Research for the Presence of Unmanned Aerial Vehicle Inside Closed Environments with Acoustic Measurements

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Article Research for the Presence of Unmanned Aerial Vehicle inside Closed Environments with Acoustic Measurements

Giuseppe Ciaburro, Gino Iannace * and Amelia Trematerra

Dipartimento di Architettura e Disegno Industriale, Università degli Studi della Campania Luigi Vanvitelli, 81031 Aversa, Italy; [email protected] (G.C.); [email protected] (A.T.) * Correspondence: [email protected]; Tel.: +39-0815010700

Received: 18 April 2020; Accepted: 20 May 2020; Published: 23 May 2020

Abstract: Small UAVs (unmanned aerial vehicle) can be used in many sectors such as the acquisition of images or the transport of objects. Small UAVs have also been used for terrorist activities or to disturb the flight of airplanes. Due to the small size and the presence of only rotating parts, drones escape traditional controls and therefore represent a danger. This paper reports a methodology for identifying the presence of small UAVs inside a closed environment by measuring the noise emitted during the flight. Acoustic measurements of the noise emitted by a drone inside a large environment (12.0 30.0 12.0 m) were performed. The noise was measured with a sound level meter placed × × at different distances (5, 10, and 15 m), to characterize the noise in the absence of anthropic noise. In this configuration, a typical tonal component of drone noise is highlighted at the frequency of one-third of an octave at 5000 Hz due to the rotation of the blades. This component is also present 15 m away from the source point. Subsequent measurements were performed by introducing into the environment, through a loudspeaker, the anthropogenic noise produced by the buzz of people and background music. It is possible to distinguish the typical tonal component of UAV noise at the frequency of 5000 Hz even when the level of recording of anthropogenic noise emitted by the loudspeaker is at the maximum power tested. It is therefore possible to search for the presence of small UAVs inside a specific closed environment with only acoustic measurements, paying attention to the typical frequency of noise emission equal to 5000 Hz.

Keywords: UAV; frequency; acoustic measurements; noise; closed environments

1. Introduction With the acronym UAV, we refer to aircraft without a pilot on board (unmanned aerial vehicle). This category of aircraft determines a drastic reduction in operating costs compared to conventional piloted aircraft. The flight is controlled remotely [1]. Originally drones were used for war purposes with the aim of creating unmanned aircraft to avoid human losses in the event of anti-aircraft defense killing the aircraft [2,3]. Technological progress and research in this field have allowed the creation of vehicles capable of realizing the most disparate applications. These results have aroused interest in this technology from the civil sector which today uses UAVs in various fields. Unlike the military sector that uses real planes equipped with destructive weapons, multi-helicopter solutions are widely used in the civil sector [4]. Multi-helicopters are characterized by vehicles with multiple propellers, from three to eight, although the most used choice is the one with four propellers. This is due to an extreme construction simplicity which substantially reduces construction and maintenance costs [5]. Small drones can be used in many sectors such as the acquisition of images, the transport of objects, and being equipped with small speakers they can be used to communicate warnings during

Buildings 2020, 10, 96; doi:10.3390/buildings10050096 www.mdpi.com/journal/buildings Buildings 2020, 10, 96 2 of 10 emergency phases. Although on some rare occasions drones have also been used for terrorist activities (Venezuela 2018 and Saudi Arabia 2019), to disturb the flight of airplanes near airports (London 2019), and for the transport of drugs or the delivery of unwanted objects in prisons; therefore, with UAVs it is possible to perform a series of illegal activities [6]. Even though all the nations of the world have very specific laws prohibiting the overflight of UAVs by creating no fly zones, we are witnessing a worrying increase in reported cases. Due to the small size and the presence of only rotating parts, the UAVs escape the traditional controls; they cannot be seen by infrared systems (thermo cameras) or electronic systems with radio waves and for this they represent a danger for the population. Furthermore, the anti-drone systems so far available are based on the two fundamental principles, intercepting the position and movement of the drone, and annihilating the same by different means [7–17]. The analysis of the airspace to be monitored takes place with four different methods:

