AIAA 2010-3911
16th AIAA/CEAS Aeroacoustics Conference
AIAA 09-0877
Noise Measurements of Tactical UAVs
Kevin Massey1 and Richard Gaeta1
Georgia Inst. of Technology/GTRI/ATAS, Atlanta, GA 30332-0844
This paper describes several noise measurement activities of GTRI to measure and
understand the noise produced by tactical UAVs. As UAVs in the tactical class have begun
to see much more use in the battlefield, it has become apparent that the noise produced by
these aircraft affects their utility in certain roles. GTRI has thus undertaken to measure the
noise of several UAVs through a series of static ground measurements, flyover
measurements, and measurements taken in an anechoic chamber. The results indicate that
propeller noise and engine exhaust noise are generally of equal importance for typical UAVs.
The difficulties in measuring flyover noise are also discussed as are methods for extracting
noise buried in back ground noise. Finally some discussions on UAV noise relative to UAV
detection is provided along with some concepts which would serve to reduce UAV noise and
thus reduce detectability.
I. Introduction
F
ROM 2000 the number of UAVs used by the Department of Defense as grown from less than 50 to over 6000
as of May 20081 with the majority of the UAVs entering into service in the theaters of Iraq and Afghanistan. The
real time imagery provided by these unmanned aerial systems has become an indispensible tool to the warfighter as
evidenced by the rapid rise in the number of flight hours as shown in Figure 1. Growth has been particularly strong
in the tactical UAV area where tactical UAVs are those which are operated by lower echelon forces as typified by
the Shadow UAV. Aircraft in this class typically have a payload in between 50 and 100 lbs and endurances on the
order of 12 hours. They typically have wingspans in the range of 15 to 30 ft, and they have been used almost
exclusively in an ISR (intelligence, surveillance, and reconnaissance) role.
While often providing actionable intelligence to commanders on the ground, and being ‘the most effective
weapon against the Taliban’ as quoted by a Taliban commander2, their utility is often limited by the fact that these
UAVs are easily detected by their radiated noise. UAV’s in this class typically have either 2-stroke or rotary
internal combustion engines with little or noise treatment which results in high noise levels being radiated by the
power plant. In addition to the engine noise, there is noise radiated by a propeller designed purely for performance
which also contributes to the far field noise. The combined propeller and engine noise can be audible by an observer
on the ground at a distance much further than the onboard sensors can detect humans on the ground. This provides
1
Senior Research Engineer, Aerospace Transportation Advanced Systems Laboratory, GTRI, Smyrna, GA, 30080,
Associate Fellow AIAA.
1
American Institute of Aeronautics and Astronautics
Copyright © 2010 by Georgia Tech Research Institute. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
AIAA 09-0877
persons of interest on the ground some time to react before the UAV can provide imagery of the subjects which in
turn reduces the utility of the UAS.
II. Prior Work
To the authors knowledge there have been few documented efforts where the noise of Tactical UAVs have been
measured. Noise measurements have been made for both propellers for manned aircraft3,4,5 and propellers for
UAVs6, but these measurements were for propellers of a scale which is an order of magnitude larger or smaller
(respectively) than that found on Tactical UAVs. The closest work in terms of propeller diameter and vehicle size is
by Heller, et al7, on ultralight aircraft. Noise measurements on internal combustion engines of the same horsepower
class have certainly been made, but as these engines would be typified by recreational vehicles such as ATVs,
snowmobiles, etc. there has been very little impetus to measure engine noise at altitude or at the cruise speeds of
UAVs.
There has been considerable amount of flyover noise measurements for manned aircraft, and the flyover noise of
small manned aircraft has some similarities to that of UAVs, though there is a significant difference in the size of the
aircraft. (Insert references and discuss)
III. Field Measurements of a Tactical UAV
Noise measurements were made for a tactical UAV at a government run airfield by Georgia Tech Research Institute
(GTRI) personnel. These measurements consisted of static noise measurements and flyover data. The flyover data
was obtained for several different altitudes and flyover speeds as military personnel operated the UAV.
