Abstract
This work describes the testing involved in generating an acoustic signature profile of a small multi-rotor unmanned aircraft system. A typical multi-rotor unmanned aircraft system, with a weight of approximately 2.1 kg, was used for sound pressure level measurements. This study established a relationship between distance, altitude and sound pressure level, finding that the sound decays approximately in line with 6 dB(A) reduction for a doubling of distance. The effect of the orientation of the multi-rotor unmanned aircraft system was also investigated. It was determined that the sound profile does not vary significantly around the periphery of the multi-rotor unmanned aircraft system in the propeller-plane. However, when measured with the observer underneath the multi-rotor unmanned aircraft system, the sound pressure level was found to vary by as much as 10 dB(A), with the greatest sound pressure level at approximately 45° from horizontal. Finally, an acoustic array was used to measure key frequencies for the main sound sources: motors and propellers. It was found that extraneous noise from the multi-rotor unmanned aircraft system frame vibration and mounting methods was also common. Despite relatively low levels of sound being measured (especially when compared with conventional aircraft and rotorcraft), the increasing numbers of unmanned aircraft systems in urban environments, close to humans and dwellings, suggests that increasing complaints are likely. Thus, further research was suggested, including expanding the range of multi-rotor unmanned aircraft system to be tested, introducing DGPS, improving the mounting for indoor testing, and psychoacoustic analysis of the sound.
Background
Multi-rotor unmanned aircraft systems (MUAS) are becoming increasingly popular, in both military and commercial applications. The European Union’s Remote Piloted Aircraft System (RPAS) Roadmap states that if unmanned aircraft systems (UAS) are ‘flown low and in great numbers, [they] will become a nuisance’. 1 Flying low, in the vicinity of housing is common for many suburban hobbyists. There are a growing number of complaints of noise in living environments, and the increased use of UAS will only add to this.
Much work has been undertaken on larger manned military and commercial aircraft, including noise reduction programs, federal regulations and standards, and changes to flight paths to reduce annoyance in civilian airports. However, there appears to be no research in the public domain on the sound signatures of MUAS.
Research objectives
The objective of this research is to map the sound level emitted by a standard MUAS in typical surroundings. In order to properly characterise the sound, three main tests are conducted. Firstly, an examination of the sound pressure level (SPL) for the chosen MUAS at varying heights and distances is made. Secondly, the sound radiating from the MUAS is measured in two planes: that in the same plane as the propellers, and a cross sectional plane to map the sound underneath the MUAS. This is described in more detail in Figures 5 and 6. Thirdly, an acoustic array is used to determine the frequencies of interest and the mechanism causing these frequencies.
Research scope
For this investigation, testing was only conducted on days which were considered to be calm or have low wind levels. Multi-rotors are well known to fly on days ranging from calm, with little to no wind, up to winds of 10 m/s (35 km/h). 2 To reduce a potentially very large test matrix, testing was only conducted on days where the local wind speed was recorded as less than 1 m/s, or Force 1 on the Beaufort Wind Scale. 3 Additionally, when flying in higher wind speed conditions, it is difficult to distinguish between wind noise and aircraft noise with SPL meters.
Computational aeroacoustics (CAA) have not been used in this study. While CAA has progressed significantly in previous years, it is still extremely difficult to accurately predict all variables that an UAS might be exposed to in a real-world test environment. Instead, experimental testing was elected to be conducted in a typical operating environment.
For this investigation, one typical quad-rotor unmanned aircraft system (UAS) was used. It was assembled completely from commercial off-the-shelf parts, using a four-cell power system, 13 inch propellers and an overall weight of 2.1 kg (more details in section ‘Field testing’).
For these tests, only the flight mode of hover was tested. If the UAS has to make any adjustments or manoeuvres during flight, this leads to a change in RPM of one or more propellers and motors. As a result, there is a change in sound produced. This is another reason low-wind conditions were used for testing. Significant perturbations from wind cause the UAS to shift its position and it must make small adjustments to accurately keep its station.
