## 1 Introduction
Even in cases where ETICS application results in a net increase of \(R_{W}\), the dip in the insulation curve, which occurs typically at rather low frequencies, can pose a considerable problem when insulation against traffic noise is concerned [28; 29; 30; 31]. The sophisticated numerical prediction model to determine ETICS cladding system transmission loss was introduced based on transfer matrix method [32; 33]. Discussions on harmonization and standardization of assessment procedures of the sound insulation in dwellings resulted in EU networking action [34]. The particular impact of noise from road, rail and air traffic is expressed in the spectrum adaptation terms \(C\) and \(C_{tr}\)[20]. In some countries, spectrum adaptation terms are not considered, but the contribution of low frequencies is included by means of extended assessment range, i.e., from 50 to 5000 Hz [35; 36; 37; 38; 39]. In [13; 40], the insulation of low frequency noiseproduced by traffic is considered by calculation of single number quantities \(R_{W}+C_{tr}\) or \(R_{W}+C_{tr,50\text{-}5000}\). In the case of ETICS, the adapted single number quantities rate the insulation performance substantially lower than \(R_{W}\), due to the dip in low frequencies. In some cases, the adapted single number quantity is lower after applying ETICS walls than before. This motivates the research on how to affect the ETICS dip by tuning the resonance frequency of the system into less audible lower frequencies [41] or by reducing the depth of the dip. In current engineering design practice, the influence on the sound insulation due to a planned refurbishment of an outer wall \(\Delta R_{W}\) (sound reduction index improvement) is often predicted by empirical relations. Based on extensive investigations by [PERSON] et al. [13], a semi-empirical approach involving the spectrum adaptation terms \(C\) and \(C_{tr}\) has been implemented in the recent draft of ISO 12354-1 [42]. This has led to sound insulation values that are more adequate in relation to subjective perception. The estimated \(\Delta R_{W}\) due to the ETICS is then added to the weighted sound reduction index value \(R_{W}\) (or \(R_{W}+\) spectrum adaptation term) of the bare original wall to get the resulting sound reduction index of the entire assembly. To assess the effects of ETICS on the perception of outdoor noise transmitted into building interior and thus its impact on the indoor acoustic comfort, assessment of the sound transmission properties is needed in the whole audible range, which is not always the case. In this paper, we report on laboratory sound insulation measurements using standardized sound pressure measurement techniques as well as vibrometry method [42]. Until now, no measurement techniques have been standardized or proposed that would allow for reliable quantification of sound transmission at low frequencies (i.e., lower than Schroeder frequency of receiving room, which is typically around 250 Hz) [43]. Here, we exploit the advantages of vibrometry to assess the impact of the installation of ETICS on the airborne sound insulation down to 50 Hz. The resonant behavior at low frequencies that goes along with the use of ETICS is accurately determined, unaffected by room acoustic aspects of the laboratory transmission rooms. In vibrometry, surface vibrations are mapped, resulting in operation deflection shapes (ODS). From the detailed information about vibrations, the sound power radiated by the measured element can be determined directly avoiding the influence of room resonances [44; 45]. This approach is more reliable than conventional acoustic (i.e., microphone-based) methods, as, in the latter method, a diffuse acoustic field is assumed, which is not the case in the lower frequency range. Vibrometry also allows the determination of the radiation efficiency and sound reduction index with high spectral resolution [46], including the effect of the finiteness of the test object on the radiation efficiency and sound reduction index [47; 48].