A. Measurement Equipment
All microphones used in the measurements were B&K Type 4939 ¼-inch condenser type. These microphones were
power conditioned through the use of preamplifiers of B&K Type 2669, and powered through NEXUS 2690
amplifiers. Each microphone was supplied with a wind sock to reduce wind noise and was mounted on a boom to
reduce reflections as shown in Figure 2. The data was acquired using a Data Physics Abacus multichannel signal
analyzer. Each microphone was calibrated to a known reference signal in the field to insure that the noise levels
measured were accurate to within 1 dB. Remote power was supplied through the use of deep cycle marine batteries
and an AC inverter. The AC inverter was small but was found to produce some noise at 60 Hz.
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B. Static Noise Setup
A total of four microphones were used for the static UAV measurements. Using the hub of the propeller as a
reference point, the four microphones were spaced 25 ft away from the aircraft off of each wing, the nose and tail as
shown in Figure 3. These noise measurements were made with the aircraft parked in front of the ground station.
Noise data was acquired for all four microphones simultaneously for three different engine RPMs.
Each
microphone was pointed at the aircraft and was approximately 5 ft above the runway surface.
The noise spectrum at maximum power setting is shown in Figure 4 as measured by the microphone off the
starboard wing. The first two harmonics are over 40 dB higher than the broadband noise and are roughly at 106 and
213 Hz. These tones are a combination of engine and prop noise as the UAV has a two bladed propeller and a 2
cylinder, 2 stroke engine. Additional harmonics are also evident though they are at lower amplitude. It should be
noted that this location was relatively close to the generator for the ground station. An attempt to shield the
microphones from this noise was made by parking a large SUV close to the generator, which audibly reduced the
noise at the microphone locations. However the 90 Hz tone of the generator was still present in the static noise
measurements as shown in Figure 4. In this figure the noise of the power inverter is also apparent, but these tones
are easily distinguishable from the noise generated by the aircraft.
In Figure 5, the noise data is presented for all four microphone locations for the same engine power setting. There
are some significant differences which indicate that there is some directivity associated with the noise radiated by
the aircraft. For the first harmonic, the noise level is virtually identical for all locations. This is a strong indication
that the noise at this frequency is dominated by the engine noise which is radiating from the engine exhaust which is
effectively a point source. The propeller noise was expected to exhibit high directivity and this type of noise
radiation is evidenced by the second harmonic where the noise varies by over 20 dB from the nose to the starboard
measurement location. This is a strong indication that the second harmonic is dominated by propeller noise.
Although classic propeller noise directivity would predict that the propeller noise off the left and right wing would
be the same amplitude, the noise measured off the starboard wing is typically 5+dB higher than that off the port
wing. Also, the propeller noise was expected to be much higher in the plane of the propeller than behind the
aircraft, but this is not always the case.
Noise measurements were made for three different power settings which corresponded to idle, cruise, and max
power throttle settings. A noise comparison is made for all three power settings in Figure 6 as a function of engine
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firing order (i.e. the frequency is normalized by the frequency of the engine firing frequency). It is clear that the
noise levels are a strong function of engine RPM, but these differences do vary from one harmonic to the next.
When the noise levels of the first three harmonics are plotted as a function of engine RPM, there is a nearly a linear
increase in noise as the engine RPM is increased. The noise increase ranges from 6.8 dB/1000 RPM for the first
blade pass frequency to 9.2 dB/1000 RPM for the engine fundamental as shown in Figure 7. While the data does not
follow a purely linear trend, it provides an engineering rule of thumb that the noise of the UAV can be reduced by
roughly 8 dB every time the engine RPM can be reduced by 1000 RPM.
C. Flyover Noise Setup
For the flyover noise measurements a total of five microphones were used. These microphones were set up in a
cruciform arrangement with microphone 1 at the center and microphone 3 located closest to the runway as shown in
Figure 8. The microphones, with the exception of mic 1, were spaced 100 ft away from data acquisition equipment.
