## 3 Results and Discussion
A 1 Hz increase in the fundamental frequency resulted in a 2.9 dB increase in the pressure level at the measurement point compared to the previous experiment. At 16 Hz, the sound pressure capable of extinguishing the flame was equal to 123.9 dB (an increase of 0.3 dB over the previous measurement). At 17 Hz, the recorded sound pressure level was equal to 124.7 dB (an increase of 0.8 dB over the previous experiment), while at 18 Hz and 19 Hz it increased by 1 dB each time. The highest pressure value was recorded for the highest of the operating frequencies analyzed, i.e., 20 Hz (it was equal to 126.9 dB). This means a difference in the level of sound pressure capable of extinguishing the flames at extreme frequencies of 6.2 dB. The results for the power that had to be delivered to the acoustic extinguisher to observe the phenomenon of complete extinguishing of the flames seem interesting against this background. Table 3 presents the results of the experiments obtained for the lowest order of higher harmonics, i.e., the second order. As in the case of the measured sound pressure level, an increasing trend in power is noted as the frequency of the fundamental wave increases. This means that higher-frequency acoustic waves require more electrical power to be delivered to the fire extinguisher. The lowest power that had to be delivered to the sound source to observe the phenomenon of complete extinguishment of the flames was recorded for the lowest of the analyzed operating frequencies, i.e., 14 Hz (less than 100 W). When low-frequency acoustic waves are applied, a large amplitude of oscillation is noted (incompatibility of the waveguide with the speaker). Consequently, as previously indicated, despite their high effectiveness in extinguishing flames, this factor can significantly limit their practical application (design considerations). Therefore, depending on the order of harmonics, the results were obtained from successful extinguishing attempts for different minimum fundamental frequencies, as can be seen in Figures 4 and 6. For the fundamental frequency of 15 Hz and higher harmonics of the second order, the power was approximately twice as high (it was more than 200 W). Further increases in frequency did not translate into such a sharp increase in power (with regard to adjacent measurements). Therefore, for 16 Hz, the required power was recorded at 240 W, for 17 Hz-290 W, and for 18 Hz-370 W. As one moved away from the frequency for which the fire extinguisher was designed, i.e., 17 Hz, the required level of power that had to be delivered to the sound source to observe the phenomenon of complete extinguishment of the flames increased, which is due to the outlier from the resonant optimum. For this frequency, the loudspeaker diaphragm was the most acoustically stressed and the amplitude of its oscillations was small. For 19 Hz,  
Figure 4: Diagram of the sound pressure, as a function of frequency, for a low-frequency sinusoidal waveform depending on even harmonics (the order was changed in the range from 2 to 10). there was an increase in power of 140 W compared to the previous measurement. For the last of the frequencies analyzed, i.e., 20 Hz, the measured power was equal to 580 W.  
Below in Table 4, the experimental results of the sound pressure levels obtained for the highest order of the harmonics analyzed, i.e., the tenth order, are presented. Similarly to the last measurements (Table 2), a gradual increase in the sound pressure level was noted with increasing frequency, at which the flame extinguishment was recorded. With a fundamental wave frequency of 14 Hz and higher harmonics of the tenth order, a sound pressure of 125.6 dB was noted at the point where the flames were extinguished. A 1 Hz increase in the fundamental frequency did not cause significant changes from the previous experiment (the results were almost the same). At 16 Hz, the sound pressure capable of extinguishing the flame was equal to 126.4 dB (an increase of 0.7 dB over the previous measurement).