## 2 Materials and Methods
### Realization of the Experiment  
Experimental research was carried out in such a way that its outputs were characterized by the greatest possible degree of credibility. All identified uncertainties and impacts unrelated to the defined output were minimized. The experimental conditions were as follows:  
* Given the number of performed measurements (in the order of hundreds), a constant state of the assessed building was ensured. Measurements were conducted over several weeks. * The location of the measuring points was the same for all measurements in the source and receiving room. * Background noise did not affect the measurements. This was repeatedly checked for all measurements. The choice of the building structure location was made with regard to this requirement. * Measurement chain was calibrated before and after each measurement. * For each measurement, sound pressure levels in the source and receiving room, background noise, and reverberation time in the receiving room were measured. * The building structure was homogenous (no structural openings, no media distribution, and no use of different building materials). Its construction was made of one type of material (brick--thickness 300 mm, plaster--15 mm), and it was built with emphasis on uniform properties over the entire surface of the structure. The partition under consideration was connected structurally to the side walls without the possibility of lateral sound transmission between the rooms. * In both rooms, double-glazed windows with soundproof double glazing were installed, which prevented the transfer of acoustic energy between the rooms. A double-layer door was also installed. Perfect closing of windows and doors has been ensured during the measurement. * For all set combinations of measurements (factors B and C), ten sets of measurements were performed, which is a statistically significant number. Figure 1 shows one of the measuring points. The measurement situation with respect to the floor plan of the test rooms is shown in Figure 2. Considering the fact that we also focused on the area of low frequencies (50 Hz) in the experiment, the emphasis in the experiment was on the arrangement of the measurement so that it meets the requirements for the measurement of low frequencies. For this reason, three noise source positions and at least ten measuring points in each room were selected, the distance of measuring points from the wall of at least 1.2 m was guaranteed, and the size of the room was chosen so that at least one dimension was greater than the wavelength of the assessed sound (more than 50 m\({}^{2}\)). The dimensions of the room comply with the requirements of ISO 16283-1. Five measuring points in the receiving room were selected for the measuring of reverberation time. The measurements used verified measuring chains of accuracy class 1, which are used in an accredited testing laboratory, are under continuous metrological control and linked to the international metrological standard. In order not to distort the sound field in the receiving and source room, only the measuring technician was present in the room during the measurements. White or pink noise is used for the excitation of a homogenous sound field. White noise is a random signal (or process) with a flat power spectral density. In other words, in a homogeneous environment, the signal's power spectral density has equal power in any band, at any centre frequency, having a given bandwidth. For example, the 20 Hz range between 40 and 60 Hz has the same power as the range of frequencies between 4000 and 4020 Hz. Pink noise, also known as 1/f noise, is a signal whose frequency density is proportional to the reciprocal of the frequency. When doubling the frequency, the energy falls by 3 dB. It has equal power in constant percentage bandwidths, for example, octave bands. This corresponds to the logarithmic nature of human perception of sound. Figure 1: Measuring point and measuring equipment (sound source, microphone). Figure 2: Floor plan of test rooms.