It was found that microphone 1 also picked up the noise of the wind moving a tent covering the test equipment as
well as the ac inverter as it was roughly 100 ft closer than the other four microphones. Microphone 5 happened to
be located near an access road which saw infrequent traffic, but nevertheless experienced some noise contamination.
Each microphone was mounted on a vertical pole 6 ft above the ground and was equipped with a wind sock as
shown in Figure 2.
The fly over data consisted of passes both up and down the runway at both a loiter speed of 52 kts and at a dash
speed of 70 kts. In addition, noise data was gathered for fixed radius orbits. This data was repeated for three
different altitudes. The GPS position of the UAV was recorded during the flyover measurements and the GPS was
plotted as a function of position relative to the center of the orbits and the altitude of the runway. The flight track
showing the different measurement altitudes for the tests is shown in Figure 13.
D. Outdoor Noise Measurements
Unlike acquiring data in quiescent conditions (e.g., an anechoic chamber), acquiring acoustic data outdoors
introduces significant measurement variability. Even on a clear, calm, day, measured sound pressure levels can vary
as much as 8 dB. Figure 14 shows measured data of a static warning siren producing a single tone. At a distance of
200 ft in an open field, the measured sound pressure level can vary as much as 8 dB. Variability of more than 10
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dB have been reported elsewhere8. The sources of variability are known and in some cases can be accounted for
with varying degrees of accuracy. A very brief and fundamental description of these variability sources are
discussed below. A detailed discussion is beyond the scope of this paper, however they are important to keep in
mind when reviewing the data presented.
1. Atmospheric Absorption of Sound
Sound waves can be attenuated as they travel through the atmosphere via two mechanisms: 1) viscosity and 2)
molecular relaxation. The latter is a much bigger effect than the former. The effect of molecular relaxation is to
absorb higher frequencies more than lower frequencies.
The amount of absorption is due primarily to air
temperature and relative humidity. Trends of atmospheric absorption due to relative humidity and temperature can
be seen in Figure 89.
As an example, at a temperature of 105°F and a relative humidity of 25%, a 100 Hz source at 7500 ft will attenuate
0.75 dB to an observer on the ground. A 300 Hz source at the same conditions will attenuate 5.37 dB. Of course
the higher the altitude, the more likely sound will travel through gradients of temperature and even humidity as
discussed below. All of this contributes to the variability in outdoor sound measurements.
2. Meteorological Effects on Sound
The speed of sound depends on the air temperature. As the temperature increases, the speed of sound increases.
This means that temperature gradients within the atmosphere will lead to gradients in the speed of sound. Typically
the atmosphere is cooler at higher altitudes. The result of this is that sound waves will “bend” upwards and produce
a shadow zone where sound does not penetrate although in reality, sound will enter this region due to scattering (see
Figure 9). Scattering can be caused by regions of atmospheric inhomogeneity. Environmentally this situation is
caused by air turbulence, rough surfaces, and obstacles.
The presence of wind will always introduce a gradient in the atmosphere because of the relative stagnant air near the
ground. When a wind gradient exists, sound waves propagating upwind are “bent” upwards and those propagating
downwind being “bent” downwards as seen in Figure 10. Temperature and wind gradients can result in measured
sound levels being very different to those predicted from geometrical spreading and atmospheric absorption
considerations alone. These differences may be as great as 20 dB contributing greatly to the time varying nature of
outdoor acoustic measurements. These effects are particularly important where sound is propagating over distances
greater than a thousand feet.
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3. Effects of Local Terrain on Sound
Sound traveling over the ground or reflecting off the ground will induce attenuation. The degree to which this
happens depends on the type of surface (e.g., smooth, hard, grass, trees, brush, sand). High frequency sound will be
absorbed more than low frequency sound. Furthermore, reflection from the ground can result in another mechanism
by which sound levels are either reduced or enhanced. When the source and receiver are both close to the ground,
the sound wave reflected from the ground may interfere destructively or constructively with the direct wave (see
Figure 11). With the source far overhead, this effect is most likely not a big factor, but at lower altitudes it may
become important at certain frequencies.
