## 5 Conclusions  
In this article, we focus on the determination of the acoustic properties (sound transmission loss, sound absorption and transmission coefficient) of membrane type of specimens under acoustic plane wave excitation. An overview of the measurement methods to extract the acoustic properties from impedance tube measurement data was given. It was noted that the determination of the acoustic properties (\(nTL\), \(\tau\)) in an impedance tube environment can be hampered by serious artefacts related to standing waves in the impedance tube. Many measurement methods suffer from this, obstructing the extraction of the real acoustic properties of the test specimen. The \(nTL\) measurement method least sensitive on possible backing wall reflection caused by a non-perfectly anechoic termination is the ASTM method (in accordance to standard ASTM E2611-09 [48]). In the case of the membrane sound absorption coefficient measurement, the backing wall impedance is directly influencing the resulting sound absorption. The method recommended by ISO 10534-2 [29] was used as a reference throughout the paper. Subsequently, the theoretical background of the proposed so-called mobility-based method (MM), which combines laser doppler vibrometer measurement data and microphone measurement data, was presented. The proposed MM method was compared with the conventional methods for sound transmission loss and absorption measurement. The methods were compared using numerically generated data from a finite element model. The FEM models included six different variations of the backing wall termination (\(\alpha_{1}=1.0\); \(\alpha_{2}=0.9\); \(\alpha_{3}=0.7\); \(\alpha_{4}=0.5\); \(\alpha_{5}=0.2\); \(\alpha_{6}=0.1\)). The numerical investigation focused on the assessment of the influence of the termination on the resulting values of sound absorption and transmission loss spectra of two differently shaped membranes (single layer circular shaped membrane and rectangular shaped membrane). The proposed mobility-based method was compared with the standardized ASTM (\(nTL\) measurement) and ISO (\(\alpha\) determination) methods, as well as with two other chosen methods, and the effect of the termination on the sound reflection was assessed. The ASTM method describes a measurement method that uses four microphones and one or two loads (different terminations). When dealing with symmetrical test specimen, only one load suffices. In the case if two loads method is applied almost no difference in results could be observed. Based on the FEM results, the MM gave similar results for anechoic backing wall termination as the results obtained in accordance to the ASTM E2611-09 with the local deviation up to 3 dB. This is a common phenomenon in numerical modelling, where no or very little damping is defined in the model. In the area of resonance, the results obtained from different domains may be overestimated (in this particular case, the results from the acoustic and the structural domain were compared). In absorption assessment, the deviation is up to 3% for the first membrane resonance frequencies. At other frequencies the deviation is not more than 1%, for both the absorption factor and the transmission loss. By increasing the reflection of the backing termination, the deviations increases and the effect of the front-back reflection can be recognised. The effect usually did not reach a deviation in the absorption of more than 3% and in the transmission loss no more than 5 dB (excluding the 1 st mode resonance effect). In the experimental part the circular shaped single layer PTFE foil specimen was tested and compared with the FE model fitted to the experiment. The MM was compared with the ASTM (\(nTL\)) and ISO (\(\alpha\)) measurement methods. The results were presented in narrow band spectra, to see more precisely the differences in the results. The MM method gave reasonable results for \(nTL\) spectra.