Application of Low-Frequency Acoustic Waves to Extinguish Flames on the Basis of Selected Experimental Attempts

Featured Application

For industrial-scale extinguishment, natural processes such as acoustic wave propagation can be used. As proven, a technology described in the article using modulated and unmodulated waves can be applied to extinguish flames, as well as to control them. The results presented in the article are limited to extinguishing candle flames, but in the long term, this technique can be used to extinguish flames (especially firebreaks) from various materials (Classes C and B). This inexpensive, versatile, and environmentally safe method can provide a supporting or additional means of fire protection, especially useful in the first stage of fire occurrence. This technique appears to be environmentally friendly (it does not leave stains and does not emit chemicals). Moreover, acoustic waves can be applied to reduce the intensity of smoke and harmful substances in the ambient air, the presence of which is the result of fires (future plans).

Abstract

Due to the consequences of fires, new and environmentally friendly firefighting techniques are constantly being sought. There are many methods of extinguishing flames around the world. One of them is a technique that uses acoustic waves for extinguishing, which can be seen as repeated sequences of molecular compression and dilation (acoustic waves transfer energy due to the movements of molecules and atoms). This research shows a new approach to the extinguishing of flames. In practice, the extinguishing capabilities of low-frequency modulated and unmodulated acoustic waves were tested on a laboratory station, the main component of which was a high-powered acoustic extinguisher (the nominal power was equal to 1700 W). A B&C 21DS115 woofer was applied as a sound source. A Rigol DG4102 and a Proel HPX2800 were used as an acoustic generator with a modulator and as a power amplifier, respectively. In this paper, the presented results are limited to extinguishing candle flames. The tests made it clear that flames can be extinguished using properly generated and directed acoustic waves. As the results indicate, it becomes possible to effectively extinguish flames with both low-frequency modulated and unmodulated acoustic waves, which brings many benefits.

1. Introduction

Due to the scale and enormity of the damage caused, one of the topics widely discussed in recent years is fire protection. It finds a justification in the case of both the natural environment and the enclosed space. All over the world, research is constantly being carried out in terms of fire mechanisms, the possibility of reducing its effects, and ways to reduce and fight fire [1,2,3]. Due to the multifaceted effects of fires, new techniques are constantly being sought. This is even more important because fires, depending on their location, size, or the source of flames, are extinguished using a variety of techniques [4]. The challenge is to first look for renewable energy sources [5]. From a practical point of view, the traditional methods of extinguishing flames currently in use are based on the application of gaseous, liquid, or solid extinguishing agents to a selected flame zone. For small fires within a building, various types of fire extinguishers, water fog, water, or a fire blanket are generally applied [4]. Furthermore, artificial intelligence [6,7,8,9,10,11] and mathematical modeling, which allows one to describe known phenomena using a mathematical apparatus [12,13,14,15,16,17,18,19], can be helpful in the event of a fire. The use of robots comes to the forefront, then [20]. The use of artificial intelligence for flame detection (as one of the elements of fire management), especially in open spaces, fills the gap related to the environmental limitations of the use of classical sensors and their limited range. Another field of research is the search for flame-retardant materials (concrete building materials, polymers, and composites) that can find applications, especially in construction, and work in the field of combustion analysis [21,22,23,24,25,26,27,28,29]. An example of a measurement station for research on the hydromagnetic minerals of calcite and huntite in co-presence with regard to the flame-retardant and mechanical properties of wood composites is presented in Figure 1. These flame-retardant materials can be applied, among other things, for the construction of acoustic extinguishers that can be used to extinguish flames [30]. It is significant that acoustic waves transfer energy as a result of the movements of molecules and atoms.