E. Flyover Noise Measurements
It should be noted that significant winds were present during the noise measurements. This not only increased the
background noise but also affected the operation of the aircraft. For example the noise produced by the aircraft was
likely more when it was flying downwind versus upwind as the aircraft was commanded to fly at a constant
airspeed. Thus differences may be noted in the noise during different portions of the orbits and when the aircraft
was flying in different directions.
The wind noise primarily consisted of low frequency noise which is apparent in the measured data as shown in
Figure 15. At frequencies below 100 Hz, the wind noise is exceeds that of the orbiting UAV while at higher
frequencies the noise of the UAV clearly dominates. To properly assess the flyover data in terms of UAV detection,
one must also consider the human response to the noise. As the measurements are made with microphones
specifically designed to have a flat frequency response, reporting of the measurements strictly as measured can be
misleading as the human ear does not have a flat frequency response. The most common way to reflect the fact that
human hearing falls off at both the low and the high end of the frequency spectrum is to apply A-weighting. It is
also important to consider the peak noise instead of the average noise with respect to detection. An observer on the
ground may only need a small burst of noise above the background to be tipped off to a UAV while the noise on
average may be at or below the background levels. Contrast Figure 15 which is a relatively long average with
Figure 16 which represents a peak hold measurement. The combined effect of the A-weighting and the peak hold
indicates multiple blade pass frequencies where the noise is over 10 dBA above the ambient noise even in the
presence of a strong wind.
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While it can be argued that it makes sense to average the noise during an orbit, averaging the noise during a flyover
event may not be appropriate. For the fly over noise, the noise data was reanalyzed where spectra were generated at
one second intervals for a three minute period as the UAV approached the measurement array and then flew past the
array. Doppler shifts in the frequency of the noise were clearly apparent when viewing these individual spectra.
Overlaying 180 spectra per altitude did not provide a meaningful way to examine the data, thus the Over All Sound
Power Level (OASPL) was calculated for each 1 second interval. These OASPL were then plotted as a function of
time for flyovers at six different altitudes as shown in Figure 17. There was considerable variation in the noise from
one second to the next which was generally due to the effects described above, yet there were also clear trends of the
noise increasing and decreasing as the aircraft flew towards and then past the microphone. To better capture these
trends a smoothed curve was fit to the 1 second OASPL data as shown in Figure 17. This data clearly shows that the
noise is reduced as the altitude of the UAV increases, but also appears to indicate a trend that the peak noise occurs
later in time as the altitude is increased. Some part of this time shift is due to a time delay brought about by the
propagation time, but this is clearly not the main reason for this shift. One possible explanation is that the peak
directivity of the noise beneath the aircraft occurs at an angle behind the aircraft and thus the peak noise occurs at
the time at which the UAV is at this angle relative to the microphone which would occur later in time as the altitude
is increased.
It was argued previously, that an average might make more sense for an orbiting UAV due to the fact that the range
to the aircraft is constant. Upon further investigation, it was found that considerable variation in the noise as a
function of time remained, even when the UAV was at a fixed distance. Figure 18, shows the time variant OASPL
for a three minute period as a UAV completed a single orbit. While the rise and fall of the noise as the UAV gets
closer and then further away shown in Figure 17 is absent, noise excursions of 10-15 dBA are common. While it is
impossible to explain each rise and fall in the data, this behavior is indicative of both the noise propagation effects
described above and the fact that the UAV is trying to maintain a constant airspeed in the presence of a gusting wind
while flying with the wind on one leg of the orbit and against the wind on the opposite leg.
