INTER-NOISE 2016
Acoustics Parameters theWings of Various Species of Owls
Joann KOPANIA1;
1
Department of Fundamentals of Technology and Industrial Ecology, Institute of Social Sciences and Management
of Technologies, Lodz University of Technology, 90-924 Lodz, 266 Piotrkowska Street, Poland
ABSTRACT
In nature can be found a good examples of effective noise reduction like soft pillows on the bottom of the
cat’s feet, membranous wings of bats and the specialized structures of owls feathers. These fascinating
animals are capable of some amazing physical feats. Additional species of these birds have evolved each with
the common characteristic of silent flight. Allows it the special design of owls’ wings and feathers. But in
nature we found two type of species of owls, called - good seeing and called - good hearing. There are also
differences between in shape of leading and trailing edges for the these species of owls. So in this work we
checked differences in geometry and shape the edges of the wings of owls by using the Stereo Microscope.
Also acoustical parameters were studied for wings of owls and other birds, for low-Reynolds numbers on
special constructed stand test with the outlet to the anechoic room.
Keywords: Noise, Owls
I-INCE Classification of Subjects Number(s): 51.4
1. INTRODUCTION
The construction of safe, quiet and high-speed aircraft has become an important task of aviation
engineers. From the last few years aviation engineers are considering about a new concept in aviation,
morphing aircraft, or aircraft that can fully change their shape. But this concept has been found by
nature, well before mankind. Looking to various bird species, tails and wings can completely change
shape to optimize their morphology for a given flight regime. They take off, fly with twists a nd turns,
soar and dive, and land again on a tiny branch, all with effortless precision. In 1901 Wilbur Wright
remarked, "that a bird's skill as a flier is not apparent. We only learn to appreciate it when we try to
imitate it." A bird is a perfectly controlled, natural flying machine. Propellers, wings, flaps, and
stabilizers are all secrets birds alone have shared for a very long time. Birds are unique in the animal
kingdom for two reasons. First, all birds have feathers. And second, all birds live in a h urry. A bird's
wing is the basic structure for flight. It is covered with contour feathers that are specialized for flight.
It is the shape of the wing that enables a bird to fly, and the shape is determined by the feathers (1). The
distinctive shape of a wing is known as an airfoil.
The nineteenth-century aviation researchers, inspired by birds flight, have studied avian wings as
the basics of developing man-made flight vehicles, as we seen in the work of Lilienthal (2), Magnan
(3) and Tianshu Liu (4). But through years of studies the researchers designed slightly thicker airfoil
based on theoretical and experimental methods of aerodynamics which gave much higher lift -to-drag
ratio at Reynolds numbers in airplane flight. Recently, there is renewed interest in low-Reynoldsnumber flight and flapping flight in the aerospace community due to the need of developing
micro-air-vehicles (MAVs) or morphing aircraft. So the ability of a bird to change the geometry of its
wing allows it to maximize efficiency for various flight strategies is very interesting. The predator is a
perfect example, utilizing a low energy gliding or soaring flight to travel great distances or search for
prey as well as a high-speed dive in which it sacrifices stability for increased maneuverab ility and
velocity. Stability and maneuverability are also a trade off in the natural world. Additional flight
without sound is the ability only one group of predators.
1
[email protected]
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2. SILENT FLIGHT OF OWLS
Most genera of owls (Strigiformes) have the ability to fly silently. The first report on the silent
flight these birds was given by Graham (5). The acoustical measurements of owls in flight was
presented by Thorpe and Griffin (6). Also Gruschka et al., (7), conducted acoustical flyover
measurements but on only one specimen Florida Barred Owl, in reverberation chamber. A compari son
of the flight noise from Tawny Owl (Strix aluco) and Mallard Duck (Anas platyrhynchos) was
presented by Neuhaus et al., (8). More recently published analyses of the silent owl flight were done by
Lilley (9,10,11), in which he concluded that the special feather adaptations of the owl lead to a major
noise reduction above 2 kHz. This was also proved by Sarradj at al., (12,13), who showed that the noise
for frequency bands above 1.6 kHz is significantly lower for the Barn Owl and Eurasian -Eagle Owl
than for the other non-quiet birds.
