## 3 Results and Discussion
As the frequency increased, the required sound pressure level at which flame extinguishment was observed increased. For a wave with a frequency of 18 Hz, it was equal to 121.9 dB. The maximum value of the sound pressure level was obtained for the highest of the frequencies analyzed, that is, 21 Hz (it was equal to 123.6 dB). The results obtained for the modulated waveform appear to be interesting against this background (Table 2). In the case of a modulated waveform (sinusoidal wave AM modulated by a square waveform), the sound pressure level required to extinguish the flames was greater than 122 dB and lower than 127 dB (ranging from about 122 to about 126 dB). As in the case of the unmodulated waveform, for the lowest of the analyzed frequencies, that is, 14 Hz, the smallest value of the sound pressure level at which complete extinguishment of the flame was recorded was observed (122.6 dB). As the frequency increased, the required sound pressure level at which flame extinguishment was observed increased. For a wave with a frequency of 18 Hz, it was equal to 124.2 dB. In turn, the maximum value of the sound pressure level was obtained for the highest of the frequencies analyzed, that is, 20 Hz (it was equal to 126.5 dB). The summary diagrams of SPL [dB] versus f [Hz] for modulated and unmodulated waves are presented in Figure 4.  
For the lowest of the frequencies analyzed, that is, 14 Hz, the difference in sound pressure level at which the effect of complete extinguishment of the flames was observed was equal to 6.4 dB. For the middle frequency, that is, 18 Hz, it was equal to 2.3 dB, while for the highest of the analyzed frequencies (20 Hz for modulated waveform and 21 Hz for unmodulated waveform, respectively), it was almost equal to 3 dB. For the assumed conditions, the necessary sound pressure level for a modulated wave was slightly higher than for an unmodulated wave. It is worth noting that this does not always have to be the rule (depending on the circumstances, the class of fire, the source of the flames, etc. ), as further work in this area is required. The results obtained experimentally confirmed the validity of the theory that modulated and unmodulated waveforms can be applied successfully to extinguish flames. In practice, low-frequency acoustic waves can be effectively used to extinguish flames. The lower the frequency of the acoustic wave, the greater the turbulence in the flame, which is beneficial from the point of view of flame extinguishing. In practice, some design constraints arise from the waveguide length (the lower the frequency, the longer the waveguide). \begin{table}
\begin{tabular}{c c} \hline \hline
**f [Hz]** & **SPL [dB]** \\ \hline
14 & 116.2 \\
18 & 121.9 \\
21 & 123.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Required sound pressure (SPL) values necessary to extinguish flames as a function of frequency (f) for a sinusoidal wave. \begin{table}
\begin{tabular}{c c} \hline \hline
**f [Hz]** & **SPL [dB]** \\ \hline
14 & 122.6 \\
18 & 124.2 \\
20 & 126.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Required sound pressure (SPL) values necessary to extinguish flames as a function of frequency (f) for a sinusoidal wave AM modulated by a square waveform (f\({}_{\text{MOD}}\) = 0.125 Hz). The results presenting the minimum electrical power that had to be supplied to the acoustic extinguisher to observe the phenomenon of complete extinguishment of the flames seem interesting against this background. Table 3 shows the results obtained for an unmodulated wave (i.e., sinusoidal), while Table 4 presents the results for a sinusoidal wave with amplitude modulation (sine wave AM modulated by a square waveform, f\({}_{\text{MOD}}\) = 0.125 Hz). An analysis of the results obtained allows us to note that as the frequency increases, an increase in the minimum electrical power values that had to be delivered to the sound source to observe complete extinguishment of the flames was noted.