## 4 Results of Research and Testing
### Sound Absorption  
The quantitative dependences between the noise level after crossing the barrier and the rubber crumb amount were obtained from the results of testing the developed sound-absorbing composites. They are shown in Figure 10.  
Three compositions with different amounts of rubber crumb were tested. The first control composition contained no rubber crumb, the sixth composition contained only rubber crumb as aggregate. The graph shows that the sound absorption capacity of the material increases as the rubber crumb quantity increases. The results of determining the coefficient of the sound absorption of the material are shown in Table 4.  
One can conclude that the most effective in terms of sound absorbing properties is the composition of concrete using only rubber crumb as an aggregate. The result was achieved due to the addition of the fractioned rubber crumb of fractions from 5 to 2.5 mm in a quantity of 6%; fractions from 2.5 to 1.25 in a quantity of 29%; fractions from 1.25 to 0.63 in a quantity of 29%; fractions from 0.63 to 0.315 mm in a quantity of 28%; and fractions from 0.315 to 0.16 mm in a quantity of 7%. The use of fractioned rubber crumb enables us to obtain the necessary sound absorption since the large specific surface area of open pore walls contributes to the active conversion of sound vibration energy into thermal energy due to friction losses. The use \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Composition Number**} & \multicolumn{2}{c}{**Percentage of Rubber**} & \multirow{2}{*}{**Initial Noise Level, dB**} & \multicolumn{2}{c}{**Noise Level after**} & \multirow{2}{*}{**Coefficient of Sound**} \\  & **Crumb Amount** & & & **Crossing the Barrier, dB** & \\  & **Relative to Aggregate** & & & \\ \hline
1 & 0\% & 83.80 & 69.2 & 0.17 \\
6 & 10\% & 83.80 & 57.1 & 0.31 \\
7 & 100\% & 83.80 & 45.0 & 0.46 \\
4 & 100\% & 83.80 & 48.2 & 0.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Sound absorption of developed composites. Figure 10: Results of the samples test for sound absorption (v-coefficient of variation). of rubber crumb with larger grains significantly reduces the compressive strength of the developed material. The proposed grain composition of the rubber crumb was stated as a result of evaluating the strength and sound absorption properties of the material. The proposed composite is made from secondary resources, which contributes to the development of resource-and energy-saving technologies [41; 42]. The gypsum-cement-pozzolan composition and the results of the sample tests are shown in Table 5.  
The proposed gypsum-cement-pozzolan composition allows us to increase the sound absorption coefficient, which rises from values of 0.31-0.48 to values of 0.46-0.70 in the studied frequency range in comparison with the composition without rubber crumbs. These results confirm the published data in [43], in which it was shown that the sound absorption coefficient of cement concretes with rubber crumbs increases at high frequencies. Summarizing the results obtained and the previously published data of other authors one can conclude that the sound absorption coefficient of gypsum and cement composites rises significantly with the increase in amount of rubber crumb from used car tires and it depends less on the properties of this rubber crumb. The mechanical properties of gypsum and cement composites significantly depend on both the amount and the properties of the rubber crumb. In paper [43] the sound absorption at the low (63, 125, 250 and 500 Hz) and high (1000, 2000, 4000 and 5000 Hz) frequencies of small-scale panels based on cement and rubber crumb was determined. Studies have shown good results for the sound absorption of the material with the higher rubber crumb amount (15%). The sound absorption was evaluated for samples with different amounts of crumb rubber (7.5% and 15% of the fine aggregate mass) and with four different grades of crumb rubber that was previously subjected to freezing and heating.