However, there is a large family of owls. The species of these birds differ in place to live and
foraging, method of hunting, structure of body, size, color, and structure of feathers and wings. While
many people think that all owls are nocturnal, many are actually diurnal or crepuscular, meaning that
they may be active at any time of day, though most likely the hours around dawn and dusk. Cieślak et
al. (14) shared the owls into two groups: good-seeing, they are active only night and good-hearing they
are active partly by day and at night but especially at dusk. Stricly nocturnal owls are:
x Barn owl (Tyto alba) - this bird belong to a family called Tytonidae, which comes from the
Greek word tuto, which means, “night owl”. She is a medium-sized white to light brown
owl with heart-shaped face, long ivory to pinkish colored beak, and relatively small
(compared to other owls), dark eyes. She hunts mostly by low quartering flight over open
habitats. Detects prey using excellent hearing and low-light vision;
x Boreal owl (Aegolius funereus) - word “boreal” means northern. This owl is circumpolar,
found in boreal and subalpine forests around the world. Boreal owl is secretive, spending
her winters hidden away in mature forests, hunting by night and roosting by day, usually
well camouflaged in dense cover. The owl will scan the ground by moving its head slowly
from side to side, listening for movement of potential prey, as they hunt primarily using
their excellent, directional hearing.
The owls active partly active by day and at night, (but especially at dusk), called diurnal, are:
x Northern Hawk Owls (Surnia ulula) - resemble hawks with their long, tapered tails, smaller
heads, and even their behavior. This owl fly with a mix of slow wing beats and long glides,
much like hawks. She is often seen perching like hawks too-on the tops of tall trees, often
near clearings-always watching for their potential food. Northern Hawk Owl tend to inhabit
areas far from cities and towns. She is diurnal and nocturnal. She can seize prey in flight,
plunge into snow, and detect prey on sound alone;
x Little owl (Athene noctua) - she facial disc is not well defined, and is greyish-brown with
light mottling. Cere is olive-grey and bill is greyish-green to yellowish-grey. Upper parts
are dark brown, with many whitish spots. Flight feathers are barred whitish and dark brown.
The belly is plain whitish. The Little Owl is most active at dusk, but also partly active by
day, and at night. Often roosts by day in dense foliage or openings of holes. Sometimes
perches in exposed sites such as fence posts, telephone poles, bare branches or mounds of
earth or rocks. When leaving the perch, the owl drops down and flies low over the ground
before sweeping up to another perch. Flight is undulating with rapid wingbeats alternating
with gliding.
The different type of life these birds caused that they have various microstructures present on the
wings. In these studies the influence of microstructures present on the leading edge of t he different
species of owls to the aeroacoustical parameters of wings was studied.
3. MATERIALS AND METHODS
3.1 Studied owls wings
The prepared owl wings of four different species of owls: Barn owl (Tyto alba), Northern hawk-owl
(Surnia ulula) Boreal owl (Aegolius funereus) and Little Owl - (Athene noctua) were examined in the
experiments described in the present paper. The prepared owl wings were obtained from private
collection of prof. Marian Cieślak (biologist of owls) - Table 1.
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Table 1. Size and characteristic parameters studied wings.
Species
Side of
Length of wing
Area [m2]
Chord in measured
wing
[m]
Barn owl
Right
0,343
0,0525
0,15
Northern hawk-owl
Left
0,340
0,0399
0,15
Boreal owl
Right
0,175
0,0253
0,10
Little owl
Left
0,171
0,0211
0,10
place [m]
The geometry of the wings is described by the wings area which was measured by platometre (it is
a measuring instrument used to determine the area of an arbitrary two-dimensional shape) and by
length and chord in the place where microphones were located.
A photograph of each wing can be seen in Fig.1.
Figure 1. Wings birds used in present experiment: a) Northern hawk-owl; b) Barn owl; d) Boreal owl,
d) Little owl
When using prepared wings for aeroacoustic measurements in special constructed wind tunnel, it is
obvious that such specimen do not exactly represent the wings of living birds in gliding flight in their
three-dimensional shape in every detail. The wings are rather reduced models that can be used. In the
present case, the basic process of the preparation was the following: the wings were separated from the
bodies, manually spread out and dried at 90 degree. It has to be kept in mind that differences in
elasticity and tension of muscles and tendons between living and dead birds are generally unavoidable
and remain a source of error. In contrast to the shape of the wings of a flying bird, which is actively
adapted by the muscles as are response to changes in the flow field, the shape of the prepared wings
can not be changed actively. Additionally, the flexibility of the wing specimen, as opposed to rigid
airfoil models, may also affect the acoustics. However, if measurements on bird wings in a wind tunnel
with outlet to the anechroic room are desired, then the process of preparation is inevitable and its
influence on the experimental results has to be discussed accordingly. The wing shape is assumed to
correspond approximately to the shape of a wing during the gliding phase of the flight, and effects like
natural twist or aerodynamically induced changes of the wing shape could not be accounted for.
