Sound power level and sound pressure level characterization of a small unmanned aircraft system during flight operations

Abstract

Unmanned aerial vehicles, actually Federal Aviation Administration calls them unmanned aircraft systems, have aroused a great deal for many applications in the scientific, civil, and military sectors. The goal of this article is to evaluate the acoustic emissions of some small unmanned aircraft systems during normal flight operations (e.g. take off, landing, turning, and hovering) related to electric engines and propellers. This analysis will be useful for developing a system (unmanned aircraft systems Tracking and Reconnaissance System), which will be able to locate a small unmanned aircraft system using its sound power level, in a prefixed area. The investigation on sound power levels and sound pressure level is based on EN ISO 3745, which specifies measurement method. The acoustic measurements were carried out in an anechoic room, and the results of each unmanned aircraft system have been presented and discussed.

Keywords

Unmanned aircraft system acoustic tracking flight mechanics noise environmental impact

Introduction

Unmanned aircraft systems (UASs) play an important role in various military and civil applications. The numerous military applications include reconnaissance, surveillance, battle damage assessment, and communication relays.

Possible civil applications for UAS include the monitoring and surveillance of areas (urban traffic, coast guard patrol, border patrol, detection of illegal imports, archeological site prospection, etc.), climate research (weather forecast, river flow, etc.), agricultural studies, air composition and pollution studies, inspection of electrical power lines, monitoring gas or oil pipe lines, entertainment and TV, and so on. Most civilian uses of UASs require the air vehicle to fly at speeds lower than 50 kts (70 km/h) and at low heights, and many applications need the ability of the aircraft to hover (e.g. for power line inspection, subsurface geology, mineral resource analysis, or incident control by police and fire services).^1–3

In this work, a small quad-rotor was chosen. This type of UAS has a good ranking among VTOL (vertical take-off and landing) vehicles, yet it has some drawbacks. Employing four rotors to create differential thrust, the quad-rotor gains flexibility, swift maneuverability, and increased payload. A useful comparison of different types of VTOL miniature flying robots (MFR) can be found in Valavanis.¹ Table 1, adapted from Valavanis,¹ gives quality indexes (from 1 = bad, to 4 = very good) for some design issues pertaining to different VTOL vehicle concepts, namely, bird-like (A), single-rotor (B), tandem rotors (C), insect-like (D), axial rotor (E), blimp (F), co-axial rotors (G), and quad-rotor (H).

Table 1.

Comparison among different VTOL concept and design issues (for further details see Valavanis¹).

Design driver	A	B	C	D	E	F	G	H
Mechanics simplicity	1	1	1	1	3	4	3	4
Aerodynamics complexity	1	1	1	1	1	3	1	4
Low-speed flight	2	4	3	2	3	4	4	4
Stationary flight	1	4	4	2	4	3	4	4
Control cost	2	1	2	1	1	3	4	3
Payload/volume	2	2	3	1	2	1	4	3
Maneuverability	3	4	3	3	2	1	2	3
High-speed flight	3	2	2	3	4	1	1	3
Miniaturization	2	2	2	4	3	1	4	3
Power cost	3	2	2	3	2	4	2	1
Survivability/endurance	2	1	1	3	3	3	3	1
Total quality index	22	24	24	24	28	28	32	33

VTOL: vertical take-off and landing.

The noise, in the same way, will be proportional to vehicle size and so with its power-source; in particular, it is mainly created by two sources: (a) propulsion system (i.e. electric engine and internal combustion engine) and (b) aerodynamic noise.

Aerodynamic noise of aircrafts, in general, is mainly made from vortices at the tips of wings, rotors, or propellers and airflow around the control and aircraft fuselage surfaces. This increases with wing or blade span loading and speed, so that low values enhance acoustic stealth.¹ Noise generally increases with power-source usage level, so that keeping the mass and aerodynamic drag of the vehicle as low as possible is a good first step to achieving low noise generation.

The aim of this work is to evaluate the acoustic emissions of some small UASs, during normal flight operations (e.g. take off, landing, turning, and hovering), useful for future design of an UAS tracking and reconnaissance system.

