## 2 Materials and Methods
The measurement station presented in Figure 2a is universal, so it becomes possible to change the parameters of the acoustic signals, in particular the frequency of the sine wave (fundamental frequency), as well as higher harmonics (the total multiple of the fundamental oscillations), as expressed by the order of harmonics. This is undoubtedly an advantage of the acoustic method over the use of disposable firefighting agents (gases, foams, and extinguishing powders), which are quickly exhausted. All the tests included in this paper were conducted in an open space (outdoors). The instantaneous measured SPL (Sound Pressure Level) value for the background noise accompanying the measurements was equal to 59.1 dB. This value allows one to determine the sound pressure level before starting the measurements. In practice, the noise level is a random variable. Therefore, it is possible to determine the probability that its temporary level falls within a given range. Ln% is understood to be a statistical level (boundary level) which is a certain limiting level surpassed by the background noise in no more than n% of the period of observation (see Table 1). Acoustic measurements were made using a SVAN 979 (SVANTEK Ltd., Warsaw, Poland) class 1 m. Sound level measurements were performed for a profile with a Z-correction characteristic (which is why a letter Z appears on the graphs). In this case, a parallel detector--Slow (S) with time constant (1000 ms) was applied. Based on the data in Table 1, it can be seen that during the measurements, the 60.7 dB background noise level was exceeded by no more than 10% of the period of observation, while the 58.6 dB limit level was exceeded by no more than 50% of the period of observation, and the 56.1 dB limit level was exceeded by no more than 95% of the period of observation. A visual representation of the resulting data, as a supplemented diagram, is provided in Figure 3. \begin{table}
\begin{tabular}{c c} \hline
**Statistical Level** & **Profile (Z, Slow)** \\
**Ln** & **Leq Ln Spectra [dB]** \\ \hline L10 & 60.7 \\ L20 & 60.0 \\ L30 & 59.4 \\ L40 & 58.9 \\ L50 & 58.6 \\ L60 & 58.2 \\ L70 & 57.9 \\ L80 & 57.4 \\ L90 & 56.7 \\ L95 & 56.1 \\ \hline \end{tabular}
\end{table} 
Table 1: Equivalent sound level values Leg [dB] using Z-correction characteristic and limit levels (Ln%) exceeded by the temporary noise level during no more than n% of the period of observation. For safety, the power amplifier had thermal protection ('Protect'), so in these experiments the maximum power level delivered to the sound source did not exceed 1000 W. For each harmonic (depending on the frequency of the wave), the value of the power that had to be delivered to the fire extinguisher to observe the complete extinguishment of the flames and the SPL values at the point where the flames were extinguished were measured. During the experiments, multiple values, including instantaneous peak values, as well as minimum, maximum, and instantaneous rms (root mean square) values were also measured simultaneously. The results presented refer to the instantaneous rms sound level values. The measurement time was the same regardless of the frequency and the order of the higher harmonics (each time its value was equal to 5 s). The acoustic measurements included in the article are for instantaneous rms values of SPL according to the IEC 61672-1:2013 standard.