optical, or video cameras capable of distinguishing a drone from a natural bird and operating • both day and night thanks to infrared vision; radiofrequency, such as scanners that continuously listen to the radio spectrum typically used by • drones and their respective remote controls; using a radar of limited dimensions expressly dedicated to identifying small flying objects with • remote piloting and with a radius of action of a few kilom; and using acoustics signatures. • In literature there are several references to studies that have used the noise produced by UAVs for their identification and location [18–36]. This work reports a methodology for the search for the presence of drones inside a closed environment by measuring the noise emitted during the flight. Acoustic measurements of the noise emitted by a drone inside a large environment were performed. The noise was measured with a sound level meter placed at different distances (5 m, 10 m, and 15 m), to characterize the noise in the absence of anthropic noise. In this configuration, a typical tonal component of drone noise was highlighted at the frequency of one-third of an octave at 5000 Hz due to the rotation of the blades [37]. This component was also present 15 m away from the source point. Subsequent measurements were performed by introducing into the environment, through a speaker, the anthropogenic noise produced by the buzz of people and background music. This noise was recorded inside an atrium of a shopping center. The level of noise introduced into the environment under test was gradually increased and in the presence of the drone noise, acoustic measurements were carried out at the points previously considered.

2. Materials and Methods For this work the authors adopted a four-propeller radio-controlled drone. The hull of the UAV was made of high-quality acrylonitrile-butadiene-styrene (ABS), while the radio control used a frequency of 2.4 GHz. The balance during the flight of the UAV was guaranteed by a 6-axis gyroscope that ensures its stability with a weight of 2.1 kg. Each quadcopter propeller had two blades, while the four propellers rotate alternately: two propellers rotate counterclockwise and the other two clockwise. Figure1 shows the dimensions of the UAV as well as showing how the four propellers of the UAV are arranged. The blades are the elements that allow the actual generation of the traction force. The dimensions of the UAVs characterize the type of noise emitted; it is made up of a component due to the airframe and a component due to the aircraft system (the engine noise and propeller noise are negligible) [38–40]. The vortex generated by the propellers and their rotation are the two main components of the noise generated by UAVs. The noise generated by the rotation of the blades at high speed is periodic in nature and is characterized by discrete frequencies obtained by multiplying the number of blades of the propellers by the rotation speed measured in cycles per second. These frequencies represent the fundamental frequencies, to which are then added the higher order harmonics obtained from integer multiples of these frequencies. For the correct identification of the spectral signature it is BuildingsBuildings 20202020, ,1010, ,x 96 FOR PEER REVIEW 33 of of 10 10 necessary to distinguish the noise produced by the UAV from the environmental noise. We therefore carriednecessary out toacoustic distinguish measurements the noise producedduring the by UAV the flight UAV fromin presence the environmental of two different noise. anthropogenic We therefore noisecarried noises out acoustic(voices and measurements music) measured during inside the UAV in hall flight of a in commercial presence of center. two di Tofferent generate anthropogenic the noise producednoise noises by (voicesthe UAV, and a music)tripod was measured used on inside which in hallthe ofUAV a commercial was anchored. center. The To sound generate level the meter noise forproduced the acoustic by the measurements UAV, a tripod was usedplaced on at which different the UAV distances was anchored. (5 m, 10 m, The and sound 15 m), level while meter the for acousticthe acoustic measurements measurements were was carried placed out at with different a sound distances level meter (5 m, type 10 m, Larson and 15 Davis m), while type the LxT. acoustic measurements were carried out with a sound level meter type Larson Davis type LxT.

FigureFigure 1. 1. FrontFront and and top top view view and and dimensions dimensions of of the the unmanned unmanned aerial aerial vehicle vehicle (UAV) (UAV) used. used.