Although acoustic waves have many applications, a relatively new application of acoustic waves is the use of them to extinguish flames. Acoustic technology may ultimately find an application in flame extinguishing that would be very difficult or impossible to extinguish with traditional techniques [31]. In reality, computer vision-based approaches could replace conventional techniques based on temperature or smoke detectors [32,33]. An advantage of using acoustic extinguishers equipped with a flame detection module that uses deep neural networks is the ability to detect and extinguish fires in real time. The advantage of the solution proposed by combining both techniques is that there is no need for human involvement during the extinguishing action. By the term infrasound, one means acoustic waves whose frequency is too low to be heard by humans. It should be noted that the most common side effects for people are fatigue, headaches, mood swings, and vibroacoustic disease [34]. The lower limit of audible frequencies, at the same time the upper limit of infrasound, is conventionally considered to be 20 Hz. However, it should be noted that the audible range is an individual matter. Under laboratory conditions, when a sine wave is played at a very high volume, the human sense of hearing can identify low sounds (as low as 12 Hz). Certain people (such as opera singers) may have a sense of hearing that allows them to hear sounds at lower frequencies relative to the assumed values [35]. Therefore, it is assumed that a person should not be exposed to infrasound or sound pressure levels higher than the pain threshold of the human ear (110 dB). So, there is no risk if a person is not present when the fire is extinguished. Another issue is the need for research on the effects of low-frequency and high-sound-pressure acoustic waves on building structures. This is particularly relevant for waves of very low frequency and very high sound pressure. These aspects require further research. Another potential application of acoustic waves is to reduce the intensity of smoke and harmful substances in the ambient air, the presence of which is the result of fires, and also the use of scientific and technological advances (i.e., polluting engines). Due to the subject matter of this article, these issues have been omitted.

If flames are detected, the fire extinguishing system can be activated automatically (without human involvement). Artificial intelligence may be applied for visual flame detection. A low-cost smart module that allows this (as one of the optional components of an acoustic extinguisher) has been developed in collaboration with researchers in Bulgaria, i.e., [36]. Furthermore, in the event of a fire, data can be captured and transmitted wirelessly to relevant services [37,38], in the case of inaccessible locations, using satellite links.

The acoustic technique for extinguishing flames is relatively new, so little work has been carried out in this area (some examples are [39,40,41,42,43,44,45]). In recent years, authors have conducted research to develop new technology that will allow flames to be extinguished with low-frequency acoustic waves generated by a high-power sound source [4,31,36,46,47,48]. For this purpose, different frequencies and waveforms have been analyzed. This is because the technique has not yet been fully discovered. According to experiments that have already been conducted, unmodulated and modulated waves can be used to extinguish flames. The benefit of using an acoustic extinguisher is that it can be universally tuned for a specific flame source, so that the extinguisher may be as effective as possible in extinguishing flames originating from the combustion of a specific material. In this regard, Class B and C fires are indicated, i.e., flammable liquid and gas fires. In the case of solid fires, there is a difficulty in taking heat away from the interior of the material. However, this method can be used supportively. Due to the limitations in the volume of this article, information on this topic (as an extension of knowledge) can be found in other articles by the authors included in the bibliography of this article [4,31,36,46,47,48]. Therefore, more research is needed for other frequencies and waveforms.

