## 2 Materials and Methods
To be able to transmit sounds below audible frequencies, a suitable cabinet was designed in the construction stage of the extinguisher to ensure high efficiency and work out the desired frequency range. The length of the waveguide in which the acoustic wave is formed was selected to match the operating frequency of the acoustic extinguisher (it is derived from 1/4 of the wavelength that is in this waveguide). As a result, there is an enhancement of efficiency for this low frequency (i.e., the operating frequency). The author realizes that better sound-transmission conditions would be obtained in a straight waveguide--the refraction of the waveguide was only intended to reduce the dimensions of the enclosure, resulting in undesirable resonant phenomena inside the waveguide. However, this was one of the design considerations. Therefore, the extinguisher uses a closed (i.e., one-sided closed) waveguide. From a physical point of view, the generated acoustic field interacts with the fire source, as a consequence of which, the flames are deflected, deformed, and ruptured until completely extinguished (depending on the flammability limit and critical frequency). It is easier to extinguish detached, separated, and deflected flames than an undisturbed original flame. In practice, acoustic extinguishers may apply additional support mechanisms, such as constructive interference, and have additional components (exemplification includes connection ports, pressure-boosting caps, transducers and energy banks, baffles and wave-reflecting elements, barriers, or attachments for precise wave targeting, etc.) [32; 52]. The output of the extinguisher narrows towards the waveguide output and is directed toward the flames, which is shown in Figure 2.  
In practice, the acoustic extinguisher was designed for a frequency of 17 Hz, and therefore all the results presented will include this frequency. The benefit then is the maximum mechanical load on the speaker's diaphragm and the effective use of its power capabilities (a local maximum of sound pressure was recorded at 17.25 Hz) [46]. Flame extinguishing is also possible for other frequencies, i.e., higher and lower than 17 Hz. To select the frequency, first, a diagram of sound pressure as a function of frequency was obtained for a  
Figure 2: (**a**) Components of the fire extinguisher: (1) generator, (2) modulator, (3) power amplifier, (4) waveguide output (extinguisher tip), (5) sound source (e.g., woofer), beginning of the waveguide, (6) sound level meter; (**b**) measurement station of the fire extinguisher. fixed distance. In practice, the frequency response was derived in the range of 10 to 90 Hz with a step of 1 Hz. Then, the minimum impedance was determined (Z = 11.4 \(\Omega\)) [46]. It is the impedance of the components that make up an acoustic extinguisher. A higher impedance means that a higher supply voltage is needed to achieve the analogous (same) power delivered to the extinguisher. By measuring the voltage and current, it was possible to determine its value as a function of frequency. It should be noted that the use of lower-frequency acoustic waves is associated with limitations on the technical side (design considerations). Unfavorable phenomena are significant deflections of the diaphragm of the sound source (the extinguisher works without the acoustic load of the waveguide). The technical part of the measurement station (instrumentation), in addition to the acoustic extinguisher, which is equipped with a B&C 21 DS115 (B&C Speakers s.p.a., Bagno a Ripoli, Italy) woofer installed at the beginning of the waveguide, as shown in Figure 2a, consists of a Rigol DG4102 generator and modulator, a Proel HPX2800 (Proel s.p.a., Sant'Omero, Italy) power amplifier, electrical quantities meters (ammeter, voltmeter, and power meter), a sound level meter, a laptop for reading and analyzing data, and the necessary cabling (Figure 2b).