## 4 Results  
### The Normal Incidence Sound Transmission Loss (Simulations)  
The \(nTL\), \(\tau\) and \(\alpha\) as determined by the different methods (for \(nTL\)-Seybert, Salissou, ASTM and the Mobility-Based method, for \(\alpha\)-ISO and for \(\tau\)-Mobility-Based method) were compared. All the methods were compared for the circular shaped single layer membrane. For the rectangular shaped membrane, the Seybert and Salissou methods were considered for reasons of brevity. Measuring the acoustic properties (\(nTL\), \(\tau\) and \(\alpha\)) in an impedance tube environment serious artefacts can be caused by standing waves in the impedance tube. Obviously, these effects are unwanted in the measurement of the acoustic properties of the sample. As these artefacts are known to be strongly dependent up the acoustic termination of the impedance tube, terminations were considered having six different values of the sound absorption coefficient \(\alpha\) of: \(\alpha_{1}=1.0\); \(\alpha_{2}=0.9\); \(\alpha_{3}=0.7\); \(\alpha_{4}=0.5\); \(\alpha_{5}=0.2\); \(\alpha_{6}=0.1\). In a number of cases, a negative \(nTL\) was obtained at some frequencies, which is obviously non-physical, and also related to standing waves in the impedance tube. These negative areas are marked by a pale red colour (see figures below). The most accurate method, which does not show a large dependency on the amount of reflection of the backing, was the ASTM method. For this reason the ASTM method with 100% absorption backing wall has been considered as the reference. [PERSON]'s method, as mentioned above, is a one-load method and its results are dependent on the termination sound absorption (Figure 7a). By increasing the reflection coefficient of the back termination, the spectra of \(nTL\) are strongly affected by multiple reflection between the sample and the termination. Due to these effects, this method requires an anechoic termination. The frequency location of \"unwanted\" dips in the spectra can be predicted from the relation \(f_{x}=n\cdot c_{0}/(2\cdot l_{2})\), where \(n\), \(c_{0}\) and \(l_{2}\) are an arbitrary natural number, the speed of sound in the air, and the length of the downstream tube, respectively. Beside the dips and peaks related to the standing wave modes in the downstream tube, also the dips which are related to the membrane resonances and the peaks which are related to the anti-resonances, are affected. The worst highest and lowest backing absorption are compared to the ASTM method in Figure 7f. Almost negligible peaks/dips in \(nTL\) spectra are occurring in case with the 100% absorptive termination (the thick blue curve), showing the real acoustic properties of the membrane structure under test. By decreasing the termination absorption to 20% the differences increased up to 14 dB. The wave decomposition (WD) by [PERSON] [55] is a two-load method (Figure 7b). The results that are obtained with this method also strongly dependent on the load termination absorption. In order to obtain reasonable results (less artifacts due to resonances in the impedance tube), at least one of the two loads needs to have an high absorptive termination. Nevertheless, the method of [PERSON] cannot eliminate the effect of the multiple termination reflections effectively. In the worst case with termination absorption ratio 0.2: 0.1 (veryreflective composition), the \(\Delta nTL\) reaches the differences more than 20 dB in comparison to the anechoic termination. Figure 7: Cont. The most accurate method (independent on the termination) for the determination of \(nTL\) is the method in accordance to ASTM E2611-09 [48] (Figure 7c). Two transfer matrix (TM) methods (single or two loads method) are described in the standard. In the case of a phase synchronized and amplitude calibrated system, the two-loads method can properly and accurately determine \(nTL\) while effectively neglecting the influence of the backing wall termination on the sound absorption.