Noise amplitude is generally considered to scale with the square of the distance, but in the real world this does not
always hold true as atmospheric absorption and other propagation effects affect the noise amplitude. An attempt
was made to quantify the effect of altitude on the measured UAV noise. One way of examining this effect was to
plot the peak OASPL at each flyover altitude as a function of altitude, however, it can be seen from Figure 17 that
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determining the ‘peak’ OASPL is somewhat subjective particularly at higher altitudes. Nevertheless when the peak
OASPLs are plotted on a logarithmic scale as a function of altitude, there is a fairly consistent trend as shown in
Figure 19. On this plot, an inverse square proportionality with distance is also plotted for reference. It is clear that
there is a greater reduction in noise with altitude than can be explained by distance. Figure 19 shows that for a
tactical UAV the trend with altitude is consistent with other aircraft noise data gathered over 60 years ago510. One
can deduce that the UAV is very loud when considering the difference in engine power and aircraft weight.
F. Spectrograms
Spectrograms of both flyover and loiter events were also created. The spectrograms provide a useful way to
visualize acoustic data and capture many of the same features shown in the plots above in a single graphic. Figure
21 is a spectrogram of a flyover event. The spectral lines indicate that the UAV can clearly be detected at multiple
frequencies for nearly the entire track. It can also be observed that the noise levels increase as the UAV approaches
the observer and then rapidly decrease. The frequency is also observed to change due to a Doppler shift as the UAV
approaches the observer. The noise levels are clearly above the background, but bursts of noise in the background
can be seen especially at low frequencies due to the wind.
A spectrogram of an orbiting UAV is shown in Figure 22. Similarly the UAV can be detected for nearly the entire
orbit. A clear frequency shift is noticed about 100 s into the orbit which accompanied by an increase in the noise
level. This is indicative of an increase in the RPM of the engine and propeller and is likely due to the UAV turning
such that the wind is at its back which requires the UAV to increase power to maintain airspeed. In both
spectrograms the noise levels are time variant indicating the various atmospheric effects upon the noise at the
ground.
III.
UAV Noise Measurements at GTRI
In an effort to reduce the noise of tactical UAVs and thus enhance their utility to the war fighting community, GTRI
has initiated a program of effort to reduce both the engine noise and the propeller noise of this class of UAVs. This
work includes active research in muffler design, quiet propeller design, active noise control, and the development of
research facilities capable of simultaneously measuring the noise and performance at simulated flight velocities.
The latter effort represents some unique challenges, but represents a capability necessary to validate the performance
of various noise reduction efforts before first flight. Due to the highly coupled nature of the various components of
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the propulsion system, it is often found that seemingly straightforward noise reduction solutions such as adding on a
muffler do not find their way to the field due to the steep losses in performance. Further, solutions which may work
in a static situation on the ground have often not been as effective during flight.
A. Propulsion System Noise Test Rig
A specialized facility is required to assess the noise impacts of modifications to the engine and propeller. GTRI has
created a propulsion noise test rig specifically for its forward flight anechoic chamber. This facility is capable of
producing an airstream in an anechoic environment with velocities up to 300ft/s (100m/s). Typically there are two
microphone arcs which allow the simultaneous measurement of far field noise in two planes. A graphical view of
the facility is shown in Figure 23. The main motor housing allows for an electric (or if needed a hydraulic) motor
which can turn a propeller up to 10,000 rpm.
An axial force sensor, rpm sensor, and a torque sensor are also
installed in the main body. Thus propeller performance can be acquired simultaneously with acoustic performance.
The GTRI facility is an anechoic wind tunnel that uses an open jet to produce the simulated flight velocity. Since
the propeller’s performance must be assessed in clean, uniform free stream flow, the size of the open jet limits the
size of the propeller that can be tested. Furthermore, in order to measure some forward arc observer locations, the
propeller rotation plane must be placed some distance downstream of the open jet nozzle exit.
For the GTRI
facility, polar arc measurements can be made from 600 to 1400.
