## 3 Results and Discussion
As in the case of sound pressure measurements, the highest common fundamental frequency analyzedfor 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 noted minimum extinguishing power level was equal to 580 W, while for harmonics of the eighth order it was equal to 900 W (an increase of more than 300 W). This means that, as both the fundamental frequency and the order of higher harmonics increases, the required power to be delivered to the extinguisher increases, as does the observed increase in the level of sound pressure capable of extinguishing the flames. To sum up, all analyses based on actual firefighting attempts allow one to conclude that the process of extinguishing the flames is influenced in addition to the fundamental frequency also by the order of harmonics. For illustrative purposes, to help with the selection of the flame-extinguishing parameters, empirical formulas are provided below in Figure 7, which can serve as a reference to estimate the power that had to be delivered to the acoustic extinguisher to completely  
Figure 5: Predicting SPLs for a low-frequency sinusoidal waveform depending on even harmonics (the order was changed in the range from 2 to 10). Figure 6: Diagram of the minimum power that had to be delivered to the acoustic extinguisher to extinguisher the flames, 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). extinguish the flames. As last time, a regression function was applied for this purpose (a polynomial of order three was used). The graphs obtained at the extinguishing site, showing the dependence of sound pressure as a function of frequency, seem to be interesting against this background--Fast Fourier Transform (FFT) graphs. A Hanning window and linear FFT averaging were applied. On this basis, it is possible to show which frequencies are present in a vibration during a certain period of time. In other words, it becomes possible to determine how much of each frequency is present in a sample (the frequency was determined along the x-axis, in turn, the amplitude of the sound pressure level along the y-axis). Spectrum calculations were made in accordance with the ANSI S.12.2-2008 standard. The FFT bandwidth was 78.125 Hz, the number of FFT lines was equal to 1601 (counting from zero), and the FFT sampling frequency was equal to 51,200 Hz. For illustrative purposes, the graph obtained for a fundamental frequency of 20 Hz and the smallest order of harmonics for which the experiment was performed, i.e., the harmonics of the second order, is provided in Figure 8.  
This graph shows the local maxima of the sound pressure level as well as the effect of even harmonics on the resulting waveform used for flame extinguishing. The measured sound pressure level applied to the site of the extinguished flames was equal to 126.9 dB  
Figure 8: Fast Fourier Transform (FFT) graph. Figure 7: Predicting the minimum power that had to be delivered to the acoustic extinguisher to extinguish the flames for a low-frequency sinusoidal waveform depending on even harmonics (the order was changed in the range from 2 to 10). (100 percent occurrence rate). The equivalent sound level values as a function of the limit levels are presented in Table 6.  
In this case, it can be seen that during the measurements, the 126.9 dB sound level was exceeded by no more than 10% of the period of observation, while the 126.5 dB limit level was exceeded by no more than 50% of the period of observation, and the 126 dB limit level was exceeded by no more than 95% of the period of observation. In practice, graphs and tabular lists were obtained in an analogous way for other fundamental frequencies, including the highest of the analyzed frequencies for which it was possible to extinguish flames using harmonics up to the tenth order, i.e., 18 Hz.