## 4 Acoustic Testing and Results
### Measurement of Internal Pressure of Resonator Cells  
Further insight should be gained by the installation of three microphones flush with the lower cell wall in three consecutive cells. Two examples are shown in Figure 10.  
Here, the upstream and downstream cells were inactive, exhibiting only a small pinhole for ventilation of mean pressure in the surface. The middle cell was an active cell.  
For the reference liner with rigid walls, the sound pressure in the active mid cell was much higher than for the neighbouring inactive cells over the entire frequency range. The small differences below 800 Hz might be owed to non-symmetrical pinholes in the upstream (Us) and downstream (Ds) cells. The phase difference for upstream and downstream cells coincide for a frequency of 204 Hz. Using a simple one-dimensional acoustic network model and assuming a pinhole of 0.5 mm diameter, and taking the further geometry of face sheet and cell into account, a phase difference for an acoustic wave entering the inactive cell compared to the active middle cell of around 50\({}^{\circ}\) is obtained. However, this value strongly depends on the geometry of the pinhole, as it is coupled to the low frequency  
Figure 10: SPL values (**upper row**) and phase information referenced to the middle cell (**lower row**) for the reference sample (**left**) and FlexiS liner with TPU1170-0.3 (**right**); (acronyms: Us—upstream, Mid—midstream, Ds—downstream, AC—active cell, IC—inactive cell).  
resonance of the inactive cell. Above 1000 Hz, where the phase shift for the middle cell occurs, the canonical behaviour of \"early arrival\" of the signal at the upstream cell and \"late arrival\" at the downstream cell can be observed, meaning that the higher frequencies can enter the cell via the pinhole, although, as visible from the SPL plot, strongly damped. For the reference liner, there is no interaction between neighbouring cells across the cell walls.  
For the FlexiS liner, the behaviour is different. There are deviations in the SPL only for higher frequencies. That means that via a deflection of the flexible walls a high SPL is also generated in the inactive cells, only with frequencies above 1200 Hz not being able to produce significant deflections. Considering the small axial distance between neighbouring cells (20 mm from one to the next), an inactive cell will be subject to nearly simultaneous pressure increase from both neighbouring active cells, and subsequently simultaneous decrease again in the considered frequency range. Thus, the SPL in the inactive cells is mainly determined by the active cells.  
It is rather difficult to interpret the phase information for this configuration in detail. For low frequencies, the direct coupling of active and inactive cells via the flexible cell walls is directly visible through the zero/low phase difference between neighbouring cells. The expected divergence for upstream and down-stream cells for higher frequencies can be well observed. Changes in the slope of the phase curve may relate to resonances in the cavity (and/or the flexible wall) which manifest in (local) dissipation maxima (compare Figure 8).