Review of regulations and noise measurement standards
Federal aviation regulation (FAR) 36 4 specifies the type of testing, measurement procedures, and subsequent calculations which must be made and submitted in order to certify an aircraft. FAR 36 for helicopter testing specifies three flight modes for acoustic testing: take-off, flyover, and approach. The helicopter subsection is the most similar category in these regulations to MUAS but the scale of the type of testing required is far greater than anything that would apply to standard use of an MUAS. For example, in flyover measurements, a helicopter should maintain an altitude of 150 ± 9 m and for approach measurements, the altitude should be 120 ± 10 m. Both these altitudes are greater than currently permitted by the Civil Aviation Safety Authority (CASA) as a hobbyist. It is also impractical for a small UAS to be flying at altitudes such as this, since it is very difficult to visually determine orientation from an observation point on the ground when the UAS is at such a height. Speeds of more than 120 km/h are also recommended for flyover testing, which is beyond the limits of small MUAS.
There are several Roadmaps aimed at integrating UAS into civil and commercial airspace over the coming years. Circular 328 from the International Civil Aviation Organization (ICAO) makes note of the possibility that noise from many UAS sources could become a problem in future: ‘As new products and aircraft come into use, it may become apparent that additional noise and emission standards are necessary’. 5 The FAA UAS Roadmap has yet to include a section on UAS noise. The RPAS Roadmap also touches on the potential for noise issues in future, but focuses on the fact that their use in commercial applications may be more beneficial than current non-electric aircraft. 1 This forethought shows that the governing agencies are beginning to consider noise standards for UAS.
For now, with the fairly limited regulation of small-scale UAS, particularly with respect to noise, it seems that the most restrictive regulations may be those presented by the Environment Protection Authority (EPA), or the equivalent body in each respective country (for example, the European Environment Agency (EEA) in Europe). These regulations can vary between countries, states, and areas. Of those surveyed, most EPA regulations prohibit noise from electrical equipment (which would include electric UAS) outside daylight hours (before 7 a.m. and after 8 p.m.) most days. 6 As the use of UAS becomes more and more accessible, people will more commonly use them in their backyards. This can be a nuisance to neighbours. Depending on the residential situation, the Police can become involved when a neighbour believes there is an ‘unreasonable’ noise level.
The problem with determining what is considered to be an ‘unreasonable’ noise level is that the perception of noise is subjective. Some people will be bothered by noises that others may not even notice. Although psychoacoustical methods could be applied to this problem, 7 people are complex, and their reactions to sound are very difficult to predict, and therefore legislate around.
Review of existing aircraft noise research
Aircraft noise testing has a long history, spanning more than 60 years. During this time, propellers have been analysed at all size ranges, and although limited, this includes the size relevant to this work.8–14 There has been very little investigation into or profiling of the sound made by small MUAS.
One of the few examples of complete UAS acoustic measurement that could be found was conducted by Massey and Gaeta 15 on a tactical UAS in the 20 to 60-kg weight range. This weight range is an order of magnitude larger than what is being considered in this study. This UAS was a fixed wing with a liquid fuelled engine (non-electric). Flyover measurements and static measurements were taken outdoors for this UAS. The authors were able to categorise the noise signature made by the UAS such that it could be clearly detected outside the useful range of on-board sensors.
A second example of complete UAS testing involving aircraft noise signature detection, used measurement of the Doppler effect to determine altitude, speed and RPM values of the aircraft. 16 However, this was also a fixed wing UAS, not a multi-rotor UAS.
Leslie et al. 17 have done some propeller investigation at a relevant size (chord length of 15–30 mm) at low Reynolds numbers. This was done to determine if the sound of a propeller could be significantly reduced by tripping the boundary layer. The removal of the separation bubble reduced the broadband noise of the propeller by 6–7 dB(A). While advocating further testing, it seems that this is, in part at least, a viable sound reduction mechanism.