Going back into history, it is worth noting that documented attempts to extinguish flames using acoustic waves have been conducted in the US. In 2012, the DARPA Agency presented a video showing the impact that sound can have on the combustion process [49]. It should be noted that the DARPA Agency has developed two prototype fire extinguishers. The first is an electromagnetic fire extinguisher. This technique uses an electrode to extinguish flames from the combustion of methane and liquid fuels. The electrode was covered with a glass coating as an insulator, so a current does not flow between the electrode and the environment. Due to the vibration of the electromagnetic field particles and a series of violent streams, the ignition zone was separated from the fuel source and, consequently, the flames were extinguished. This method has some limitations in application for example, it is not suitable for extinguishing fires on the surface of liquids. The operation of this extinguisher is quite different from the environmentally friendly technology for extinguishing flames using low-frequency acoustic waves, which applies different mechanisms and laws of physics to extinguish flames. In the case of solids (such as a coal), although it was possible to suppress the flames with acoustic waves, they returned because, unlike many other flame extinguishing techniques, the heat from inside the material was not taken away. The transmission factors are different in open and closed spaces [50]. It is assumed that this technique may find application especially in the case of extinguishing firebreaks. Since oxygen concentration is one of the key factors affecting the course of a fire (according to the combustion triangle), it is presumed that the acoustic technique may find application primarily to extinguish flames inside facilities (in halls or industrial plants). In open space, because of the range of operation, this technique has a limited effect. However, tests conducted in Poland to extinguish flames outdoors have also been successful (up to a distance of a few meters). In addition, the acoustic technique can be applied successfully to control the combustion of materials. Another direction for the use of acoustic waves may be the elimination of pollutants from the air resulting from fires, among other things, while this requires further research. Undoubtedly, all measures to protect the environment serve human life and health, which is important in times of energy transformation [51,52].

Because there is still a need to fill the literature gap in terms of understanding the extinguishing properties of acoustic waves (this applies mainly to low-frequency waves, which exhibit the best capabilities of fire extinguishers), this article consists of several parts on fire protection. First, the state of the art is presented second, the methodology is described (including the physical apparatus); third, the components of the laboratory station are shown; and finally, the original results of the experiments are presented. The data obtained are included in tabular form and diagrams. The potential benefits of this technology, its limitations, and directions for further research are also indicated. The article concludes with a brief summary that succinctly presents the most important conclusions.

2. Materials and Methods

As indicated earlier, this paper deals with one aspect of fire management, i.e., acoustic fire extinguishing. This subject forces the use of appropriately selected research methods, because the main purpose of this article is to analyze the possibility of applying acoustic waves to extinguish flames. Since the article is a research paper, the experimental method, the case-by-case method, and the analytical method were used in addition to the literature review method. The results obtained from the laboratory station were compared with each other. Modeling and prediction methods (based on acquired data from the laboratory station) can also be applied for research purposes. At the stage of constructing the research problem, the effectiveness of each method was analyzed by studying the feasibility of its use to achieve the stated objectives. For the formulation of the conceptual framework, elements of system analysis were used, which allowed the creation of a separate structural space for the topic under consideration.

The experimental part used a prototype environmentally friendly high-powered acoustic fire extinguisher with a modulation function suitable for low-frequency operation (Figure 2). It turns out that an acoustic method can be applied to fight flames. The acoustic extinguisher presented in Figure 2 includes components used in electronics: a signal generator, power amplifier, sound source, and meters of electrical and non-electrical quantities. For acoustic measurements, a professional SVAN 979 m was used (all acoustic measurements were performed with the accuracy of first-class instruments).

The acoustic generator is one of the components of the laboratory station. For the purpose of the study, a Rigol DG4102 generator was used, which has a modulation function (it is a standalone component of the laboratory station). In practice, the test station is universal (it is possible to change the frequency, waveform shape, and modulation), which is shown in this article. A B&C 21DS115 woofer was applied as a sound source. A Proel HPX2800 was used as a power amplifier. The attempts were conducted outdoors, in an open area, under windless conditions. The recorded background sound level was about 65 dB. For the purpose of extinguishing, both unmodulated waves (measurement time of 5 s) and modulated waves (measurement time of 15 s) were used to extinguish the flames coming from the candle (flame height was about 2 cm). Flames from a gas mock-up were also successfully extinguished using the acoustic method. However, in the case of a dispersed fire source, it was more difficult to clearly determine whether the flames had been extinguished (they often returned). That is why a candle was used in the experiments in this article. The fire source was placed behind the outlet of the acoustic extinguisher at a distance of 50 cm. In this experiment, during measurement in an open space, the maximum power supplied to the fire extinguisher was 700 W. Due to technological limitations, a rectangular cross section and a closed end waveguide were applied.