The measures were related to electric engines power level and propeller spin velocity. Each data collection was carried out in an anechoic room through a certified phonometer. Investigation on sound power level (SWL) and sound pressure level (SPL) was based on Standard EN ISO 3745, which specifies the measurement method. The results of each UAS have been presented and discussed.

Theoretical framework

The determination of SWLs of noise sources using sound pressure precision methods for anechoic and hemi-anechoic rooms due to UNI EN ISO 3745⁴ has been carried out. The microphone positions were associated with equal partial areas of the test sphere (or hemisphere), and the following equation shall be used to obtain the surface SPL

{\bar{L}}_{p f} = 10 \lg (\frac{1}{N} [\sum_{i = 1}^{N} 10^{0.1 L_{p i}}]) dB

(1)

where ${\bar{L}}_{p f}$ is the surface SPL over the test sphere, in decibels (ref. 20 µPa); L_pi is the SPL corrected for background noise resulting from the ith microphone position, in decibels (ref. 20 µPa); and N is the number of microphone positions.

The SWL was calculated through equation (2), where C₁ and C₂ were the correction factors related to the room temperature and pressure. It should be noted that all values reported are A-weighted⁴

L_{W} = {\bar{L}}_{p f} + 10 \lg (\frac{S_{1}}{S_{0}}) dB + C_{1} + C_{2}

(2)

where S₁ is the area of the test sphere (4πr²) and S₀ = 1 m² and C₁ and C₂ are

C_{1} = - 10 \lg [\frac{B}{B_{0}} (\frac{313.15}{273.15 + θ})] dB

(3)

C_{2} = - 15 \lg [\frac{B}{B_{0}} (\frac{296.15}{273.15 + θ})] dB

(4)

B₀ and B are, respectively, the reference barometric pressure (101,325 Pa) and the barometric pressure during the measurements (Pa). The air temperature θ considered during the acquisitions was in degrees Celsius and it is important to specify that equation (2) is applicable in a temperature range 15°C ≤ θ ≤ 30°C.

Test bench and instrumentation

The data collection was made in an anechoic room considering two different small quad-rotors as test bench. The noise intensity was recorded using a sound meter level, positioned in a series of fixed positions.

Anechoic room

The chamber (Figure 1) consists of an inner room (4.40 m × 4.40 m × 4.50 m) covered by absorptive fiberglass wedges providing a very quiet environment and outer shell of reinforced concrete. The room has a very free acoustic reverberation at any frequency and fully absorbent. The cut-off frequency of the chamber is 100 Hz.

Figure 1.

Anechoic room utilized in this work.

Sound level meter

Measurements were carried out using a sound level meter 01 dB type Solo SLM (Figure 4), a precision data-logging sound meter level. Further technical specifications have been reported.⁵ The equipment was calibrated by means of the 01-Metravib acoustic calibrator “CAL21” (Figure 2).

Figure 2.

01 dB type Solo SLM during calibration with the CAL21.

The SOLO sound level meter is a class 1 multipurpose metrological instrument that is suitable for the measurement environmental noise. During the experiments, equivalent continuous levels L_eq and pressure levels L_p, with A frequency weight ((dB(A)) and L_in (dB), were measured. It can also record 1/3 octave spectral frequencies.

For good acquisition, a fixed system is necessary (source-receiver), which reduces the error of positioning between the source and the receiver. Thus, the SLM was fixed on a tripod in different positions (i.e. sphere or hemisphere pattern⁴) during the data collection.

The data were stored in the non-volatile memory and then uploaded for reading in the dBTRAIT software.

UASs

Two different models of small quad-copters were analyzed:

Syma X5C;⁶

RC Eye One Xtreme.⁷

It was important to consider that these small UASs were chosen for their robustness, lightweight, low cost, and fast response time properties.