The acoustic properties of UAS propellers have been investigated by Sinibaldi and Marino, 18 where an acoustically optimised propeller was created through a theoretical analysis and then tested compared to a standard, ‘conventional’ propeller. The results were promising; however, the investigation stopped short of testing the propeller as a means of propulsion for an UAS.
Of the aircraft where significant testing has been accomplished, MUAS are most similar to helicopters. Both are vertical take-off and landing (VTOL) aircraft. Where helicopters are concerned, the impulsive sound of helicopter ‘blade slap’ is considered to be most annoying to listeners, 19 although it is not always present in all flight regimes. The impulsive noise generated by helicopter rotors is not something that can be accurately predicted for a new design, nor is it something that is easily suppressed by common reduction methods. Lower overall rotor speeds, and therefore lower tip speeds, are less likely to produce blade slap. If more lift is required, then more propellers can be introduced. It remains to be seen if the comparatively high RPM values, but smaller blade diameter of MUAS, also produce this phenomenon, and if it is detectable to the human ear.
Helicopter noise mostly emanates from the rotor blades, engine, and gearbox. In the case of MUAS, gearbox noise does not apply, but motor and propeller noise is relevant, as well as the way the noise emanates from vibration of the structure. Apart from the obvious size differences between full-scale helicopters and multi-rotor UAS, the powerplant is the next greatest difference. Helicopters are mostly driven by internal combustion engines, whereas multi-rotor UAS are electric. Despite the differences between fixed-wing aircraft and helicopters, the report by Molino 19 concluded that there is ‘no need to measure helicopter noise any differently from other aircraft noise’. Whether this statement should also apply to MUAS remains to be seen.
Field testing
The following experiments were designed to determine the effect of distance on the noise levels, as well as to characterise the sound. For all tests, the quad-rotor craft in Figure 1 was used.
Quad-rotor MUAS used in testing.
The frame dimension was 650 mm in diameter. The MUAS used four 4108 Sunny Sky motors, with APC 13 × 4.7 inch propellers. The system was powered by a 5000 mAh 4S battery, and had a total system weight of 2.1 kg. The flight controller was a Pixhawk and the ground control station was a laptop running Mission Planner.
Two separate SPL meters were used for testing: a Brüel and Kjær Type 2235 SPL with a 1/2 inch condenser microphone and the Digitech QM1592 SPL. A Testo 435 anemometer was also used to measure local wind speed. The two SPL meters were calibrated against a frequency of 1000 Hz at 90 dB(A) and returned a deviation of ±0.8 dB(A). The anemometer was calibrated in the RMIT University industrial wind tunnel. During testing, it was discovered that the two SPL meters continually gave readings within ± 2 dB(A) of each other, and for further testing only the B&K meter was used.
A-weighting in decibels was considered the most appropriate measure since it attempts to replicate human perception of sound.
Testing location was a park at the top of a small hill as pictured in Figure 2. Testing was conducted away from the residential estate. Trees border the park, which partially shield it from external noises as well as wind. The test ground was nominally flat and covered with grass. Distance measurements were taken out to a horizontal distance of 30 m.
All measurements were taken with SPL meters at a height of 1 m from the ground.
Panoramic view of field test area. Grass length approximately 100–150 mm.
All tests were conducted early morning to avoid any disturbances from traffic, before the wind speed increased, and with minimal external sounds. One sound source that was beyond control of the experimenters was wildlife noise. Birds (cockatoos) often made noises which peaked at around 80 dBA. When this occurred, extra values were taken for averaging of that data point.
Distance measurements and consideration of errors
Using Mission Planner to measure distance and altitude, SPL readings were taken at a range of distances for several altitudes. Initially, this testing was done with both SPL meters, but was reduced to use only the B&K meter after determining there was little difference between them. Figure 3 shows the SPL varying with horizontal distance (as measured across the ground by Mission Planner) for four different altitudes: 5 m, 10 m, 15 m, and 20 m. This can be compared with Figure 4, which shows the same data against total shortest distance to the UAS from measurement point. For each altitude and distance, a minimum of six instantaneous values were recorded. This was repeated a minimum of three separate occasions and the results were averaged to the data points visible in Figures 3 and 4.