The operation of the acoustic extinguisher is based on the dispersion of the fire over a large area due to the use of acoustic waves. Exposure of flames to acoustic waves results in their extinction. However, to effectively use this method, it is necessary to emit appropriate frequency acoustic waves. The tests show that the sound wave, increasing the speed of air movement at the edge of the flames of the fire, has the effect of reducing the area over which the combustion process takes place. In practice, flames are extinguished in an acoustic field produced by at least one sound source, such as a subwoofer. The outlet of the acoustic extinguisher is directed toward the source of the flames. Thus, we are dealing with two dynamics. First, as indicated above, the acoustic field increases the air velocity. In turn, when the air velocity increases, the flame boundary layer appears, where the combustion process occurs, which has a beneficial effect on the disruption of the flame surface. Second, as a result of exposure to the sound field, higher fuel evaporation is observed, which expands the flame but also lowers its total temperature. Due to the fact that the same amount of heat is spread over a larger area, it influences the combustion process [4,53]. Consequently, it is possible not only to significantly weaken the flame and control it (another application) but also to completely extinguish it. As indicated previously, frequency plays a key role. The phenomenon of extinguishing flames using acoustic waves is presented in Figure 3.

Figure 3a shows an illustration of flame extinguishing using acoustic waves at a frequency different from the critical frequency (as we can see, the flame is not torn apart). Figure 3b presents the effect on the flame of acoustic waves close to the critical frequency, while Figure 3c illustrates the effect on the flame of acoustic waves equal to the critical frequency. At first, the exhaust fumes are separated from the combustible mixture, and then there is a flame-breaking phenomenon that is finally extinguished (the mixture is separated from the flame). As proven, technology using modulated and unmodulated waves can be used to extinguish flames, as well as to control them. As a result of the acoustic field, a change in the shape of the flames is observed (their deformation, cooling, breaking into parts, flattening parallel to the direction of the acoustic wave), reduction, and consequently, complete extinguishment.

3. Results and Discussion

From a practical point of view, the extinguishing abilities of acoustic waves of different frequencies and modulations are not thoroughly analyzed (state of the art), especially with the use of high and very high powers. Therefore, it was necessary to partially fill the gap in this area. To test the extinguishing capabilities of acoustic waves, the laboratory station presented in Section 2 (in Figure 2) was used, the main component of which is an acoustic extinguisher with a high-power sound source (the nominal power was equal to 1700 W).

For the research presented in this article, the applied frequencies of the acoustic extinguisher were not chosen coincidentally (the mechanical load on the sound source and the acoustic efficiency were taken into account). First, the relationship between sound pressure and frequency was analyzed at a distance of 100 cm from the extinguisher outlet. This made it possible to determine the operating frequency of the acoustic extinguisher. Between 17 and 18 Hz, a minimum impedance of 11.4 Ω was recorded. Based on the response to this frequency, a local maximum pressure level was obtained [47]. Therefore, the 17.25 Hz frequency was called the operating frequency.

In practice, low-frequency acoustic waves have better extinguishing abilities compared to high-frequency waves, which is due to greater flame turbulence. However, technical considerations limit their use (vibration of the speaker diaphragm as a result of the fact that the waveguide does not interact with the speaker at low power). The applied frequencies of the acoustic extinguisher were in the range of 14 to 21 Hz to show the extinguishing capabilities of the device for three selected points that cover frequencies close to the operating frequency: 14, 18, and 20 or 21 Hz (for modulated and unmodulated waveforms, respectively). They were chosen for the safety of the speaker (its mechanical load). For the unmodulated waveform, the highest frequency was 1 Hz higher than for the modulated waveform and was equal to 21 Hz. For the modulated waveform, the highest applied frequency was equal to 20 Hz (above this frequency, there was a risk of deflection of the speaker’s diaphragm, threatening to damage it). Due to the design limitations of the extinguisher (sound source), the lowest frequency analyzed in both cases was equal to 14 Hz.