AnthropogenicAnthropogenic noise noise was was recorded recorded via via a a Zoom Zoom digital digital recorder recorder (Zoom (Zoom H6 H6 Handy Handy Recorder Recorder equippedequipped with with a aX-Y X-Y Microphone) Microphone) (Zoom (Zoom North North America, America, New New York, York, USA) USA) to to obtain obtain a a signal signal that that waswas subsequently subsequently to to be be reproduced reproduced with with a a loudspeaker loudspeaker in in a a large large environment. environment. This This procedure procedure was was performedperformed due due to to the the impossibility impossibility of of carrying carrying out out acoustic acoustic measurements measurements in in a areal real environment environment due due toto the the obvious obvious organizational organizational reas reasonsons and the didifficultiesﬃculties ofof carryingcarrying out out measurements measurements in in an an atrium atrium of 3 ofa a shopping shopping center center with with the the presence presence of of the the audience; audience; the the atrium atrium volume volume is equalis equal to 22,610to 22,610 m .m With3. With the theimpulse impulse response response method method and in and accordance in accordance with ISO with 3382 ISO [41,42 3382], the [41,42], environment the environment was characterized was from an acoustic point of view with the following reverberation time values T s. Table1 shows the characterized from an acoustic point of view with the following reverberation time30, values T30, s. Table 1reverberation shows the reverberation time in octave time band in octave measured band in meas the atriumured in ofthe the atrium shopping of the center shopping [43]. center [43].

TableTable 1 1.. ReverberationReverberation time time in in octave octave band band measured measured in in the the atrium atrium of of the the shopping shopping center. center.

Frequency,Frequency, Hz Hz 125 125 250 250 500 500 1000 1000 2000 2000 4000 4000

T30, s T30, s 1.92 1.92 2.07 2.07 2.19 2.19 2.23 2.23 2.13 2.13 1.72 1.72

The environment under test is an industrial warehouse that houses a measurement laboratory The environment under test is an industrial warehouse that houses a measurement laboratory with the following dimensions, width 12 m, length 30 m, and height 12 m (volume = 4320 m3). The with the following dimensions, width 12 m, length 30 m, and height 12 m (volume = 4320 m3). The walls are covered with smooth plaster while the floor is concrete with glass surfaces. Inside there are walls are covered with smooth plaster while the ﬂoor is concrete with glass surfaces. Inside there are equipment and machinery typical of a measurement laboratory. Table 2 shows the reverberation time equipment and machinery typical of a measurement laboratory. Table2 shows the reverberation time in octave band measured in the atrium of the shopping center. in octave band measured in the atrium of the shopping center. Table 2. Reverberation time in octave band measured in the environment test. Table 2. Reverberation time in octave band measured in the environment test. Frequency, Hz 125 250 500 1000 2000 4000 Frequency, Hz 125 250 500 1000 2000 4000 T30, s 2.5 2.8 3.2 3.3 2.9 2.4 T30, s 2.5 2.8 3.2 3.3 2.9 2.4 2.1 UAV Acoustic Characterization inside the Anechoic Chamber A measurement was first performed in the anechoic chamber to characterize the sound emission of the UAV [44–46]. The anechoic chamber with the following dimensions (4.40 m × 4.40 m × 4.50 m), allowed us to perform noise measurements in the absence of background noise and in a free field. In Buildings 2020, 10, 96 4 of 10

