## 3 Results and Discussion
At 17 Hz, the recorded sound pressure level was equal to 129.2 dB (its value increased by 2.8 dB from the previous experiment), while at 18 Hz it was equal to 127.4 dB (the slight fluctuation may be caused by short-term changes in atmospheric conditions). The maximum difference in the sound pressure level capable of extinguishing the flames for higher harmonics of the tenth order was equal to 3.6 dB. Measurements for the fundamental frequencies of 19 and 20 Hz were not carried out due to the excessive power that had to be delivered to the sound source (the thermal protection of the measurement station tripped). In practice, the power amplifier has limitations on the amount of power, the nominal parameters of which are given in accordance with the AES (Audio Engineering Society) standard applied to music. If we juxtapose the results obtained for low-frequency sinusoidal waveform and higher harmonics (the second and the tenth orders) for the smallest of the frequencies analyzed, a difference in sound pressure level of 4.9 dB was obtained. In turn, for the highest common frequency analyzed, i.e., 18 Hz, the difference was equal to 1.7 dB. Indeed, it should be noted that in the case of higher harmonics of the tenth order, no results were obtained for the highest fundamental frequencies, i.e., 19 and 20 Hz, because the 'Protect' thermal protection in the power amplifier tripped. Although in practice the nominal power of the applied sound source was equal to 1700 W (one thousand seven hundred watts), the power delivered to the speaker was about 1000 W.  
Interesting against this background seem to be the results on the power that had to be delivered to the sound source to observe the phenomenon of complete extinguishment of the flames for a low-frequency sinusoidal waveform and the highest of the harmonics analyzed (i.e., the harmonics of the tenth order). These are presented below in Table 5. In practice, there is a logarithmic relationship between the sound pressure level and the electrical power supplied to the fire extinguisher. Similarly to the last measurements, the lowest power that had to be delivered to the sound source to observe the phenomenon of complete flame extinguishment was recorded for the lowest of the analyzed operating frequencies, i.e., 14 Hz (the power was equal to 390 W). For the fundamental frequency of 15 Hz, the power was 100 W higher compared to the lowest frequency and was equal to 490 W. For the 16 Hz frequency, it was noted to be 80 W higher than in the previous experiment (the recorded power was equal to 570 W). For subsequent frequencies, the measured powers that had to be delivered to the sound source to observe the phenomenon of complete flame extinguishment were equal to 600 and 650 W, respectively, representing increases of 170 and 50 W (with regard to adjacent measurements). As the frequency increased, the required level of power that had to be delivered to the sound source to observe the phenomenon of complete flame extinguishment increased. If we compare these results with those obtained for low-frequency sinusoidal waveforms and the higher harmonics of the second order, a significant power difference of about 300 W was obtained for the smallest frequency analyzed. In turn, for the highest common frequency analyzed, i.e., 18 Hz, the difference was equal to 280 W, which allows one to notice some similarity in the differences of the measured power values depending on the harmonic order. It is worth noting that due to the tripping of the thermal protection 'Protect'in the power amplifier, in the case of higher harmonics of the tenth order, no results were obtained for the frequencies 19 and 20 Hz (the power delivered to the speaker was about 1000 W). During the measurements, depending on the electrical power delivered to the sound source, the sound pressure increased linearly only in a certain range, making it impossible to apply the classical regression function model for all data. In the linear range, it is possible to use the models described, e.g., in [77].