## 1 Introduction  
The use of composite thermal insulation systems on external walls to improve the energy efficiency of buildings has been a common method in Middle European countries for many decades. External thermal insulation composite systems (ETICS), consisting of a thermal insulation layer placed on the external side of the wall cladding of a building, covered by a thin solid layer of plaster-like material, offer an adequate solution against thermal energy losses, while keeping the installation cost low and application procedure simple. Building physics research on ETICS involves not only thermal insulation performance [1], but also moisture infiltration in existing systems in newly developed materials in terms of bio-deterioration. [2; 3; 4] Extensive studies on optimization of ETICS design were published with respect to both thermal properties and moisture penetration [1; 5]. While wall insulation layers in the 1980s were typically 40-80 mm thick, the current criteria of EU energy policy and the predictions of EAE (European Association for External Thermal Insulation Composite Systems) require (depending on EU country) a thermal insulation layer with thickness between 200 mm and 300 mm, in particular if passive house standards have to be met [6; 7]. In the case of renovations and energy performance upgrades of existing buildings, by virtue of its non-invasive character, the use of ETICS is one of the most straightforward approaches to improve thermal insulation. An important challenge for the implementation of ETICS is to improve the thermal insulation properties of exterior walls of buildings while preserving or ideally improving other essential building physics parameters and aspects of architectural aesthetics [8; 9; 10; 11]. In the case of thermal insulation systems, in which a mineral wool layer is properly placed in the cavity between two massive layers, acoustic insulation performance also improves. This is thanks to sound absorptive properties of mineral wool that leads to damping of acoustic resonances in the cavity between the two massive layers. However, when a mineral wool layer is used in an ETICS configuration, sound insulation of an external wall might paradoxically decrease compared to situation in which only one bare walls are used [12; 13]. Despite the added mass, the addition of a thin solid layer with a soft layer in between the outside air and a massive wall can have an acoustic impedance matching effect, increasing sound transmission from the air to the wall. Compared to the original acoustic insulation spectrum of the bare wall, the insulation spectrum after application of ETICS shows a dip around the mass-spring-mass (m-s-m) resonance frequency of the two ETICS layers, due to the thin solid layer acting as a responding mass, and the thermal insulation layer as spring [9; 13; 14; 15; 16; 17; 18; 19]. Typically, the resonance frequency lies in the part of the spectrum where human hearing is quite sensitive, such that, even for a small effect on the single number rating \(R_{W}\)[20], the effect of ETICS on the sound insulation is likely to be audible. In a recent revision of the Environmental Noise Directive (END) [21], it was stated that noise pollution continues to be a source of health effects [22] such as sleep disorders with awakenings [23], learning impairment [24], and hypertension ischemic heart disease [25]. The outer and inner walls of habitations represent, together with the windows, the first and most important defense for citizens from night noise, avoiding the above reported negative health effect. Thus, the use of composite thermal insulation systems also needs to be investigated from building acoustics point of view. Several studies based on sound insulation measurements according to the ISO 10140 [16; 26; 27], have reported effects of ETICS on parts of the insulation curve between \(-8\) dB and \(+19\) dB.