2.1. UAV Acoustic Characterization inside the Anechoic Chamber Buildings 2020, 10, x FOR PEER REVIEW 4 of 10 A measurement was first performed in the anechoic chamber to characterize the sound emission ofBuildings the UAV 2020,[ 1044, –x46 FOR]. ThePEER anechoic REVIEW chamber with the following dimensions (4.40 m 4.40 m 4.504 of m), 10 this way it was possible to estimate the emission spectrum of the UAV in the absence× of unwanted× soundallowed reflections us to perform due to the noise surfaces measurements of the environm in the absenceent, and ofto backgroundevaluate the noisetonal andcomponents in a free field.of the In this way it was possible to estimate the emission spectrum of the UAV in the absence of unwanted UAV.this Figure way it 2 was shows possible the trend to estimate of the sound the emission spectrum spectrum in a frequency of the UAVof one- inthird the absence octave bands of unwanted both forsound the minimum reflections reflections values due due to to and the the surfacesfor surfaces the averageof of the the environm environment,values measured.ent, and and to toevaluateThe evaluate UAV the considered the tonal tonal components components has a tonal of the of componenttheUAV. UAV. Figure Figureat a 2frequency shows2 shows the of thetrend 5000 trend of Hz. the of From thesound sound these spectrum spectrumobservations, in a infrequency a to frequency verify of ifone- ofthere one-thirdthird is aoctave UAV octave bandsactive bands bothin for the minimum values and for the average values measured. The UAV considered has a tonal theboth indoor for theenvironment, minimum valueswe could and perform for the average a meas valuesurement measured. of noise Theemissions UAV consideredwith a sound has level a tonal component at a frequency of 5000 Hz. From these observations, to verify if there is a UAV active in metercomponent and check at afor frequency the presence of 5000 of Hz.a tonal From component these observations, at 5000 Hz. to verify if there is a UAV active in the the indoor environment, we could perform a measurement of noise emissions with a sound level indoor environment, we could perform a measurement of noise emissions with a sound level meter andmeter check and forcheck the for presence the presence of a tonal of a component tonal component at 5000 at Hz. 5000 Hz.

Figure 2. Values of the minimum (at the left) and average (at the right) spectrum emitted in the

anechoic chamber by the operation of a UAV. It is possible to note the presence of the 5000 Hz tonal Figure 2. Values of the minimum (at the left) and average (at the right) spectrum emitted in the anechoic component.Figure 2. Values of the minimum (at the left) and average (at the right) spectrum emitted in the chamberanechoic bychamber the operation by the operation of a UAV. Itof isa possibleUAV. It tois possible note the presenceto note the of presence the 5000 Hzof the tonal 5000 component. Hz tonal component. 2.22.2. Acoustic Acoustic Measurements Measurements in Real in Real Enviro Environmentnment in the in theAbsence Absence Of OfAnthropic Anthropic Noise Noise 2.2 AcousticTheThe UAV UAV Measurements was was placed placed insidein Real anEnviro industrialindustrialnment inwarehouseware the houseAbsence thatthat Of houses Anthropichouses a a measurement measurementNoise laboratory. laboratory. The Theshed shed is is located located in in an an area area where where there there is nois no vehicular vehicular tra trafficffic or or other other forms forms of noiseof noise pollution. pollution. The The UAV was placed inside an industrial warehouse that houses a measurement laboratory. Themeasuring measuring sound sound level level meter meter was was placed placed at at three three di differentfferent distances distances (5 (5 m, m, 10 10 m, m, and and 15 15 m m from from the The shed is located in an area where there is no vehicular traffic or other forms of noise pollution. theposition position where where the the UAV UAV was was placed). placed). Figure Figure3 shows 3 shows the the plan plan of the of the position position of the of UAVthe UAV on the on stand, the The measuring sound level meter was placed at three different distances (5 m, 10 m, and 15 m from stand,during during the acoustic the acoustic measurement measurement phases, phases, with respectwith respect to the to positions the positions of the of sound the sound level m. level The m. first the position where the UAV was placed). Figure 3 shows the plan of the position of the UAV on the Theseries first of series measurements of measurements were performed were performed in the absence in the of anyabsence form ofof noiseany inform the environment.of noise in the stand, during the acoustic measurement phases, with respect to the positions of the sound level m. environment. The first series of measurements were performed in the absence of any form of noise in the environment.