Since acoustic waves pass through a variety of materials (both solid, liquid, and gaseous), the acoustic method can be used to extinguish flames or to support/control the combustion process, where access to traditional extinguishing agents is impossible or limited or when extinguishing with them is ineffective (hard-to-reach places). The advantage of the acoustic technique is its versatility (the parameters of the acoustic waves can be selected in such a way that the method, depending on the conditions, including the source of the flames, can be as effective as possible).

The results of the experimental measurements are included below. The results presented are for instantaneous SPL values (rms) using frequency Z-correction characteristics.

Table 1. Required sound pressure (SPL) values necessary to extinguish flames as a function of frequency (f) for a sinusoidal wave.

f [Hz]	SPL [dB]
14	116.2
18	121.9
21	123.6

shows the results obtained for an unmodulated wave (i.e., sine wave), while

Table 2. Required sound pressure (SPL) values necessary to extinguish flames as a function of frequency (f) for a sinusoidal wave AM modulated by a square waveform (f_MOD = 0.125 Hz).

f [Hz]	SPL [dB]
14	122.6
18	124.2
20	126.5

presents the results for a sinusoidal wave with amplitude modulation (sine wave AM modulated by a square waveform, f_MOD = 0.125 Hz).

Based on an analysis of the data in Table 1, it can be concluded that to extinguish the flames using an unmodulated waveform (sine wave), the sound pressure level is greater than 115 dB and lower than 125 dB (ranging from about 116 to about 123 dB). For the lowest of the frequencies analyzed, that is, 14 Hz, the smallest value of the sound pressure level at which complete flame extinguishment was recorded was observed (116.2 dB). As the frequency increased, the required sound pressure level at which flame extinguishment was observed increased. For a wave with a frequency of 18 Hz, it was equal to 121.9 dB. The maximum value of the sound pressure level was obtained for the highest of the frequencies analyzed, that is, 21 Hz (it was equal to 123.6 dB). The results obtained for the modulated waveform appear to be interesting against this background (Table 2).

In the case of a modulated waveform (sinusoidal wave AM modulated by a square waveform), the sound pressure level required to extinguish the flames was greater than 122 dB and lower than 127 dB (ranging from about 122 to about 126 dB). As in the case of the unmodulated waveform, for the lowest of the analyzed frequencies, that is, 14 Hz, the smallest value of the sound pressure level at which complete extinguishment of the flame was recorded was observed (122.6 dB). As the frequency increased, the required sound pressure level at which flame extinguishment was observed increased. For a wave with a frequency of 18 Hz, it was equal to 124.2 dB. In turn, the maximum value of the sound pressure level was obtained for the highest of the frequencies analyzed, that is, 20 Hz (it was equal to 126.5 dB). The summary diagrams of SPL [dB] versus f [Hz] for modulated and unmodulated waves are presented in Figure 4.

For the lowest of the frequencies analyzed, that is, 14 Hz, the difference in sound pressure level at which the effect of complete extinguishment of the flames was observed was equal to 6.4 dB. For the middle frequency, that is, 18 Hz, it was equal to 2.3 dB, while for the highest of the analyzed frequencies (20 Hz for modulated waveform and 21 Hz for unmodulated waveform, respectively), it was almost equal to 3 dB. For the assumed conditions, the necessary sound pressure level for a modulated wave was slightly higher than for an unmodulated wave. It is worth noting that this does not always have to be the rule (depending on the circumstances, the class of fire, the source of the flames, etc.), as further work in this area is required. The results obtained experimentally confirmed the validity of the theory that modulated and unmodulated waveforms can be applied successfully to extinguish flames. In practice, low-frequency acoustic waves can be effectively used to extinguish flames. The lower the frequency of the acoustic wave, the greater the turbulence in the flame, which is beneficial from the point of view of flame extinguishing. In practice, some design constraints arise from the waveguide length (the lower the frequency, the longer the waveguide).

