## 2 Materials and Methods  
The research part of this paper is typically experimental in nature. In particular, the literature review method and the comparative method were used to supplement the content with the most important information on the subject of the article (state of the art). Elements of system analysis appeared in the theoretical part, which made it possible to present a clear structure of the analyzed topic. The results of the experimental attempts were analyzed using the comparative method. Analytical and individual case methods were also applied for this purpose. A model experiment was also used to study the effect of combustion products on the propagation of acoustic waves, a full-scale experiment was carried out to check the functioning of acoustic systems in real conditions, and the statistical processing of the experimental data was carried out to identify regularities and dependencies. The work involved burning a number of organic substances: paraffin wax, wood, paper, plastic, polystyrene, textiles, gypsum plasterboard, and engine (motor) oils. The characteristics of some of these substances have already been given above. Paraffin wax is a mixture of hydrocarbons, consisting of hydrocarbons with a straight chain, having a carbon number mainly greater than C20 [108]. Gypsum plasterboard consists of a gypsum core (calcium sulfate dihydrate), which is sandwiched between two layers of paper and has high fire-resistant properties [109; 110]. Engine (motor) oils are a mixture of petroleum-based hydrocarbons, polyalphaolefins, and some other hydrocarbons [111]. The first part of the research involved the use of the acoustic method in the range of two carrier frequencies of the extinguisher, i.e., 15 and 19 Hz, depending on a distance of the flame source from the outlet of the waveguide, which is also the outlet of the extinguisher. These frequencies were chosen to obtain a small impedance (11.4 \(\Omega\)). Its selection translates into an acceptable level of the vibration of the diaphragm of the sound source and its effectiveness, i.e., optimal acoustic energy transfer. Therefore, these frequencies oscillate around 17 Hz (the upper value plus and the lower value minus 2 Hz). Their selection was preceded by the determination of the transmission characteristics of the acoustic system. The selection of the methods was evaluated in terms of their ability to achieve specific research goals. For the purpose of the experiment, a point source of the flame was used, i.e., a candle with a flame height of about 2 cm, whose distance from the outlet of the extinguisher was varied from 20 cm to 70 or 80 cm (with a step of 20 or 10 cm in the case of the last distance), so it was possible to determine unambiguously whether the flames were successfully extinguished (this is important because when a diffuse fire source was used, they sometimes returned). The organic substance that was extinguished was paraffin wax in these attempts. A laboratory stand (Figure 2) was built for experimental attempts to extinguish flames using acoustic waves, with an acoustic extinguisher as the overriding component. It generates acoustic waves until the flames are completely extinguished. The test stand is universal (it allows us to change the parameters of the acoustic wave, in particular, its frequency, as well as its shape and modulation, as needed). Using a laboratory stand, it is possible to test the possibility of extinguishing flames originating from various materials with acoustic waves (i.e., paraffin wax, flammable gases, etc.). The tests were carried out outdoors (in windless conditions) on the same day with a background sound level oscillating around 65 dB. During these attempts, the maximum power delivered to the acoustic extinguisher did not exceed 1000 W. In practice, the power level required to extinguish the flames and the sound pressure level at the point where the flames were extinguished were investigated.