FigureFigure 3. 3.PlanPlan of ofthe the internal internal simulation simulation environment. environment. The The positions positions of the of the elements elements used used in inthe the

simulationsimulation scenarios scenarios are are clearly clearly indicated: indicated: sound sound level level meter meter (1), (1), loudspeaker loudspeaker (2), (2), and and UAV UAV (3). (3). Figure 3. Plan of the internal simulation environment. The positions of the elements used in the simulation scenarios are clearly indicated: sound level meter (1), loudspeaker (2), and UAV (3).

Buildings 2020, 10, 96 5 of 10

In the absence of anthropic noise conﬁguration, Figure4 shows the values of the minimum and average spectrum emitted by the UAV in the environment under test in the frequency domain of a one-third octave band. The receiver–noise source distances are respectively (A equals 5 m), (B equals 10 m), and (C equals 15 m). It is possible to note the tonal component presence at the frequency of 5000Buildings Hz. 2020, 10, x FOR PEER REVIEW 5 of 10

FigureFigure 4. 4.Values Values of of the the minimum minimum (at the(at left)the andleft) averageand aver (atage the (at right) the spectrumright) spectrum emitted inemitted the position in the ofposition the UAV of the in theUAV environment in the environment under test, unde inr test, absence in absence of noise. of noise. Thereceiver–source The receiver–source distances distances are respectively:are respectively: ((A) (A equals equals 5 m, 5 m, (B )B equals equals 10 10 m, m, and and ( CC) equals equals 15 m). 2.3. Acoustic Measurements in the Environment with Anthropic Noise (Voice) In the absence of anthropic noise configuration, Figure 4 shows the values of the minimum and averageThe spectrum second series emitted of the by acoustic the UAV measurements in the environment were carried under outtest in in the the presence frequency of domain voice noise, of a previouslyone-third octave recorded band. inside The the receiver–noise atrium of a shopping source dist center,ances generated are respectively by a loudspeaker. (A equals 5 The m), recording (B equals of10 them), noise and (C emitted equals by 15 the m). voice It is possible was performed to note the in the tonal atrium component of a very presence popular at shoppingthe frequency center of on5000 a weekendHz. evening. This allowed the recording of the typical voices of people when they are present in a large environment. The purpose of the recording was to obtain a realistic audio signal that2.3 Acoustic could be Measurements compared with in the the Environment noise emission with of Anthropic the UAV. Noise For this(Voice) purpose, the understanding of the voice was not necessary, but only the typical spectrum of the sound emissions of people on the The second series of the acoustic measurements were carried out in the presence of voice noise, move. Through a loudspeaker the noise was introduced into the environment under test. The LAeq previously recorded inside the atrium of a shopping center, generated by a loudspeaker. The level in the environment was equal to 61 dBA (measured for ﬁve minutes), corresponding to the sound recording of the noise emitted by the voice was performed in the atrium of a very popular shopping level inside the atrium of the shopping center. Furthermore, since the environment under test was center on a weekend evening. This allowed the recording of the typical voices of people when they a large empty volume, the sound level was on average widespread. Figure5 shows the values of are present in a large environment. The purpose of the recording was to obtain a realistic audio signal that could be compared with the noise emission of the UAV. For this purpose, the understanding of the voice was not necessary, but only the typical spectrum of the sound emissions of people on the move. Through a loudspeaker the noise was introduced into the environment under test. The LAeq level in the environment was equal to 61 dBA (measured for five minutes), corresponding to the sound level inside the atrium of the shopping center. Furthermore, since the environment under test was a large empty volume, the sound level was on average widespread. Figure 5 shows the values of the minimum and average spectrum measured within the environment in this configuration, in the Buildings 2020, 10, 96 6 of 10