The results presenting the minimum electrical power that had to be supplied to the acoustic extinguisher to observe the phenomenon of complete extinguishment of the flames seem interesting against this background.

Table 3. Minimal electrical power values (P) necessary to extinguish flames as a function of frequency (f) for a sinusoidal wave.

f [Hz]	P [W]
14	125
18	155
21	345

shows the results obtained for an unmodulated wave (i.e., sinusoidal), while Table 4 presents the results for a sinusoidal wave with amplitude modulation (sine wave AM modulated by a square waveform, f_MOD = 0.125 Hz).

An analysis of the results obtained allows us to note that as the frequency increases, an increase in the minimum electrical power values that had to be delivered to the sound source to observe complete extinguishment of the flames was noted. For the smallest of the analyzed frequencies, that is, 14 Hz, the minimum power required was equal to 125 W. In turn, for the largest of the analyzed frequencies, that is, 21 Hz, the recorded minimum power was almost equal to 350 W.

The results obtained for the modulated waveforms are presented in Table 4.

Table 4. Minimal electrical power values (P) necessary to extinguish flames as a function of frequency (f) for a sinusoidal wave AM modulated by a square waveform (f_MOD = 0.125 Hz).

f (Hz)	P (W)
14	200
18	305
20	700

Table 4. Minimal electrical power values (P) necessary to extinguish flames as a function of frequency (f) for a sinusoidal wave AM modulated by a square waveform (f_MOD = 0.125 Hz).

f (Hz)	P (W)
14	200
18	305
20	700

Similarly to the unmodulated waveform, it can be observed that as the frequency increases, an increase in the minimum electrical power values that had to be delivered to the sound source to observe complete flame extinguishment was observed. For the lowest frequency analyzed, that is, 14 Hz, the required level of minimum electrical power was equal to 200 W. In turn, for the highest of the analyzed frequencies, that is, 20 Hz, a power level almost double that of the unmodulated waveform was recognized—it was equal to 700 W if no temporary change in weather conditions was noted (it is worth noting that if temporary changes in atmospheric conditions or gusts of wind affect the results obtained, experiments need to be repeated). The results allow us to see that there is a correlation between the increase in the minimal electrical power values that had to be delivered to the sound source to observe complete extinguishment of the flames as the frequency increases. Moving away from the resonant frequency results in a deviation from the resonant optimum, which translates into an increase in the minimal electrical power values that had to be delivered to the sound source to observe complete extinguishment of the flames. The summary diagrams of P [W] vs. f [Hz] for modulated and unmodulated waves are presented in Figure 5.

During fire extinguishing tests, the smallest tested frequency, that is, 14 Hz, showed the highest fire extinguishing effectiveness. For this frequency, the difference in power level that had to be supplied to the sound source of the extinguisher to observe the effect of completely extinguishing the flames was less than 100 W during the measurements. For low frequencies, the electrical power required to supply the fire extinguisher was the lowest. Increasing the frequency translates into an increase in electrical power that must be supplied to the acoustic extinguisher to observe the effect of extinguishing the flames. On the other hand, the benefit is then a greater directional concentration of acoustic energy, as well as practical considerations (limiting the size of the waveguide). For the middle frequency, that is, 18 Hz, the required power to extinguish the flames increased and was equal to 150 W, while for the largest of the frequencies analyzed (that is, 20 Hz for the modulated waveform and 21 Hz for the unmodulated waveform, respectively), it was about 350 W. This means almost twice the level of power that had to be delivered to observe the effect of completely extinguishing the flames compared with the unmodulated wave (i.e., sinusoidal). In practice, for the assumed conditions, the level of power that had to be delivered to the sound source of the extinguisher to observe the effect of complete extinguishment of the flames for the modulated wave was higher than for the unmodulated wave. It is worth noting that despite the low power, the amplitude of the speaker’s vibration may limit its effective use for flame extinguishing. To protect the sound source from possible damage (design considerations, diaphragm deflection), the measurements in this experiment were limited to the selected frequencies.

