## 2 Methods  
### Experimental Evaluation of the Facade Sound Insulation According to ISO 16283-3  
Facade sound insulation measurements are usually carried out according to the ISO 16283-3 standard. The evaluation can be performed either on individual facade elements or on the entire facade. Depending on the purpose of the measurement, different types of noise sources can be used (road/railway/aircraft traffic or loudspeaker). To have better control of the sound source position, this article focuses on the estimation of the facade global acoustic insulation with the loudspeaker method. The use of the loudspeaker method is necessary when facades with high acoustic performances are to be evaluated, when the real source (traffic, railway and aircraft) does not have sufficient energy for a correct evaluation or when higher repeatability of the results is required. When applied to the global facade, this method makes it possible to assess the sound reduction relative to a position that is 2 m away from the facade. In situ loudspeaker measurements according to ISO 16283-3 are carried out by positioning the noise source outdoors, the sound wave being then directed towards the center of the facade. The angle between the normal to the facade and the impinging sound wave must be \(45^{\circ}\pm 5^{\circ}\). The minimum distance, \(D\), between the facade and the loudspeaker must be at least 5 m. The loudspeaker should preferably be placed on the ground. The directivity of the loudspeakers must be approximately uniform, meaning that omnidirectional radiation is needed (Annex C--ISO 16283-3). The standardized level difference \(D_{\text{ls,2m,nT}}\) is computed as the difference between the outdoor sound pressure level measured at 2 m in front of the center of the facade (\(L_{1,\text{2m}}\)) and the space average of the sound pressure level in the receiving room (\(L_{2}\)), corrected by a factor accounting for the reverberation time of the room (\(T\)):  
\[D_{\text{ls,2m,nT}}=L_{1,\text{2m}}-L_{2}+10\text{lg}\left(\frac{T}{T_{0}}\right) \tag{1}\]  
where \(T_{0}=0.5\) s is the reference reverberation time for dwellings. The outside microphone must be placed 1.5 m above the floor of the receiving room. The weighted standardized level difference, \(D_{\text{ls,2m,nT,w}}\), is then calculated according to the ISO 717-1 standard [22]. When different positions of the sound source are used, for example when assessing the acoustic insulation of a corner room, the combined result must be calculated according to the following formula:  
\[D_{\text{ls,2m}}=\ -10\text{lg}\left(\frac{1}{n}\sum_{\text{i}=1}^{n}10^{-D_{ \text{i}}/10}\right) \tag{2}\]  
where \(n\) is the number of source positions and \(D_{\text{i}}\) is the level difference for each source-receiver combination. In the case of facades with two or more sides exposed to noise, the ISO 16283-3 standard does not detail in which positions the loudspeaker should be placed. However, in this case, two different measurements on the facade are required. Figure 1 shows four possible positioning combinations of the source for a corner room placed on the ground floor of a building. Considering the example in Figure 1, the following four combinations of a source positioned on the ground at a distance of 5 m from the facade can be considered: (a) 1a-2a; (b) 1a-2b; (c) 1b-2a; (d) 1b-2b. Case \"a\" is expected to lead to the lowest measured value of \(D_{\text{ls,2m,nT,w}}\), as in the considered positions the source is in direct view of both sides of the facade. On the other hand, case \"d\" will presumably lead to the highest measured value of the facade acoustic insulation because of the shielding effect of the building corner. Of course, if the final goal is the acoustic protection of the occupants, it is necessary to quantify the above-mentioned differences and, if possible, to provide the most representative value of the facade sound insulation.