## 3 Results and Discussion
### Results of Laboratory Measurements
#### 3.1.1 Results of the First Method  
The measurement results in seconds for the first method and for 24 measurements in accordance with the methodology described in Sections 2.1 and 2.3 are as follows:  
500 Hz  
T\({}_{1}\) = 1.176; T\({}_{2}\) = 1.519; T\({}_  
The approximation of this fragment of the decay curve was adopted using linear regression, as shown in Figure 8.  
The regression equation averaging the fragments of twenty-four decay curves based on six points on each of them has the following form:  
\[L_{p}=-3.873\cdot x+98.4117 \tag{15}\]  
where \(x\) is the number of the point lying on the curve.  
The test probability for linear estimation is of the order of \(p\ll 0.05\), which means that all regression parameters are statistically significant.  
Using Equation (11), we obtain \(L_{1}=94.5\) dB, \(L_{6}=75.2\) dB, which yields \(\Delta L=19.3\) dB.  
Since the rate of sound level drop is described by the direction coefficient of a straight line expressed with Equation (15), the standard uncertainty involving the estimation of this coefficient can be accepted as the uncertainty resulting from the averaging of all decay curves \(u(L)=0.13\) dB.  
The average time increment is \(\Delta\tau=0.39\) s, while the mean standard deviation is \(\sigma_{\tau}=0.055\) s. When determining the standard uncertainty on the basis of (10), we obtain \(u(\tau)=0.011\) s.  
Taking these calculations into account, we obtain the reverberation time determined on the basis of Formula (1) \(T_{500}=1.21\) s and the combined uncertainty based on Formula (9) equal to \(u_{c}=0.036\) s.  
Ultimately, we can write \(T_{500}=1.21\pm 0.036\) s.  
A similar approach can be used for the frequency of 1000 Hz (Figure 9).  
The regression equation averaging the fragments of twenty four decay curves based on six points on each of them has the form:  
\[L_{p}=-3.8742\cdot x+99.7908 \tag{16}\]  
where \(x\) is the number of the point lying on the curve.  
The test probability for linear estimation is of the order of \(p\ll 0.05\), which means that all regression parameters are statistically significant.  
Using Equation (16), we obtain \(L_{1}=95.5\) dB, \(L_{6}=76.5\) dB, which yields \(\Delta L=19.0\) dB.  
Since the rate of sound level drop is described by the direction coefficient of a straight line expressed with Equation (16), the standard uncertainty involving the estimation of this coefficient can be accepted as the uncertainty resulting from the averaging of all decay curves \(u(L)=0.12\) dB.  
Figure 8: Regression function from 24 fragments of sound decay curves for the frequency of 500 Hz.  
The average time increment is \(\Delta\tau=0.34\) s, while the mean standard deviation is \(\sigma_{\tau}=0.04\) s. When determining the standard uncertainty on the basis of (10), we obtain \(u(\tau)=0.007\) s.  
Taking these calculations into account, we obtain the reverberation time determined on the basis of Formula (1) and the combined uncertainty based on Formula (9) equal to \(u_{c}=0.023\) s.  
Ultimately, we can write \(T_{1000}=1.07\pm 0.023\) s.  
Similar calculations are performed for 2000 Hz (Figure 10).  
Regression equation:  
\[L_{p}=-3.903\cdot x+99.45, \tag{17}\]  
where \(x\) is the number of the point lying on the curve.  
Figure 10: Regression function from 24 fragments of decay curves for the frequency of 2000 Hz.  
Figure 9: Regression function from 24 fragments of decay curves for the frequency of 1000 Hz.  
The test probability for linear estimation is of the order of \(p\ll 0.05\), which means that all regression parameters are statistically significant. In this case:  
\(L_{1}=95.5\) dB, \(L_{6}=76\) dB, \(\Delta L=19.5\) dB, \(u(L)=0.12\) dB.  
The average time increment is \(\Delta\tau=0.34\) s, while the mean standard deviation is \(\sigma_{\tau}=0.034\) s. When determining the standard uncertainty from (10), we obtain \(u(\tau)=0.007\) s.  
Ultimately, we can write \(T_{2000}=1.046\) s, \(u_{c}=0.022\) s, and hence \(T_{2000}=1.07\pm 0.022\) s.