## 3 Results
### Sound Reduction Improvement  
Figure 6 shows, for both walls, a comparison between the radiated sound pressure level measured by microphones (ISO 10140-2, denoted in the legend as \(L_{\text{p,RR(Mic)}}\)) and calculated from ODSs (denoted in the legend as \(L_{\text{p,RR(Ray)}}\)). In accordance with ISO 10140-2, the microphone-based results were corrected for room acoustic effects by application of the factor \(-10.\text{log}_{10}\)(A) in Equation (1). Nevertheless, due to the non-diffusiveness of the room below the Shroeder frequency of the receiving room (330 Hz), differences in the \(1/3\)th octave \(R\)-spectrum with the ODS based spectrum, amount up to 8 dB (at 80 Hz). For higher frequencies, microphone measurements are not affected by room acoustics, by virtue of the high density per frequency band of room acoustic modes, which allows cancelling out their effect by spatial and frequency averaging. The accuracy of deflection shapes interpolation is related to the number of the samples per specific wave length. Vibrometry measurements however become less reliable, as they are based on a spatial scan along the wall with a finite grid spacing g = 15 cm. According to the Nyquist-Shannon sampling theorem, this only allows accurately sampling structural waves with wavelengths up to 4 g = 60 cm, which, based on the velocity dispersion curves in Figure 4, corresponds with a frequency of 1250 Hz (bending wave velocity 955 m/s). Considering the reduced accuracy of microphone and accelerometer-based measurements at low and high frequencies, respectively, we have taken the best of both and retained data above and below  
Figure 6: Sound pressure level (\(L_{\text{p}}\)) spectrum in the receiving room during measurements: Wall 1 (**a**); and Wall 2 (**b**). The area highlighted in purple indicates the spectrum below the Schroeder frequency. Blue and red curves express the \(L_{\text{p}}\) measured by standardized microphone approach in 1/3 rd octave spectrum. Green and black curves are the results of \(L_{\text{p}}\) achieved by vibrometry data processing in 1/3 rd and 1/48 th octave bands. \(f_{\text{c}}\) is the coincidence frequency of the wall;\(f_{0}\) is the ETICS resonance frequency; and \(f_{l,l}\) is the wall structural modes (i.e.,\(f_{1,1}\) is the first wall natural mode frequency). 1250 Hz for microphone and accelerometer-based measurements, respectively, thus constructing the combined spectra in Figure 7. Based on the resulting combined \(R\)-spectrum, the \(R_{\text{W}}\) value of Wall 1 is 1 dB lower than based on the microphone-based data alone (Table 2). The spectrum adaptation terms \(C\) and \(C_{tr}\) are both 1-2 dB lower. The complementarity between the microphone and accelerometer-based results allows assessing the effect of ETICS on the sound insulation spectrum in an extended frequency range, as shown by \(\Delta R=R_{\text{wall 2 }}-R_{\text{wall 1}}\) in Figure 8. The most prominent ETICS induced effect on the sound insulation curve occurs below 250 Hz, where the ETICS resonance causes a large dip in the frequency spectrum. In this range, the microphone results are affected by the spatial non-uniformity of the pressure level pattern of room acoustic modes [44], so that the \(\Delta R\) values extracted from vibrometry measurements can be considered as the more reliable.