## 4 Results
### Measurement Results  
The sound transmission and the sound absorption and transmission coefficient were determined in the impedance tube for the one PTFE foil-based membrane specimens under a plane-wave excitation environment. The mobility method results (_nTL_, \(\alpha\) and \(\tau\)) explained above were compared with results determined in accordance to the ASTM E2611 -09 approach (for _nTL_--single load) and in accordance with the ISO 10534-2 (\(\alpha\)). Additionally, the FEM model was created taking in the account the membrane material  
Figure 10: Normal-incidence sound absorption of a single leaf PTFE rectangular shape membrane determined for six different terminations (variation of termination (“term.” in the legend) absorption from \(\alpha\) = 0.1 to 1, data were obtained from FEM). (**a**) TFM-ISO 10534-2 (“ISO” in the legend); (**b**) mobility-based method (“MM” in the legend); and (**c**) difference between sound absorption obtained by means two methods expressed in percentage. properties (see Section 3.1) as well as the measured boundary condition (the tension was \(T_{0}\) = 850 N/m\({}^{2}\)). The resulting comparisons are shown in Figure 11. Additionally, the operation deflection shapes (ODS) of a vibrating membrane related to the chosen frequencies, measured by means of the LDV, are also presented in Figure 11. With focusing to identify the mode shapes, the ODSs are the spatial surface plot of the imaginary part of displacement (the excitation signal was used as the reference signal to acquire information of the phase surface phase). The measured \(nTL\) spectra corresponds to each other rather well. The highest peak of single-membrane \(nTL\) spectra at a frequency of about 510 Hz is caused by anti-resonance phenomena (the displacement of membrane surface is too low). On the other hand, the dips of \(nTL\) and peaks of \(\alpha\)\(\tau\) spectra at a frequencies of approximately 205 Hz, 562 Hz and 1078 Hz are caused by structural resonances caused by 1 st mode (0.1--the first axisymmetric mode), 4 th mode (0.2--the second axisymmetric mode) and the 9 th mode (0.3--3 rd axisymmetric mode) of the membrane system. Two unexpected dips (that are missing in the idealised simulation cases, e.g., presented in chapter 5.1) in the \(nTL\) spectra occurs at frequencies 296 Hz and 464 Hz. The dips are related to the 2 nd and 3 rd structural mode (1.1 and 2.1) of the membrane. Normally, if the membrane edge boundaries would be uniform, the influence of the 2 nd and 3 rd mode would not be visible. In the experimental case, the stretching ring from PLA material was slightly bent, which affected the stiffness at the membrane edges. The stretching ring consisted of two rings connected by means of eight screws. In this way, the rings' perimeter was divided into eight parts with specific stiffness. After mounting the specimen into the impedance tube, the rings were slightly bended, which caused the variation of the stiffness at the perimeter. Specifically, two of the sixteen perimeter parts were significantly affected. This has been verified also in the FEM tuning process where the significantly lower stiffness (the spring constant \(k_{L}\) was lower by factor \(10^{4}\)) was needed to set in the boundary system of the membrane clamping. Caused by the already mentioned nonuniformities in the tension in the stretching ring and its bending, that not possible perfectly fit all the investigated frequency spectra. Nevertheless, the FEM  
Figure 11: (**a**) Sound transmission loss spectra of the single layer 0.5 mm PTFE membrane. (**b**) Measured sound absorption and transmission coefficient s of the single layer 0.5 mm PTFE membrane. Abbreviation describing the measurements based on: ASTM—ASTM E261109; MM—mobility based method; ISO-ISO 10534-2.  
and measurement based obtained results obtained for both discussed methods correspond reasonably well with each other.