Horizontal distance (in metres) versus sound pressure level (SPL) in decibels (A-weighting) for a range of altitudes. Magnitude of distance (in metres) from observer to multi-rotor versus sound pressure level (SPL) in decibels (A-weighting) for a range of altitudes.
It is clear that for the 5 m and 10 m altitude measurement, the SPL decays approximately linearly, then flattens out around the local ambient level. The decay rate is approximately 6 dB(A) with a doubling of distance, which is in line with the theoretical prediction for sound decay in the free-field (usually 6.02 dB(A) for each doubling of distance). This trend is not so clear with the higher altitude measurements, although there is a downward trend, as would be expected. The fluctuations visible in the 15 m and 20 m altitude measurements can likely be attributed to a combination of a range of factors, but rarely due to errors in position hold. If it was observed that the MUAS shifted position by more than 1 m in any direction, the test was halted until position had been reclaimed. This is more difficult to assess visually as the distance increases. The same problem exists for any small altitude changes. If any adjustments are made by the MUAS but not clearly noticed by the observer, this can change and even increase the sound made by the MUAS. Station-keeping adjustments could potentially be partly the source of these fluctuations. Another source of the sound fluctuations could be the underside angle from which the observer hears the noise. This phenomenon is described more clearly in section ‘Sound profiles’
Atmospheric absorption of sound can have an effect on outdoor acoustic testing. 20 The amount of absorption is dependent on temperature, pressure, humidity, and frequency of the sound. No atmospheric conditions are controllable by the experimenter, only the selected test days are pre-determined. This choice was mostly based on lack of precipitation and low predicted wind levels.
Terrain can have an impact on the way sound travels. It can reflect and refract, inducing attenuation. 20 It is possible that when both the source of the sound and the receiver of that sound are close to the ground, the reflections from the ground can interfere constructively or destructively with the waves incoming from the source. The test area was grass, which should reduce some reflections, but measurements were only taken from a height of 1 m. The consistency of the results, even across different test days, shows that terrain reflections had minimal influence on the results.
It should also be noted that each battery used was identical and fully charged at the beginning of the flight, and equipped with a voltage alarm, but finished the test with a lower voltage than began. Each of the motors is powered by electronic speed controllers (ESCs) which regulate the voltage and amperage supplied. Provided the voltage for the LiPo battery stays above the minimum required (3.0 V minimum per cell to prevent damage), which it did, any effects on sound due to battery voltage changes are negligible.
Sound profiles
A sound profile for the quad-rotor MUAS was established in two planes.
The first profile was developed by placing the quad-rotor in position hold at an altitude of 1 m, taking SPL measurements, then yawing the MUAS by a set interval of 30° and repeating the measurements. This was done several times until a complete rotation had been made. This was done at a distance of 2 m and 5 m from the centre of the MUAS. This was in order to establish if there is any directionality to the sound produced by the MUAS and at which point measurements can be considered to be in the free-field. The results are presented in Figures 8 and 9. The overall average sound profile for these distances can be seen in Figure 7 and shows that at both distances, it is possible to consider the measurements taken as far-field. There are small fluctuations in the SPL as the MUAS is yawed, but these could likely be attributed to potential errors from station-keeping and other extraneous noise.
Plane measured in yaw profile, equidistant from centre of MUAS.
21
Plane measured in underside profile.
21
Average sound profile (measured in dB(A)) for the MUAS being yawed in a single location at an altitude of 1 m. Sound profile (measured in dB(A), measured from a distance of two metres) for the MUAS being yawed in a single location at an altitude of 1 m. Sound profile (measured in dB(A), measured from a distance of five metres) for the MUAS being yawed in a single location at an altitude of 1 m.