Buildings 2020, 10, x FOR PEER REVIEW 6 of 10 the minimum and average spectrum measured within the environment in this conﬁguration, in the frequencyfrequency domaindomain of of one-third one-third octave octave bands. bands. The The receiver–noise receiver–noise source source distances distances are are respectively: respectively: (A equals(A equals 5 m), 5 m), (B equals (B equals 10 m),10 m), and and (C equals(C equals 15 m).15 m). Furthermore, Furthermore, in thisin this conﬁguration configuration it is it possible is possible to noticeto notice the the presence presence ofthe of the tonal tonal component component at the at the frequency frequency of 5000 of 5000 Hz. Hz. In this In this condition condition the the Figure Figure5C (sound5C (sound source–receiver source–receiver distance distance equal equal to to 15 15 m), m), due due to to the the distance distance the the emission emission levellevel of the UAV, UAV, beginsbegins toto decreasedecrease toto approachapproach thethe emissionemission levellevel ofof thethe voice.voice. AlthoughAlthough thethe tonaltonal componentcomponent atat thethe frequencyfrequency ofof 50005000 Hz,Hz, typicaltypical ofof thethe emissionemission ofof thethe UAV, alwaysalways remainsremains detacheddetached fromfrom otherother vocalvocal soundsound components.components.

FigureFigure 5.5.Values Values of of the the minimum minimum (at the(at left)the left) and averageand aver (atage the (at right) the spectrumright) spectrum emitted emitted in the position in the ofposition the UAV of inthe the UAV environment in the environment under test, under with anthropictest, with anthropic noise (voice). noise The (voice). receiver–source The receiver distances–source aredistances respectively: are respectively: ((A) equals (A 5 m,equals (B) equals5 m, B 10equals m, and 10 m, (C )and equals C equals 15 m). 15 m). 2.4. Acoustic Measurements in the Environment with Anthropic Noise (Music) 2.4 Acoustic Measurements in the Environment With Anthropic Noise (Music) The third series of measurements were performed by generating the music previously recorded The third series of measurements were performed by generating the music previously recorded inside the atrium of the shopping center. The recorded music was emitted from the speakers for inside the atrium of the shopping center. The recorded music was emitted from the speakers for advertising purposes. The purpose of recording the musical signal was to obtain a realistic audio advertising purposes. The purpose of recording the musical signal was to obtain a realistic audio signal that could be compared with the noise emission of the UAV. Through a loudspeaker the noise signal that could be compared with the noise emission of the UAV. Through a loudspeaker the noise was introduced into the environment under test. The LAeq noise level inside the environment was was introduced into the environment under test. The LAeq noise level inside the environment was equal to 71 dBA (measured for five mins), corresponding to the sound level in real conditions inside equal to 71 dBA (measured for five mins), corresponding to the sound level in real conditions inside the atrium of the shopping center. The sound level emitted within the environment under test was the atrium of the shopping center. The sound level emitted within the environment under test was on average widespread. Figure 6 shows the values of the minimum and average emitted spectrum measured within the environment in this configuration. Moreover, in this configuration it was possible to notice the presence of the tonal component at the frequency of 5000 Hz. The receiver– Buildings 2020, 10, 96 7 of 10 on average widespread. Figure6 shows the values of the minimum and average emitted spectrum measured within the environment in this configuration. Moreover, in this configuration it was possible Buildings 2020, 10, x FOR PEER REVIEW 7 of 10 to notice the presence of the tonal component at the frequency of 5000 Hz. The receiver–noise source distancesnoise source are respectively: distances are (Arespectively: equals 5 m), (A (B equals equals 5 m), 10 m), (B equals and (C 10 equals m), and 15 m).(C equals In the 15 conditions m). In the analyzedconditions in Figureanalyzed6, the in issueFigure of 6, music the issue emission of music is always emission well is separated always well from separated the tonal from component the tonal frequencycomponent of 5000frequency Hz, typical of 5000 of Hz, the ty UAVpical emission. of the UAV It remains emission. always It remains detached always from detached the other from sonic the componentsother sonic ofcomponents the music. of the music.

