## 2 Materials and Methods
For this purpose, a Class 1 SVAN 979 meter (SVANTEK Ltd., Warsaw, Poland) was used for measurements according to IEC (International Electrotechnical Commission) 62672-1:2013. The acoustic measurement time for each attempt was equal to 5 s.  
The acoustic extinguisher shown in Figure 2, in addition to the waveguide, consists of a power supply (portable battery or mains), a Rigol DG4102 generator (Rigol Technologies, Warsaw, Poland) with a modulation function, a Proel HPX2800 power amplifier (Proel s.p.a., Sant'Omero, Italy) which is electrically connected to the waveform generator, and a sound source B&C 21 DS115 (B&C Speakers s.p.a., Bagno a Ripoli, Italy). A nominal power of the sound source was equal to 1 700 W (one thousand se  
Figure 2: (a) Elements of the acoustic extinguisher (illustrative view): (1) generator, (2) modulator, (3) amplifier, (4) waveguide outlet, (5) sound source, and (6) flame source (i.e., candle, mock-up, etc.) exposed to acoustic waves; (b) actual acoustic extinguisher. amplifier input was supplied with a voltage of 1 V RMS. The role of the generator (usually with a modulation function) is to generate a suitable low-frequency waveform, for which the signal is provided to the power amplifier. The sound source, in turn, converts the signal so that there is an acoustic field generated by emitting waves with the energy necessary to extinguish the flames (without causing unwanted effects in the extinguished areas, hence the technique is called non-invasive) [112]. The tip of the waveguide is directed toward the flames. Fire extinguishers can use additional mechanisms in their operation that affect the process of extinguishing flames (e.g., connection ports, additional energy transducers, baffles, barriers, and elements that reflect waves, elements that store the energy of acoustic waves due to additive interference, attachments for the precise direction of acoustic waves, caps that increase the acoustic pressure at the outlet of the waveguide, etc.) [113]. This is important because, in particular, the close environment may affect the effectiveness of flame extinguishing with acoustic waves [114]. Generated, amplified, and focused sound waves of low frequency and high or very high acoustic power allow the creation of local conditions in which the flame is unable to sustain itself, as its continuous exposure in the acoustic field leads to its complete extinguishment without the need for chemicals [39]. Practical considerations have influenced the extinguisher to select a one-sided closed waveguide with a length of 4.28 m, assuming a sound propagation speed of 343 m/s. A notable feature is that the rectangular cross-section of the waveguide narrows towards the outlet of the acoustic extinguisher. With the reduction in the outlet, a favorable effect of increasing the sound pressure level at the outlet of the waveguide was observed, taking into account the physical properties of sound propagation. In the other case (open version), its length would have to be doubled, which for acoustic waves of such a low frequency would be difficult to achieve for practical reasons. Therefore, the extinguisher has a one-sided closed waveguide (acoustic waves are amplified due to the phenomenon of acoustic resonance), which is used in the acoustic technique of flame extinguishing. The second part of the study, aimed at investigating the features of monitoring in a smoky room, is based on the application of an acoustic device, the principle of which is based on the use of echolocation. This approach allows for the more effective detection of objects and obstacles, which improves the ability of specialists to quickly and accurately assess the surrounding situation. The device, thanks to the analysis of reflected acoustic waves, makes it possible to detect obstacles in a space, which allows to ensure an effective level of monitoring in a smoke-filled area during the combustion of organic substances. Figure 3 shows a graphic representation of the functioning of the device.