## 3 Results and Discussion
A visual representation of the results obtained, from the experimental attempts, including not only the extreme higher harmonics of the second and tenth orders, but also of the fourth, sixth, and eighth orders, as summary diagrams, is presented in Figures 4 and 6. As noted earlier, when low-frequency acoustic waves are applied, a significant amplitude of vibration of the diaphragm of the sound source is noted, which may limit their application despite their high extinguishing capacity. Therefore, depending on the order of the harmonics, the results obtained were observed for different minimum fundamental frequencies, as can be seen in Figures 4 and 6. By analyzing all the results obtained, it can be seen that the sound pressure capable of having an extinguishing effect, at the point where the flames were extinguished, ranged from 120.7 to 129.4 dB, depending on the fundamental frequency and the order of higher harmonics. In general, as the operating frequency increased, a gradual increase in the sound pressure level was observed at the point where the flames were extinguished. This relationship is noted regardless of the order of the higher harmonics (Figure 4). Possible anomalies may be due to a short-term change in atmospheric conditions. On this basis, it becomes possible to determine the development trend function. Similar observations can be made from the analysis of the impact of the order of higher harmonics on the extinguishing effect. With an increase in the order of harmonics, a higher sound pressure capable of having an extinguishing effect was observed for the same frequency. For example, for the smallest of the operating frequencies analyzed, i.e., 14 Hz and harmonics of the second order, it was equal to 120.7 dB, for 14 Hz and harmonics of the sixth order it was equal to 124.2 dB, for 14 Hz and harmonics of the eighth order it was equal to 125.1 dB, while for this frequency and harmonics of the tenth order it was equal to 125.6 dB. A similar analogy can also be seen for other fundamental frequencies and orders of higher harmonics. The highest common frequency analyzed for which at least two measurements were obtained for different orders of higher harmonics was equal to 20 Hz. For harmonics of the second order for this frequency, the recorded sound pressure level was equal to 126.9 dB, while for harmonics of the eighth order it was equal to 127.4 dB (an increase of 0.5 dB). Despite some differences, due to the sinusoidal shape of the wave, the results are similar to those obtained for sinusoidal waveforms [46]. For illustrative purposes, Figure 5 provides empirical formulas that can serve as a reference for estimating the value of the necessary acoustic pressure level at which the flames are completely extinguished. A regression function (polynomial of order three) was applied to predict the sound pressure level from its known values. Of interest against this background appear to be the results for the minimum power level that had to be delivered to the extinguisher to observe the phenomenon of complete extinguishment of the flames (Figure 6). In practice, the curves in Figures 4 and 6 have a similar shape, since the sound pressure level and the minimum power measurements were carried out within the same extinguishing attempts (the departure from the resonant optimum for the sound pressure and power measurements was the same). In general, as the fundamental frequency increased, a successive increase in the minimum required extinguishing power was noted. This relationship was observed regardless of the order of the higher harmonics (as in the case of sound pressure, an increasing trend of function is visible). Similar observations can be made from the analysis of the influence of the order of higher harmonics on the extinguishing effect (analogous to the sound pressure level). As the order of harmonics increased, the need to supply more power at the same frequency to the extinguisher was noted to achieve the extinguishing effect.