To properly asses the noise of UAV engine it must be loaded with either a propeller or a device which mimics the
torque of a propeller. In general these engines are air cooled which means that if a propeller is not providing the
airflow around the engine then other arrangements must also be made to cool the engine. In light of these factors,
the simplest solution is to test the engine with the propeller, however, this means it is difficult to isolate the engine
noise as the propeller noise will also be contributing to the overall noise. One solution to this problem is to make
two separate noise measurements; one where the noise of the propeller is isolated and one where both the engine and
propeller noise is measured; and then subtract out the propeller noise to determine the engine noise from the
combined measurement. Another solution is to measure the engine exhaust noise using a dynamometer that replaces
the propeller load with a water brake. In all cases, engine power (or axial thrust produced by the propeller) should
be measured. This is essential when trying to assess noise reduction concepts. Most noise reduction concepts come
at a price on performance and thus needs to be quantified. Furthermore, a common (and useful) design feature of a
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quiet propeller is low tip speeds. This means that propeller twist angles are high to generate the desired lift.
Without the use of a variable pitch propeller, this means that under static conditions these “quiet” propellers will
stall readily (and produce large drag). Thus these types of propeller designs must be assessed aerodynamically and
aeroacoustically at flight conditions where the free stream flow reduces the inflow air angle to the propeller. This is
accomplished in a wind tunnel or in flight test – static testing is not sufficient itself to assess the noise of such
propellers.
B. Quiet Propeller
The key to reducing propeller noise is slowing down the tip speed and reducing the individual blade loading. Using
a 2-bladed propeller from a tactical UAV as a baseline, GTRI developed a 6-bladed design of the same diameter to
operate at 60% of the baseline rpm. Werner and Gehlhar (Ref. 5) tested a 6-bladed propeller on a larger scale for a
single place aircraft and realized a peak OASPL noise reduction of 10dB. The new propeller is shown in Figure 25
mounted on a tactical UAV engine which in turn was mounted on an engine dynamometer. The chord and twist
distribution for this fixed-pitch propeller were determined using the method for optimum propeller design presented
in Adkins and Liebeck11. This propeller is designed to produce the same thrust and torque as the baseline propeller,
but at a much lower tip speed. The propeller is currently undergoing performance and acoustic testing. Initial
results indicate that the noise level is substantially reduced from the baseline 2-bladed propeller.
C. Engine Noise Reduction
While there are substantial gains in engine noise reduction that can be realized through proper engine/airframe
integration, a significant amount of the engine exhaust noise still occurs with a properly designed muffler. For a two
stroke engine, back pressure is a critical parameter. Most exhaust pipes on these types of engines are designed for
performance. They act as a scavenging device to boost power output. By sizing first a diffusing area to create an
expansion wave back at the cylinder head and then a compression wave with a converging section downstream, the
exhaust pipe can augment the power by bringing in a fresh air charge and then closing the exhaust port during
compression12. This does not generally reduce the noise and indeed most likely increases the noise. Reducing the
radiated noise requires absorbing or canceling the finite pressure wave that exits the chamber at the primary firing
frequency. This is accomplished with either a reactive or resistive muffler or a combination of the two. A reactivetype muffler uses acoustic resonance to cancel the offending wave. A resistive muffler uses bulk absorbing material
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(e.g., fiberglass, glass wool) to viscously dampen the acoustic wave. A good source for design methodology that
examines the effects of basic parameters like volumes, pipe lengths, and areas can be found in Davis, et al13.
Using this muffler design approach, GTRI is developing muffler system for a large 2-stroke engine that has a
reactive component to address the fundamental firing frequency and a larger expansion can is lined with a bulk
absorber that is the resistive component.
duct lengths.
The muffler is modular in order to vary critical design parameters like
The performance the muffler will be first demonstrated on a dynamometer so the affect on engine
performance can be quantified along with the noise reduction.
configuration and adapt to an applicable UAV airframe.
Plans are then to close in on a proper design
Final testing will consist of ground static and flyover
measurements.
IV. Conclusion
The noise of tactical UAVs presently limits their functionality on certain missions. Field measurements have been
made to characterize this noise for various altitudes, power settings, and flight paths. The measured data clearly
indicate that the UAVs can clearly be detected on the ground beyond the useful range of their sensors. The better
understanding of the various noise contributions gained from these measurements have led to multiple noise
reduction concepts. GTRI has developed an experimental rig to quantify the performance of several noise reduction
devices in a forward flight anechoic chamber. This facility will be able to provide both thrust and noise data at a
simulated flight condition in a controlled environment. It is envisioned that the technologies developed will find
their way onto future tactical UAVs thus improving their utility.
