties and construction details of a piezo-ceramic driven flat-panel loudspeaker are given. For the acoustical measurement, a system consisting of a microphone type 2669, a power supply module type 2826, an input module type 3041 and a controller module type 7540-A by Bruel and Kjaer were selected. The microphone was placed in 1 m distance of the structure focusing toward the center of the panel (see Figure 11).  
Figure 10: (**a**) Piezoelectric actuator placed on the carbon fabric just before embedding, and (**b**) textile-reinforced composite after curing.  
Figure 9: (**a**) Local fixation of the piezoelectric actuator, and (**b**) connecting cables in the mold cavity by means of a fast curing adhesive.  
The results of the individual pressure level of each specification are shown in the following Figure 12. The results are smoothed with 1/3 per octave to increase comparability due to the high deviation of the frequency response. As shown in Figure 12, all three specifications have different frequency-dependent behavior, which illustrates the property range of the manufacturing process. All specifications have a lower frequency limit of approx. 300 Hz, which corresponds to a midrange/tweeter driver. This frequency limit is the result of the small radiating area, which gives a sufficient decoupling, but an increased stiffness for low-frequency radiation. For the full frequency reproduction, an additional woofer is needed.  
The reached acoustical properties are summarized in Table 1 and expressed with the following parameters: lowest frequency, mean sound pressure level (SPL) and frequency deviations.  
Figure 11: Microphone placed in 1 m distance to the panel.  
Figure 12: Comparison of the individual sound pressure levels of all three specifications: A—standard configuration, B—without additional reinforcement, C—with local carbon fiber reinforcement.  
Specifications A and B have nearly similar mean SPL, whereby specification B is with 76.2 dB, the most sensitive one, which corresponds to the lower mass and lower stiffness of the membrane. Furthermore, specification B has the lowest frequency limit, with 300 Hz. Through the soft membrane, SPL difference at 100 Hz of approx. 20 dB is possible and the low-frequency behavior is improved. However, this specification has not only advantages. The deviation of specification B is much higher compared to specification A, which results in a more uneven frequency response and more DSP-filtering is needed.  
Furthermore, similar deviation results are achieved by the stiffer specification C. This implicates a certain optimum of modal density for a given structure, defined by the area, ratio and material properties. In general, specification C is too stiff for this application. This results in a higher corner frequency of 393 Hz and a 6.5 dB lower mean SPL. This is the result of the higher mass, due to the additional reinforcement structure and the increased stiffness, which decreases the efficiency of the piezo-ceramic.  
In summary, this section emphasized the variety of properties, which can be achieved with the LFI-manufacturing process. An actuator at one position can lead to significantly different acoustical properties depending on the type of integration. In this case, an unstiffened specification provides the lowest frequency and the highest sensitivity. The flattest behavior can be achieved with specification A and non-modified properties. A structure that is too stiff results in lower sensitivity and missing low-frequency level. Reinforcement is not recommended for this speaker configuration. In the following steps, the low-frequency behavior needs to be improved as well as the frequency deviation, to be more comparable to conventional speaker systems.