# Vibroacoustic Assessment of an Innovative Composite Material for the Roof of a Coupe Car
## 5 Evaluation of the Vibroacoustic Behavior of the Alfa Romeo 4C Car Roof
### Vibroacoustic Transfer Functions  
The Vibroacoustic Transfer Functions, FdT, (also called \"P/F\") represent the ratio between the sound pressure levels at the listening points inside the passenger compartment and the force levels that, applied at the body entry points, generated them. They characterize the acoustic response of the car body evaluated at the standard listening points (from a customer perspective), in correspondence of the driver's right ear and the passenger's left ear. In the case in question, since the car is the Alfa Romeo 4C, the rear passenger is not present.  
The coordinates of the driver listening points are calculated with respect to the seats (Figure 25) as in the followings:  
* X: Back seat intersection,
* Y: Center line back + 20 cm,
* Z: Back seat intersection + 70 cm.  
In order to observe and compare the acoustic contribution in the aforementioned points provided by the PP23 CF OPTIMIZED or TCA pavilion, the point where exciting the structure was chosen in the pavilion center (Figure 26).  
Figure 24: Analysis of potential coupling between the cavity and structure modes, considering the pavilion made of PP23 FC OPTIMIZED (red line) and TCA (blue line).  
Figure 25: Positioning of listening point inside passenger compartment.  
The vibroacoustic transfer functions were calculated with the Nastran SOL 111 Modal Frequency Response Analysis (according to FCA internal rules).  
From the graph in Figure 27, it is possible to observe:  
* an improvement in almost the entire frequency range provided by the new material,
* a worsening limited to the very low frequencies, where the PP23 CF pavilion strongly vibrates, when exciting the pavilion in the center.  
Moreover, in order to reproduce the working conditions of the vehicle, it was decided to excite the shock absorber attachment point, which is the most critical point, on which all the weight of the vehicle rests (Figure 28).  
Figure 26: Hall center input where exciting the structure and measurement point for sound pressure level (MIC 2).  
Figure 27: Vibroacoustic transfer functions (TF) from the car roof center to the ear node for TCA (black line) and PP23 CF OPTIMIZED (red line).  
The vibroacoustic transfer functions were assessed in the same way, evaluating both the contribution of the pavilion alone (Figure 29) and the vibroacoustic transfer functions, which include all the contributions of the panels adjacent to the cavity (Figure 30).  
Figure 28: Force input at front shock absorber connection (in red) and measurement point for sound pressure level (MIC 2).  
Figure 30: Vibroacoustic transfer functions (TF) from the shock absorber attachment to the ear node (MIC 2)—contribution of all panels, for pavilion made of TCA (black line) and PP23 CF OPTIMIZED (red line).  
Figure 29: Vibroacoustic transfer functions (TF) from the shock absorber attachment to the ear node (MIC 2)—contribution from the pavilion only, considering the two materials: TCA (black line) and PP23 CF OPTIMIZED (red line).  
From the graphs in Figures 29 and 30, it can be observed that:  
there is no longer a deterioration at low frequencies because the involved vibration modes are not excited,  
there is an improvement in the whole frequency range, with improvement peaks of 9%.