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Predators and
Global Hawks
Shadows, Hunters
and other TUAV
GAO-09-175
Figure 1 Number of flight hours for DOD’s UAS.
(from Ref. 1)
Figure 2 Typical microphone setup showing
microphone boom and wind sock.
Figure 3 Microphone setup for static noise
measurements.
Figure 4 Noise measured off starboard wing for
static engine run up at max power.
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Staticc Test - 6500 RPM
M
(25 ft Distance)
110
Tail
Port
Nose
Starboard
90
-5
SPL, dB (re. 2 x 10 Pa)
100
80
70
ure 8 Microphoone setup for flyover
Figu
meassurements.
60
50
200
0
400
600
Frrequency, Hz (f =2 Hz)
800
1
1000
Figure 5 Static
S
noise da
ata for tacticall UAV.
Static Test
(25 ftt Distance- Starboard)
110
Max Power
Loiter
100
90
-5
SPL, dB (re. 2 x 10 Pa)
Idle
ure 9 Trends in
n atmosphericc sound absorp
ption
Figu
due to
t atmospheriic temperature and humiditty.
80
70
60
50
1
10
3
4
F
Firing
Order (f =2 Hz)
2
Figure 6 Order
O
tracking
g of three diffferent throttle
settings foor starboard microphone.
m
105
100
Peak Noise foor First Three Harmonics
H
as a Funcction of Engine RPM
R
ure 10 Notion of
o refraction of
o sound in
Figu
atmoosphere with temperature
t
g
gradients.
Engine Noise
Blade Pass
Blade Pass2
95
-5
SPL (re. 2 x 10 Pa)
6.8 dB/1000 RPM
90
85
ure 11 Effect of wind gradien
nts on sound
Figu
prop
pagation.
8.3 dB/1000 RPM
80
2 dB/1000 RPM
9.2
75
70
65
3000
3500
4000
4500
5000
55
500
6000
6500
RPM
ure 12 Phasingg differences between
b
sourcee and
Figu
observer due to paath differences near ground
d.
Figure 7 Peak
P
noise as a function of engine
e
RPM
for first 3 harmonics.
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70
Peak-Hold and A-weighted
Acoustic Spectrum
SPL - Peak Hold, dBA
60
10000
Altitude (ft)
5000
50
40
30
Blade Pass Frequencies Dominate
10+ dBA above background
20
10
0
200
400
600
800
1000
Fequency, Hz (f = 4 Hz)
A-Weighted
Peak Hold Method
0
-5000
60
Figure 16 Peak hold flyover noise spectrum.
4000
2000
Xp
osit 0
ion
(ft)
0
-2000
-4000
Figure 13 Track of orbit tests.
1070 ft MSL
1500 ft MSL
2500 ft MSL
5000 ft MSL
7500 ft MSL
10000 ft MSL
Background Ambient
90
OASPL [dBA; re: 0.00002 Pa]
5000
100
(ft)
ion
osit
YP
80
70
60
50
40
30
20
0
20
40
60
80
100
120
140
160
180
Time [seconds]
Figure 17 Fly over OASPL data of tactical UAV.
Noise During 3 min of Orbit at 6700 ft
60
70
60
Averaged Acoustic Spectrum
Noise Variations can not be explained
by atmospheric effects alone
55
OASPL, dBA
1 second intervals, 10 avg
Figure 14 Example of variability in outdoor
acoustic measurements.
50
45
50
215 Hz
SPL, dB
Average Background Noise
40
0
40
40
80
120
160
Time (s)
Figure 18 Time variant noise for orbiting tactical
UAV.
30
20
10
0
200
400
600
Fequency, Hz (f = 4 Hz)
800
1000
Figure 15 Averaged flyover noise spectrum.
