## 4 Conclusions  
In the experiments, the two most commonly used types of noise--pink and white--were chosen for excitation in the source room. The analysis of experimental research leads to several conclusions:  
* Regression Model I, which was focused on assessing the effect of frequency and excitation interval on the apparent sound reduction index, shows the independence of the results from the excitation interval for frequencies above 120 Hz. This means that, in the case of routine assessments (for example, in the indicative determination of the airborne sound insulation of building structures), it is not important to pay attention to the intensity of sound field excitation in the source room, while the basic recommendations given in technical standards must be observed. * Regression Model II, which was focused on assessing the effect of frequency, excitation interval and type of noise used on apparent sound reduction index, shows the independence of the results from the type of noise used in two frequency intervals (Interval II, Interval III). Again, the independence  
Figure 8: Scatter plot of residuals for both types of noise (regression Model II). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multicolumn{3}{c}{**Frequency interval**} \\ \cline{2-4}  & **Interval I** & **Interval II** & **Interval III** \\ \hline \multirow{3}{*}{**Frequency**} & Changing the frequency value **significantly** & Changing the frequency value **significantly** & Changing the frequency value **affects** the apparent sound reduction index & **The average amount sound reduction index \\ \hline \multirow{3}{*}{**Excitation interval**} & Changing the excitation interval **significantly** & Changing the excitation interval **does not affect** & Changing the excitation interval **does not affect** & **The average amount sound reduction index \\ \cline{1-1}  & **affects** the apparent sound reduction index & Changing the frequency value **affects** the apparent sound reduction index & **The average amount of sound reduction index \\ \hline \multirow{3}{*}{**Noise type**} & Changing the type of noise **affects** the apparent sound reduction index & Changing the type of noise **does not affect** the apparent sound reduction index & **The average amount of sound reduction index \\ \cline{1-1}  & **offects** the apparent sound reduction index & Changing the type of noise **affects** the apparent sound reduction index & **The average amount of sound reduction index \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of the effect of parameters on the apparent sound reduction index. of results from the excitation interval for frequencies above 120 Hz and a significant dependence at frequencies up to 120 Hz are confirmed, which is evident from the scatter plot of residues for both types of noise (see Figure 8). The impact of frequency on the results of determination of airborne sound insulation of building structures is significant in all frequency intervals. On the other hand, in the area of low frequencies (below 100 Hz, Interval I), the results of determining the apparent sound reduction index are significantly affected by the values of the investigated frequency interval and the used excitation interval. The influence of the noise type also manifests itself in this frequency interval. For practice, it is important to realize that, if it is necessary to assess acoustic properties of building structures for frequencies below 100 Hz, it is important to consider the assessment using both types of noise. For evaluation and prediction, a lower value of the sound reduction index should always be used. From the conclusion of the evaluation, the authors came to the objectives of further research, which consists in replacing the currently used noises in the generation of the sound field with a real sound occurring in the assessed areas (loud sound of a television set, music of various genres, loud speaking).