Syma X5C

The Syma X5C (Figure 3) comes with a 6-Axis Gyroscope which allows for strong balance in the air and precise hovering, even when there is a light wind due to four-axis structure.⁵ For beginner, there is a 360° Eversion control that allows the X5C to roll continuously in flight without sacrificing control. The spread spectrum control (SSC) allows for greater flight distance, responsiveness, and duration with less interference than radio control. Thrust is generated due to four DC (direct current) motors. In addition to flight control, the Syma X5C comes with a 2 MP 720P HD Camera for taking fantastic pictures and videos from the air. If flying at night, the Syma X5C also comes with green and orange lights that spot it at night and keeps the user oriented. However, in this work, the UAS was fixed on a tripod and in order to have much more battery endurance the camera and the lights were disabled.

Figure 3.

Syma X5C quad-rotor.

This UAS is not a toy, but in everyday life, main users are children. Thus, it was important to consider its influence on environment through noise power emission evaluation. The Syma X5C was chosen for its low cost, solid structure, easily reachable spare parts, and fast time of response.

RC Eye One Xtreme

Another UAS, used for this research, was the RC Eye One Xtreme (Figure 4), a micro-quad-rotor; it is one vehicle of a new series.⁶ In this case, this UAS was smaller than X5C, but better in terms of performance (i.e. stability and control). Micro-quad-rotors will be the future of UAS technology.

Figure 4.

RC One Eye Xtreme quad-rotor.

Latest 6-axis gyro stabilization technology and outstanding brushless motor-driven flight control (better than X5C DC motors) are embedded within a robust yet stylish frame design. The sturdy lightweight construction is an ideal platform for flight applications ranging from aerial surveillance, imaging, or simply unleashing acrobatic fun flight excitement. It is possible to select two flight modes: beginners and experts flight enjoyment alike. In this work, beginner’s mode was used.

A 2.4-GHz receiver equips quad-rotor; therefore, it is possible to configure a 2.4-GHz transmitter, in order to control and manage it. As X5C, also RC Eye One Xtreme was fixed on a tripod during data collection.

Both UASs use different colors (e.g. for propellers, led, and legs) to distinguish the nose from the tail; this has been important for the referencing during data collection. Each point acquired needs to be referenced with respect to a reference coordinate system. Both UAS data collection and measurements were evaluated in a reference system, where the origin was the centroid of the vehicle, and the axes were orientated like the UAS body axes (Figure 5).

Figure 5.

Reference coordinate system.

Test bench

In accordance with UNI EN ISO 3745:2012⁴ and Figure 5, each UAS data collection considers several phonometer positions, evenly spaced along a semi-sphere with a diameter of 1.2 m. Figures 6 and 7 show the scheme of the acquisition points (e.g. phonometer), respectively, for the Syma X5C and the RC One Eye Xtreme.

Figure 6.

Scheme of the sound level meter positions for Syma X5C.

Figure 7.

Scheme of the sound level meter positions for RC Eye Xtreme.

The SLM positions were several for each quad-rotor, in particular 5 points for Syma X5C and 9 points for RC One.

Data collection and results

The SWL was obtained by integrating sound intensity (or more accurately the average sound pressure) over a hypothetical spherical surface assuming plane wave conditions. The main formulation is based on equation (1). This standard requires an anechoic or semi-anechoic room, that is, free field conditions. The standard also limits the minimum number of microphones and their locations (Figures 6 and 7).

The SWL in dB (linear) and dB (A), for each small UAS, was calculated and compared.

The UASs in the anechoic room with the instrumentation utilized for the data collection were depicted in Figure 8.

Figure 8.

(a) Syma X5C and (b) RC ONE Eye with the instrumentation in the anechoic room.

The noise measurements, respectively, for RC Eye Xtreme and Syma X5C, consider different power levels of the engines in ascending order (Table 2).

Table 2.

Engine power level during data collection.

UAS	Engine power level (% full throttle)
Syma X5C	25–50–75–100
RC ONE Eye Xtreme	25–65–100

UAS: unmanned aircraft system.

The power levels of the four engines are expressed in percentage of the full power (i.e. vertical thrust) of the UASs (full throttle).