14
American Institute of Aeronautics and Astronautics
AIAA 09-00877
May 14, 2008
Peak Fly-Over OASPL [dBA; re: 0.00002 Pa]
100
2
1/R Law Prediction
n
Peak OASPL [dBA] Mic #3
90
Peak OASPL [dBA] Mic #4
80
Atmospheric Absorptio
on and Meterological Efffects
70
60
50
40
Mea
an Background Ambient Noise
N
30
3
3.2
3.4
3.6
3.8
4
Log [Altitude]
10
945 ft MSL
M
1401 ft MSL
395 ft MSL
23
50
032 ft MSL
Figu
ure 22 Spectrogram for UAV
V orbiting at 7500
7
ft MSL.
7612 ft MSL 10,168 ft MSL
Figure 19 Relationsh
hip between alltitude and
OASPL off tactical UAV
V.
ure 23 Propulssion noise meaasurement rig in
Figu
GTR
RI’s forward flight
f
anechoicc chamber.
V noise with
h
Figure 200 Relationshiip with UAV
altitude with
w
compariso
on to light trrainer aircraftt
from referrence 10.
Figu
ure 24 Close up
p of propeller rig.
Figure 21 Spectrogram for UAV flyover at 7500 ft
MSL.
15
Americcan Institute off Aeronautics and
a Astronautiics
AIAA 09-0877
8
Downing, Micah Propagation Issues Presented at
the DARPA/NASA Rotor-Propeller Acoustic
Infrastructure Workshop, NASA LaRC, March 9-10,
2010.
9
Handbook for Acoustic Ecology, 2nd Edition , Barry
Truax, editor; Cambridge, 1999
10
Regier, Arthur, “Effect of Distance on Airplane
Noise” NACA-TN-1353, 1947.
11
Adkins, C.N. and Liebeck, R.H., “Design of
Optimum Propellers,” Journal of Propulsion and
Power, Vol. 10, No. 5, Sept.- Oct. 1994, pp. 676-682.
Figure 25 Low noise propeller mounted on engine
dyno.
12
Jennings, G “Two-Stroke Tuner’s Handbook”
Copyright Gordon Jennings, 1973.
13
References
1
United States Government Accountability Office
Report, UNMANNED AIRCRAFT SYSTEMS
Additional Actions Needed to Improve Management
and Integration of DOD Efforts to Support
Warfighter Needs, November 2008, GAO-09-175.
Davis, D., Stokes, G., and Moore, D. “Theoretical
and Experimental Investigation of Mufflers with
Comments on Engine-Exhaust Muffler Design”
NACA Report 1192, 1954.
2
‘Porous Pakistani Border Could Hinder U.S.’, New
York Times, May 4, 2009.
3
Regier, A. and Hubbard, H., Status of Research on
Propeller Noise and Its Reduction, J. Acoust. Soc.
Am. Volume 25, Issue 3, pp. 395-404 (May 1953).
4
Parry A., Crighton D., Asymptotic theory of
propeller noise. I - Subsonic single-rotation propeller,
AIAA JOURNAL Volume: 27 Issue: 9 Pages:
1184-1190 Published: SEP 1989.
5
Werner, D. and Gehlhar, B., The noise from piston
engine driven propellers on general aviation
airplanes,AIAA-1997-1708,
AIAA/CEAS
Aeroacoustics Conference, 3rd, Atlanta, GA, May
12-14, 1997.
6
Leslie, A., K C Wong, K. C., Auld, D., Broadband
noise reduction on a mini-UAV Propeller, AIAA2008-3069, 14th AIAA/CEAS Aeroacoustics
Conference (29th AIAA Aeroacoustics Conference),
Vancouver, British Columbia, May 5-7, 2008.
7
Heller, H., Dahlen, H., and Dobrzynski, W.,
Acoustics of Ultralight Airplanes, JOURNAL OF
AIRCRAFT 1990,vol.27, no.6 (529-535).
16
American Institute of Aeronautics and Astronautics