With these tests seeming to be both within the free-field, and therefore leaving orientation irrelevant, the sound profile in Figure 10 was produced. It shows the sound profile of most interest to every day listeners: that which one might hear standing underneath the MUAS (referred to as ‘underside profile’). Measurements were taken from a height of 1 m. Initially, the MUAS was 4 m away at an altitude of 1 m. In order to maintain this 4 -m radius and continue the underside profile, the altitude of the MUAS was incrementally raised, while the measurement point was moved progressively closer to the MUAS starting point. This ended with the MUAS directly over the SPL meter. This was repeated several times, with three test runs completing the same procedure additionally in reverse to confirm the assumption of symmetry.
Sound profile (measured in dB(A)), measured underneath the MUAS, mirrored from one side to the other.
Figure 10 shows all of the half measurements mirrored to the other side by symmetry, as well as the full underside measurements plotted together.
From these profiles, it is interesting to note that an observer who is at an equal altitude to the UAS (i.e. next to it) will experience slightly less noise than one located beneath the MUAS.
Some factors beyond the control of the experimenter limited the accuracy of these particular tests. One such example is the software used to adjust the position of the MUAS. Mission Planner only allows altitude hold at increments of 1 m and the accuracy at which a specified altitude is held depends on the quality of the barometer. It was noted during some tests, that a small increase in wind speed (gusts recorded were not greater than 2.5 m/s) would affect the barometric reading, and consequently, the MUAS would alter its altitude. For all of these tests, many data points were taken and the result averaged. This was done to minimise the effect of any spread in results due to this inaccuracy of altitude. It is recommended for future testing that the barometer is pneumatically damped to reduce the impact of small wind gusts.
The aforementioned inaccuracy is compounded by the small drift in position hold. Position hold, for location rather than altitude, depends on GPS and inertial sensors. Testing was halted if drift was observed to be more than 1 m in any direction. Nonetheless, many data points were taken and averaged to reduce the effect of drifting.
Laboratory testing
An LMS acoustic array was used to determine the main sources of sound, and their associated frequencies. Four configurations were used, with 10 runs of each configuration – propellers on, off, and with the top and bottom of the UAS facing the camera, respectively. A standard recording of the MUAS with motors at 50% throttle (approximately 3600 r/min) was made for each run. The results were analysed within Test.Lab software. The setup was as shown in Figures 11 and 12, with the aircraft mounted such that the propeller plane is parallel to the array plane.
Test setup for LMS acoustic array testing. Array pointed at MUAS, parallel to plane of interest. Distance between LMS acoustic array and MUAS on frame is 730 mm.
Mounting the MUAS parallel to the plane of the acoustic array proved problematic. The MUAS frame was attached to a square frame (yellow) of dimensions 1300 × 1300 mm with nylon cord.
The frame was placed 730 mm from the array. The battery used to power the UAS, despite comprehensive tethering, meant that the weight distribution was not completely even, causing a small tilt top rearward (toward the array – slightly visible in Figure 12). This meant that for some tests, where the bottom two arms on the MUAS were pushed away from the camera, and when the MUAS was turned around, the effect worked in the opposite direction.
Array results
Figure 13 shows an average room decibel level of approximately 28 dB(A), which is extremely quiet. Even a peak reading of approximately 45 dB(A) is considered to be quiet in most conditions. Most of the sound is low frequency and can be attributed to the air conditioning running.
Ambient frequency levels in quiet test room.
Figures 15 to 22 show the sound mapped onto an image of the MUAS, with corresponding frequency distribution on the right.
Ambient sonogram for quiet test room. Test with all motors running but no propellers. Top of MUAS facing array. Frequency range shown: 520–2260 Hz.
