## 3 Description of Measurements  
### Description of Measurement Setup  
In this section, the measurement setup and the specimen examined in the study is described. The measurement setup was created based on the impedance tube principle as described in the ASTM or ISO standards [25; 44]. The core part of the construction was a set of rigid steel tubes of diameter 0.1 m, which gives us an upper cut-off frequency of about of 2 kHz. To allow an LDV-scan of the vibrating membrane the impedance tube was accessible from one side surface of the chosen specimen. In this paper only the single membrane experiment is presented so the scanning from one side, the muffler side, was sufficient. However, the measurement setup gives opportunity to perform scanning measurements from both sides of the sample (also from the source side). This would find the application in cases of more complex specimen (which were not investigated here). Therefore, the loudspeaker (on the source side) was mounted on the (cylindrical) wall of the impedance tube as shown in Figure 4b.  
The 6 speaker type \"Eminence ME6-7586408 ohm\" was mounted on a rigid plate. The speaker was driven by a Mono Power Amplifier (Vincent SP-996). On the other side of the impedance tube apparatus, a low frequency muffler with an open end was mounted. The muffler consisted of a sheet-steel wall-based hollowed cone (upstream diameter = 0.75 m; downstream diameter = 0.1 m; length = 0.97 m), filled with mineral wool (see Figure 4a  
Figure 4: The measurement setup modification description. (**a**) Photo of full setup; and (**b**) the axis perpendicular side connection of LS.  
right end). The muffler was designed in accordance to the standards ISO 5136 and ISO 7235 [61, 62]. The measured sound absorption of the muffler is shown in Figure 5a. One can see that the sound absorption has a straight tendency as the function of the frequency in the range from 200 to 1100 Hz.  
Due to this, the presented measurement results were also evaluated only in this frequency region. Several holes were available in the tube apparatus for flush mounting the measurement microphones. The chosen pair distance (for the upstream and downstream part for nTL as well as \(\tau\) and \(\alpha\) determination) was 75 mm in both cases. A scheme of the total setup is shown in Figure 6. The microphones that were used for the measurements were a pair of \(\frac{1}{2}\)\" ICCP pre-polarized microphones (BSWA MA231), conditioned by the multichannel measurement system (Soft db--Tenor 24 bits, 8--channel data-logger). The ASTM and ISO based results were obtained by means of the impulse response measurement. As the excitation signal, for microphone measurement-based methods, a sine sweep was used. The Matlab(tm) postprocessing routines were created for the data processing.  
To determine the mobility-based sound transmission loss, a scanning laser Doppler vibrometer Polytec PSV 400 controlled by PSV Acquisitions SW was used. The measuring perpendicular mesh of 253 points was generated by SW tool and scanned. The reference excitation signal for the mobility-based method was pink noise generated by means of portable signal generator type Minirator-MR1 (NTI). The scanning grid used for spatially averaged velocity (or displacement data) consisted of 305 points.  
Figure 5: (a) The sound absorption coefficient spectra of the muffler measured in accordance to ISO 10534-2; and (b) test specimens—a single-leaf 0.5 mm PTFE membrane.  
Figure 6: Scheme of measurement setup with LDV scanning head. Where Mic 1–4 denote the microphone positions, \(s\) is distance between microphone positions, \(x_{1}\)–\(x_{4}\) are distance of the microphones to the specimen surface, \(<\)\(\omega\)\(>\) is the spatially averaged displacement measured at the specimen surface, LS is loudspeaker, LDV is laser doppler vibrometer.