FigureFigure 6. 6Values. Values of theof the minimum minimum (at the (at left) the and left) average and aver (atage the right)(at the spectrum right) spectrum emitted in emitted the position in the ofposition the UAV of in the the UAV environment in the environment under test, under with anthropic test, with noise anthropic (music). noise The (music). receiver–source The receiver distances–source aredistances respectively: are respectively: ((A) equals (A 5 m, equals (B) equals 5 m, B 10 equals m, and 10 ( Cm,) equalsand C equals 15 m). 15 m). 3. Discussion 3. Discussion The purpose of this work was the search for a UAV inside a closed environment in the presence of The purpose of this work was the search for a UAV inside a closed environment in the presence an anthropogenic noise (voice and music) with acoustic measurements carried out with a sound level of an anthropogenic noise (voice and music) with acoustic measurements carried out with a sound meter. First, a preliminary measurements session in anechoic chamber was executed, then acoustic level meter. First, a preliminary measurements session in anechoic chamber was executed, then measurements were performed in an industrial warehouse and using a loudspeaker, and then the acoustic measurements were performed in an industrial warehouse and using a loudspeaker, and vocal and subsequently the music that had been recorded inside the hall of an atrium of a shopping then the vocal and subsequently the music that had been recorded inside the hall of an atrium of a center was spread. The UAV was placed on a tripod and during operation the sound emission values shopping center was spread. The UAV was placed on a tripod and during operation the sound were measured with a sound level meter placed at fixed points (5 m, 10 m, and 15 m). The results of emission values were measured with a sound level meter placed at fixed points (5 m, 10 m, and 15 the acoustic measurements were reported in a frequency in one-third octave bands. The measurements m). The results of the acoustic measurements were reported in a frequency in one-third octave bands. The measurements were carried out in the presence of noise emitted from the audio registration of the audience who frequent the atrium of a shopping mall, or from music inside the atrium. The sound levels due to the emission of vocals or music within the environment under test were on average widespread in the sense that they did not change when the measurement position changes. This was due to the large size of the environment and the consequent values of the time reverberation Buildings 2020, 10, 96 8 of 10 were carried out in the presence of noise emitted from the audio registration of the audience who frequent the atrium of a shopping mall, or from music inside the atrium. The sound levels due to the emission of vocals or music within the environment under test were on average widespread in the sense that they did not change when the measurement position changes. This was due to the large size of the environment and the consequent values of the time reverberation measured (T30 = 3.3 s at the frequency of 1000 Hz). These acoustic measurements highlight the presence of a tonal component at the frequency of 5000 Hz, which represents the typical sound emission of the UAV during the flying operating phase. The identification of this frequency is the sign of the presence of a UAV in the environment. Depending on the conditions of acoustic measurements performed with the voice and with the music, it should be noted that the spectra in the two conditions are different from each other. The spectrum in the condition of the emission of the voice at the frequency of 500 Hz reached a maximum and then slightly decreased, while the emission spectrum of the music was flat in the frequency range considered.

4. Conclusions The purpose of this paper was the search for a UAV in the studied environment in the presence of anthropogenic noise (voice and music) with acoustic measurements carried out with a sound level meter. The search for the presence of a UAV was somewhat complex and not simple for its small size. Given the difficulty of making measurements in a real environment for organizational security needs in crowded places, the acoustic measurements were simulated in an industrial environment. The paper focused on the noise emissions of the UAV during the operating phases inside a large environment simulating flight activities inside the atrium of a shopping center. The analysis of the acoustic measurements showed a tonal component at the one-third octave at the frequency of 5000 Hz. This tonal component was present both when the voice was present in the environment under test, and when the music was present. Therefore, simple acoustic measurements and a frequency analysis can confirm the presence of UAVs in the studied environment.

Author Contributions: Conceptualization, G.I. and G.C.; methodology, G.C.; investigation, G.C.; measurements G.I. and G.C.; software G.C.; post processing data, G.I. and G.C.; data curation, A.T.; writing—original draft preparation, A.T.; writing—review and editing, A.T.; visualization, G.C. and A.T.; supervision, G.I.; references study, A.T.; and setup measurements, G.I. and G.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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