Figure 15, maps the sound source for the frequency range 520–2260 Hz onto the image of the MUAS. The underside of the MUAS is facing away from the camera and the sound is clearly concentrated on the bottom two motors. In this case, the propellers were not attached, but the motors were running. This noise concentration can be attributed to the difficulty in having the array exactly parallel to the measured surface. The battery used to power the MUAS for these tests caused a slight tilt of the MUAS. This meant that the bottom motors in Figure 15 are closer to the array than the top ones, leading the array to attribute the majority of the sound to these locations. Despite this, the array was able to determine the overall shape of the sound pattern and it is still kept close to the motors, even the top ones. The frequency spectrum for this configuration is shown in Figure 16. The frequency range plotted on the map (Figure 15) does not include the largest spike visible at approximately 2400 Hz, which was determined to be noise from the mounting method. Further spikes above 4000 Hz have been determined to relate to resonance in the metal frame around the MUAS and harmonics thereof. Figure 17 also shows a frequency range of 520–2260 Hz, but in this case, the dominant noise source is the mounting technique. The dark red region (highest decibel level) is centred on the right hand side restraining cord. The interaction of the propellers with the mounting mechanism seems to have caused this particular event. From the frequency spectrum in Figure 18, it is possible to see a repetition of harmonics in frequencies below approximately 2000 Hz. It is suspected that these frequencies belong almost entirely to the mounting scenario. Propellers and motors are also in this frequency band but the noise generated by the vibration of the cord is overpowering that of the MUAS.
Frequency levels for test with all motors running but no propellers. Top of MUAS facing array. Test with all propellers and motors running. Top of MUAS facing array. Frequency range shown: 520–2260 Hz. Frequency levels for test with all motors and propellers running. Top of MUAS facing array.
When looking at the underside of the MUAS with the propellers and motors running, Figures 19 and 20 are the result. When viewed from the bottom, the MUAS presents a different noise pattern to that presented when viewed from the top (Figure 17). In this case, the noise of the motors and propellers is at a higher decibel level than that of the mounting mechanisms. For the same frequency band (520–2260 Hz), the acoustic array was able to place the location of the sound at the centre of the MUAS. (The circle is slightly offset toward the top due to the misalignment discussed previously, causing the top two motors to be closer to the array than the bottom two.) This view is the one that most listeners would be subjected to (motors and propellers running, underside facing observer). The overall decibel level is considerably higher than tests without propellers (in some frequencies by more than 30 dB(A)) and also higher than the test where the top of the MUAS was facing the array (by an audible level of 5 dB(A) according to the map produced in Figure 19). The frequency spectrum (Figure 20) shows the same underlying harmonic pattern visible in Figure 18, but considerably more prominent low-frequency sound. The array was unable to precisely locate the origin of noise in the 70–500 Hz range, only providing the information that the entire area was equally loud.21,22 Heilmann et al.
22
supports the idea that frequencies below 500 Hz (with wavelengths in the range of metres) are very difficult to localise. Heilmann et al.
22
states that for these low frequencies ‘the wavelength becomes so large that an array’s angular resolution is too small to achieve a sufficient map’.
Test with all motors and propellers running. Underside of MUAS facing array. Frequency range shown: 520–2260 Hz. Frequency levels for test with all motors and propellers running. Underside of MUAS facing array.
The small peak in the frequency plot (Figure 20) at approximately 3300 Hz was analysed and determined to be MUAS frame vibration noise. It is suspected that this stems from the propellers and motors transmitting vibrations through the arms and into the frame-plates of the MUAS. Such a relatively small area, with many fixed points may vibrate at such a frequency.
Figures 21 and 22 show the MUAS with motors running but no propellers attached. The map in Figure 21 shows a frequency range of 650–2110 Hz, slightly different to that shown previously. However, it is still clear that the overall shape of the sound pattern respects the configuration of the MUAS, with the majority of the noise being concentrated around the motors. When comparing the underside frequency spectrum in Figure 22 with the top-facing frequency spectrum in Figure 16, it is possible to see that the overall frequency pattern is very similar and the sound levels are within a few decibels of each other. From this, it is possible to conclude that the main increase in sound level is caused by the addition of propellers, and not by observing the MUAS from underneath rather than above.
