## 3 Results and Discussion  
From a practical point of view, the extinguishing abilities of acoustic waves of different frequencies and modulations are not thoroughly analyzed (state of the art), especially with the use of high and very high powers. Therefore, it was necessary to partially fill the gap in this area. To test the extinguishing capabilities of acoustic waves, the laboratory station presented in Section 2 (in Figure 2) was used, the main component of which is an acoustic extinguisher with a high-power sound source (the nominal power was equal to 1700 W). For the research presented in this article, the applied frequencies of the acoustic extinguisher were not chosen coincidentally (the mechanical load on the sound source and the acoustic efficiency were taken into account). First, the relationship between sound pressure and frequency was analyzed at a distance of 100 cm from the extinguisher outlet. This made it possible to determine the operating frequency of the acoustic extinguisher. Between 17 and 18 Hz, a minimum impedance of 11.4 \(\Omega\) was recorded. Based on the response to this frequency, a local maximum pressure level was obtained [47]. Therefore, the 17.25 Hz frequency was called the operating frequency. In practice, low-frequency acoustic waves have better extinguishing abilities compared to high-frequency waves, which is due to greater flame turbulence. However, technical considerations limit their use (vibration of the speaker diaphragm as a result of the fact that the waveguide does not interact with the speaker at low power). The applied frequencies of the acoustic extinguisher were in the range of 14 to 21 Hz to show the extinguishing capabilities of the device for three selected points that cover frequencies close to the operating frequency: 14, 18, and 20 or 21 Hz (for modulated and unmodulated waveforms, respectively). They were chosen for the safety of the speaker (its mechanical load). For the unmodulated waveform, the highest frequency was 1 Hz higher than for the modulated waveform and was equal to 21 Hz. For the modulated waveform, the highest applied frequency was equal to 20 Hz (above this frequency, there was a risk of deflection of the speaker's diaphragm, threatening to damage it). Due to the design limitations of the extinguisher (sound source), the lowest frequency analyzed in both cases was equal to 14 Hz. Since acoustic waves pass through a variety of materials (both solid, liquid, and gaseous), the acoustic method can be used to extinguish flames or to support/control the combustion process, where access to traditional extinguishing agents is impossible or limited or when extinguishing with them is ineffective (hard-to-reach places). The advantage of the acoustic technique is its versatility (the parameters of the acoustic waves can be selected in such a way that the method, depending on the conditions, including the source of the flames, can be as effective as possible). The results of the experimental measurements are included below. The results presented are for instantaneous SPL values (rms) using frequency Z-correction characteristics. Table 1 shows the results obtained for an unmodulated wave (i.e., sine wave), while Table 2  
Figure 3: (**a–c**) Illustration of flame extinguishment using acoustic waves depending on frequency: (A) direction of wave propagation, (B) flame source, (C) flame, (D) flame rupture, and (E) exhaust fumes. presents the results for a sinusoidal wave with amplitude modulation (sine wave AM modulated by a square waveform, f\({}_{\text{MOD}}\) = 0.125 Hz). Based on an analysis of the data in Table 1, it can be concluded that to extinguish the flames using an unmodulated waveform (sine wave), the sound pressure level is greater than 115 dB and lower than 125 dB (ranging from about 116 to about 123 dB). For the lowest of the frequencies analyzed, that is, 14 Hz, the smallest value of the sound pressure level at which complete flame extinguishment was recorded was observed (116.2 dB).