# Sound Radiation of Aerodynamically Excited Flat Plates into Cavities
## 4 Experimental Results
### Flow-Induced Sound Radiation into the Cavity  
To determine the influence of the single measuring configurations and to compare and assess those effects, there are only one or two varying parameters. All figures are related to microphone position number 5 which is each located in the middle of the rear, side and top panel of the cavity (Figure 10). Unless otherwise specified, the maximum volume and the microphone position on the rear panel is chosen in the following. The lower cut-off frequency of the cavity as an acoustic chamber with maximum volume is 2866 Hz. Above this frequency, a strong superpositioning of natural frequencies occur which is apparent in the following sound pressure level depictions. \begin{table}
\begin{tabular}{c c c c c c c c c  
Figure 11 shows the effects of different microphone positions on the sound pressure levels. The measuring planes (Figure 11a), the distance from the flexible plate (Figure 11b) and finally the microphone position in the rear panel (Figure 11c) are varied. The frequency range is from 100 to 1000 Hz because that is the interesting area where significant modifications arise. The sound pressure level spectrum is plotted against the frequency for the three microphone positions on the different cavity walls. Figure 11: Sound Pressure Level at variation of the microphone position (plate thickness \(t=1\) mm). (**a**) Different measuring surfaces; (**b**) On the cover panel in \(z\)-direction; (**c**) On the rear panel in \(x\)-direction (minimum volume). Figure 10: Sketch of the microphone positions on the cavity walls. Common peaks appear in Figure 11a at 310 Hz, in the range of 400 Hz, 550 Hz and at higher frequencies, too. Below 300 Hz, the curve characteristics differ considerably because the frequency is below the frequency of the first room mode of the cavity and thus not influenced by the room. The second figure (Figure 11b) depicts the different distances from the flexible plate with position 2 as the nearest position to the plate. Between 300 and 500 Hz, the sound pressure level measured at position 8, located at half of the cavity length, is significantly lower than those of the other positions. This is because special acoustic waves, whose wavelength correspond to the half room length, form nodes and thus sound pressure minima. A greater distance to the flexible plate leads to a lower sound pressure level at low frequencies with the exception of a few peaks. Finally, different \(x\)-positions, describing the distances from the nozzle outlet in streamwise direction, are selected and located on the rear panel. The distance increases with a decreasing microphone number while number 5 exactly represents the perpendicularly displaced center of the flexible plate. There is a lower sound pressure level between the microphone positions 4 and 6 compared with 5 between 330 and 450 Hz and between 770 and 900 Hz. The responsible axial modes run in negative \(z\)-direction from the flexible plate and produce sound pressure minima at position 5. The curve shapes up to a frequency of 330 Hz in the low-frequency range are similar to each other. Changing the rear panel position and thus the cavity volume implicates different developing sound pressure levels (Figure 12). The maximum at 210 Hz only occurs with maximum volume although cavity modes do not propagate below the first axial mode at approximately 400 Hz. This sound superelevation is dependent on the room size and position. The small peak at a frequency of 387 Hz can be explained by the second axial room mode for the minimum cavity volume. The next step was to study the influence of plate thickness variation which is depicted in Figure 13. There is a considerable difference in the development of the sound pressure level at low frequencies for a plate thickness of 1 mm and the remaining thicknesses. The shape modes of the thinnest plate are excited and radiate sound into the cavity. Furthermore, the sound transmission through this plate is the highest among the other thicker plates.