According to UNI EN ISO 3745:2012,⁴ Table 3 shows the sound power values in dB (linear) and dB (A), considering four power levels of the RC Eye Xtreme. Each condition was made considering a time acquisition of 60 s for the Syma and 30 s for the RC ONE. The duration depends on the endurance of the UAS and also strongly related to the power of the engines. These conditions reproduce a hovering flight condition, namely, when the UAS is in a fixed position in the air.

Table 3.

SWL for Syma X5C.

Condition		dB	dB (A)
No.	(% full throttle)	dB	dB (A)
1	10	86.6	81.9
2	25	86.9	83.9
3	50	86.9	82.2
4	100	86.7	82.4

SWL: sound power level.

Table 3 shows that if the engine power is increasing, the level of sound power emitted increases. It is clear that in the same time the endurance decreases.

Successively, the SWL per 1/3 octave band, respectively, for conditions 1, 2, 3, and 4 (engine power levels—ref. Table 2) are depicted in Figure 9.

Figure 9.

Syma X5C—SPL 1/3 octave band for each condition listed in Table 2.

As done for the Syma X5C, Table 4 shows the SWL in dB and dB (A) extracted during the data collection, considering the three conditions (Table 2) for RC One Eye Xtream. The results for this UAS are similar to the first one, and also in this case, if the engine power increases, the level of sound power emitted increases.

Table 4.

SWL for RC ONE Eye Xtreme.

Condition		dB	dB (A)
No.	(% full throttle)	dB	dB (A)
1	25	72.0	55.9
2	60	75.8	70.2
3	100	79.2	72.1

SWL: sound power level.

For the RC One, the graphs depicted in Figure 10 show the values of SPL. For the first condition, the maximum sound emission exists in a range of frequencies from 630 Hz to 2 kHz with a maximum at 1 kHz, while at low frequencies, the SPL is not significant. Instead, for the second and third conditions, the maximum of sound emission is included in a range of frequencies from 315 Hz to 8 kHz, and also in this case, the SPL at low frequencies is not significant, except at frequencies around 125 Hz.

Figure 10.

RC One—SPL 1/3 octave band for each condition listed in Table 2.

In addition, a directivity analysis was done. Figure 11 shows the SWL directivity values at the frequency 1/3 oct of 1.0 kHz, while Figure 12 shows the SWL directivity values at the frequency 1/3 oct of 2.0 kHz, in the three conditions considered for RC One.

Figure 11.

SWL directivity values, at the frequency 1/3 oct of 1.0 kHz, for the three conditions considered.

Figure 12.

SWL directivity values, at the frequency 1/3 oct of 2.0 kHz, for the three conditions considered.

Conclusion

In this article, the extraction of the SWL and SPL of small UASs was done, in accordance with UNI EN ISO 3745:2012.^4,8 A further investigation on the directivity was done. This analysis will be useful in further works related to UAS noise impact in outdoor or indoor environments, during flight operations and for a system that will be useful for UAS reconnaissance and tracking.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

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. Advances in unmanned aerial vehicles—state of art and the road to autonomy. Rotterdam: Springer, 2007.

Austin

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Office of the Secretary of Defense. Unmanned Aircraft Systems Roadmap: 2005-2030. Washington, DC: Office of the Secretary of Defense, 2005.

UNI EN ISO 3745:2012. Acoustics. Determination of sound power levels of noise sources using sound pressure Precision methods for anechoic and hemi-anechoic rooms.

01db Solo SLM Technical Specifications, http://www.01db.com/ (version March 2007, accessed 2016).

Syma Toys. Syma X5C manual, http://www.symatoys.com/product/show/1901.html (accessed February 2016).

RC Logger. Eye One Xtreme manual, http://www.rclogger.com/RC-EYE/RC-EYE-One-Xtreme/RC-EYE-One-Xtreme-Mode-2.html (accessed February 2016).

Papa

Iannace

Del Core

. Determination of sound power levels of a small UAS during flight operations. In: Proceedings of the 45th international congress and exposition on noise control engineering (INTER-NOISE): Towards a Quieter Future. 2016. pp. 216–226.