It was experimentally proved that the propagation of acoustic waves with assumed parameters, including frequency and modulation, affects air particles. From the point of view of physics, any propagated acoustic wave causes local pressure changes, with the phenomenon occurring if the level of the resulting turbulence is sufficient to extinguish the flame (once the critical frequency value is reached) (see Figure 3). In practice, turbulence depends on the frequency and the modulation used. This is due to the fact that sound pressure depends on the power applied to the speaker, and this relationship is not linear (linearity is observed only in a limited range). The wave motion of molecules also interacts favorably with the surface of the flame (by propagating the acoustic wave and increasing the speed of air movement at the edge of the flame, there is a reduction in the area over which the combustion process occurs, thus spreading the flame over a larger area). In addition, as previously mentioned, the wave motion of the particles has a beneficial effect on the temperature of the flame (if it decreases and is lower than the ignition temperature, extinguishment will occur). The consequence of acoustic waves is the distribution of heat over a larger area, and thus the disruption of combustion (continuous exposure of the flame to acoustic waves will result in its deviation from its original position, breaking into pieces, and then being completely extinguished).

As shown, the extinguishing effect is affected by the frequency of the wave and modulation, which translates into a difference in the necessary power that must be delivered to the sound source for complete extinguishment of the flames to occur. It is worth noting that both attempts to extinguish the flames with modulated and unmodulated waves were successful. In this experiment, the sound pressure level at which the phenomenon of acoustic flame extinction was observed ranged from about 116 dB to 127 dB. Because the extinguishing capabilities of acoustic waves have not been fully recognized to date, this technique is still undergoing testing and development. The presented method may make it possible to replace traditional fire protection measures with it, especially in the case of extinguishing firebreaks, in the future.

4. Conclusions

As presented in the article, modulated and unmodulated low-frequency acoustic waves can be applied successfully to extinguish flames, especially in the first stage of fire occurrence (firebreaks). It is significant that acoustic waves transfer energy as a result of the movements of molecules and atoms through a medium. The experiment showed the extinguishing capabilities of acoustic waves in the low-frequency range both for modulated and unmodulated waveforms. In both cases, the attempts were successful. In practice, the lower the frequency of the acoustic wave, the greater the turbulence in the flame.

The acoustic technique appears to be cheaper to use than traditional methods to extinguish flames. Its advantages are the lack of need to pressure test the tank, the unlimited amount of extinguishing agent, its environmental friendliness (the waves are not a chemical product), and the versatility of use. Because acoustic waves pass through a variety of materials (both solid, liquid, and gaseous), the acoustic method can be used to extinguish materials or to support/control the combustion process in places that are difficult to access. It can be applied to extinguish flames from materials that are difficult or impossible to extinguish by classical means. This is even more important because there is ongoing research around the world aimed at finding flame-retardant materials that can find many applications [54].

Furthermore, it can provide an additional supportive, non-invasive, and safe measure for the protection of the environment. It is expected that extinguishing flames in an indoor space will be easier due to the lower oxygen concentration in the air. Therefore, it is assumed that acoustic waves may find applications for the extinguishing of firebreaks in enclosed spaces in the future. For example, acoustic extinguishers can be permanently installed on the foundations or structures of a particular device and suppress or extinguish flames until the arrival of specific services [4]. It should be noted that, in the event of flames, the acoustic extinguisher can be activated fully automatically (without the need for human presence during the extinguishing action), which is particularly important when using low-frequency waves and significant levels of acoustic pressure to extinguish flames, the use of which is a controversial issue due to the impact on human health. This is especially interesting given the use of artificial intelligence and the search for flame-retardant materials that can be extinguished by this method. Imaging techniques can be helpful in this regard [55,56,57,58,59,60]. In the future, it is expected that it will also be possible to use acoustic waves to fully control the combustion process, control flames, and reduce the intensity of smoke and harmful substances in the air, the presence of which is the result of fires.