## 5 Discussion  
In engineering practice, there are present both natural sources of excitation (earthquakes, winds), as well as man-made sources of excitation (traffic, construction, blasting). As cited in BS ISO 4866:2010 [24]: \"The frequency range of interest depends upon the spectral content of the excitation and upon the mechanical response of the structure\". Therefore, in [24] standard, frequencies from a very wide range: From 0.1 Hz to 500 Hz are indicated as important from the point of view of the structure receiving vibrations. In the above standard, however, a record can be found stating that most structural damage from man-made sources occurs in the frequency range from 1 Hz to 150 Hz. From the point of view of the possible impact of recorded vibrations on newly designed buildings, we are basically interested in reliable measurement of vibrations propagated in a soil medium with frequencies in a slightly narrower frequency band from 0.5-1.0 Hz to 100 Hz [19; 25]. In this frequency band the measuring bases with transducers mounted on them should not show any natural frequencies. Analyzing the results of numerical simulations carried out in Ansys Engineering Simulation Software, it should be stated that the weight of accelerometers mounted on them has a dominant impact on the natural frequencies of the measuring bases. The higher the total weight of transducers--the lower the values of the natural frequencies of the measuring bases anchored in the ground, and vice versa, the lower the soil stiffness--the higher the natural frequencies of the measuring bases. For the most accurate and also the heaviest uniaxial seismic accelerometers (type 8340 from Bruel & Kjaer with a sensitivity of 10 V/g and a weight of 0.775 kg), the lowest natural frequencies of the measuring base depending on the adopted Young's modulus of soil (E\({}_{0}\) = 75-125 MPa) are as follows (Tables 2, 5 and 8):  
* 150-181 Hz--for the measuring base No. 2 (L30 \(\times\) 30\(\times\)4 mm);
* 197-236 Hz--for the measuring base No. 1 (2 \(\times\) L30 \(\times\) 30 \(\times\) 4 mm);
* and 249-304 Hz--for the measuring base No. 3 (L50 \(\times\) 50 \(\times\) 5 mm). The lowest natural frequencies of the same measuring bases fixed with much lighter single-axis low-frequency accelerometers (Dytran Instruments type 3187D with a sensitivity of 0.5 V/g and a weight of 0.122 kg) are higher by at least 100 Hz, as listed below (Tables 3, 6 and 9): \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode** & **E\({}_{\mathbf{o}}\) = 75 MPa** & **E\({}_{\mathbf{o}}\) = 100 MPa** & **E\({}_{\mathbf{o}}\) = 125 MPa** \\ \hline
1 & 587.4 & 668.6 & 738.5 \\
2 & 597.5 & 683.8 & 758.4 \\
3 & 608.2 & 701.5 & 783.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Natural frequencies—the measuring base No. 3 (L50 \(\times\) 50 \(\times\) 5) with a miniature triaxial accelerometer: PCB Piezotronics type 356B18. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode** & **E\({}_{\mathbf{o}}\) = 75 MPa** & **E\({}_{\mathbf{o}}\) = 100 MPa** & **E\({}_{\mathbf{o}}\) = 125 MPa** \\ \hline
1 & 249.0 & 278.5 & 303.6 \\
2 & 268.6 & 302.0 & 330.1 \\
3 & 407.6 & 468.0 & 520.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Natural frequencies—the measuring base No. 3 (L50 \(\times\) 50 \(\times\) 5) with a set of 3 uniaxial seismic accelerometers: Bruel & Kjaer type 8340/Dytran Instruments type 3191A. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode** & **E\({}_{\mathbf{o}}\) = 75 MPa** & **E\({}_{\mathbf{o}}\) = 100 MPa** & **E\({}_{\mathbf{o}}\) = 125 MPa** \\ \hline
1 & 456.8 & 514.9 & 564.5 \\
2 & 478.7 & 543.2 & 598.1 \\
3 & 560.5 & 646.4 & 721.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Natural frequencies—the measuring base No. 3 (L50 \(\times\) 50 \(\times\) 5) with a set of 3 uniaxial seismic accelerometers: Dytran Instruments type 3187D. * 256-312 Hz--for the measuring base No. 2 (L30 \(\times\) 30 \(\times\) 4 mm);
* 326-396 Hz--for the measuring base No.