Test with all motors running but no propellers. Underside of MUAS facing array. Frequency range shown: 650–2110 Hz. Frequency levels for test with all motors running but no propellers. Underside of MUAS facing array.
The sonograms corresponding to the previous images are shown in Figures 23 to 26. From these figures, it is possible to see faint vertical stripes. These vertical stripes indicate a slight intermittency, or beating, in the nature of the sound. This could be attributed to small inaccuracies between the four ESCs running the motors. Sometimes it is possible to hear this intermittency which possibly stems from a slight phase difference between propellers. These data can then be compared with the ambient room recordings taken, visible in Figures 13 and 14.
Sonogram showing frequency levels per time for test with all motors and propellers running. Top of MUAS facing array. Sonogram showing frequency levels per time for test with all motors and propellers running. Underside of MUAS facing array. Sonogram showing frequency levels per time for test with all motors but no propellers running. Top of MUAS facing array. Sonogram showing frequency levels per time for test with all motors but no propellers running. Underside of MUAS facing array.
The main limitation for this test is that it had to be conducted indoors. While every effort was made to ensure a quiet test space, it is difficult to minimise sound reflections from the ceiling and/or floor. Nearby objects which represented a potential reflecting surface were moved as far away from the test area as possible in order to reduce the likelihood of such reflections.
Conclusions
Spatial flow mapping was undertaken for a small MUAS in laboratory and field tests under steady-state hover. The SPL was measured while varying; (1) the distance from, and above, the observer at various altitudes, (2) the orientation of the MUAS to the observer in the horizontal plane and; (3) the angle underneath the MUAS as viewed by the observer. At a radial distance of greater than about 2 m, the SPL was found to be relatively invariant to the angle between the MUAS and the observer in the horizontal plane, but in the vertical plane stronger sound radiation was noted at about 45° compared with 0 and 90. Although the distance–altitude relationship generally proved the theoretical 6 dB(A) reduction with a doubling of distance, this effect was thought responsible for the variation in the SPL values at higher altitudes.
To investigate the sound sources, including frequency distributions, an acoustic array was focused on the MUAS while it was suspended on a frame in the laboratory. The throttle setting replicated which was used in the field (approximately 50%, or hover). The main sound contribution arose from the propellers, followed by the motors, but other significant sound sources included the structure and extraneous noise was found arising from the suspension methods.
Further work is suggested, with emphasis on expanding the range of UAS included in testing, and the psychoacoustical effects of UAS noise. Although the noise levels emitted by small UAS are relatively low compared to conventional aircraft and rotorcraft, the increasing popularity of small UAS enforces the need to know more about them.
Further work
For future outdoor testing using position hold, it is recommended that the accuracy be improved by the use of differential-GPS (DGPS).
The tests conducted only look at the steady state of hover. Future work could include multiple flight modes, including fly-over or fly-past at a range of speeds and altitudes. The effects on MUAS sound during ascent and descent, in particular, vortex ring state, which affects multi-rotors as it does helicopters, 23 should be investigated.
An alternate mounting method for the MUAS during acoustic array measurements could also be considered for further investigation. This could also include the use of an anechoic chamber to reduce the potential for reflections and the effect of ambient noise.
A range of UAS should be tested in future to determine any relationship between take-off weight and sound, or size and sound. Very small (‘racing quads’) are becoming popular among enthusiasts and were noted to emit a very different quality of sound compared to some of the larger UAS. Capturing the extreme ends of the hobby scale for multi-rotors would show the outer limits in the quality and level of sound possible.
It is extremely complex to determine if a sound will be perceived as pleasant or unpleasant due to the subjective nature of the relationship between sound and listener. For this reason, the perception of the sound, as heard by a listener, and whether it is deemed to be ‘noise’ is not within the scope of this paper but may be considered for future work. 7
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.