Notwithstanding the foregoing the selected wings were found to be best suited for the intended
experiments despite some minor restrictions - for example certain wings show small imperfections
(the missing seventh and sixth primary feather of the Little owl wing, Fig.1d).
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Measure the angle of attack is a limitation of this study because variations of the angle of attack
caused by flow-induced motions of the wing were not taken into account. Of course, efforts were made
to the wings during the measurements were set at the same angle of attack to the incoming flow.
Therefore, special care has been taken to adjust the angle in a similar way for each prepared wing by
using level in alula area of the wings- Fig.2.
Figure 2. Setting of the wings used in present experiment (example for Barn owl) and spirit level within alula
it is small wing which is reclining during landing bird )
However, the purpose of this study is the comparison of the noise generation at the wings of silently
flying birds, like different species of owls, at comparable angles of attack.
3.2 Stereo microscope observations
The leading edge of the wings were observed using the Stereo Microscope (SM). The geometrical
characteristics were measured by using the measurement software based on the SM photos by using the
Pixel-Fox programs. The photos were made by 10×22 ocular with 1,5 enlargement.
3.3 Aeroacoustic measurements
The measurements were performed on the special constructed stand with the outlet to the anechoic
room, as we see at the Fig.3 The outlet was circular with a diameter of 250 mm.
Figure 3. The photograph of the stand used to study the wings in anechoic room.
Flow was induced by a fan mounted on the inlet of the stand and regulated by the power inverter.
The fan type D4D250-BA02-01 with parameters: rotor diameter 2500mm, maximum power input
1140/1210W, speed/rpm 1240/1410 min -1 was selected. The outlet of test stand was in the anechoic
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room at the Aeroacoustics Laboratory of Institute of Power Energy in Lodz. The anechoic test chamber
is cubic, approximately 350m 3 in size and has walls that are acoustically treated with foam wedges
providing a reflection free environment. To measure the far-field noise was made by SVAN 958. The
microphone was located above the leading edge (perpendicular to the direction of the flow) positioned
at a distance 300mm from the edges - Fig.3. The second microphone was located behind the wings at
a distance 200mm. To provide isolation from wind noise, wind socks were placed on this microphone.
The microphones was calibrated before commencing the acoustic test. Spectra 1/3 octave were
measured.
Acoustic measurements were taken at five flow velocities ( 2m/s; 4m/s; 6m/s; 8m/s; 10m/ s) by
adjustable through an inverter connected to the fan. The velocities distribution in outlet of the stand
test were measured by using a Pitote probe and calculated by using log -Chebyshev method. The static
pressure and temperature in the channel were measured and recorded by the data acquisition station SAD-2, equipped with the ADAM modules 4000+, an integrated PC with the application GeniDAQ. In
each measured points the data were recording by 10s with resolution 0,1s. The level of turbulences
were not greater than 2%.
4. RESULTS AND DISSCUSION
4.1 Microscope observations
As shown at Fig.4 the Barn owl and Boreal owl have “teeth” on the leading edge of the wing (Fig 4
and Fig 6). These teeth have curved edges and they are not planar but slightly inclined away from the
level of the wings. Each “teeth” is formed by the tip of a single barb which can be divided into
proximal base and distal tooth-shaped tip. The teeth are thin for Boreal owl. Each serration is curved in
a certain way, i.e. the tip points toward the proximal end of the feather. The tooth-shaped tip has
different length along the edge in the range 2-5 mm. This is according with the researches carry on by
Yang et al. (15) and Cieślak et al. (14). The structures which are presented on the leading edge of
Northern hawk-owl and Little owl have got the different shape. They are like ellipses (arcs). They are
rather flat according with the wing position. The height of these structures is in range 0.2 - 1 mm. They
are arranged in direction of top of wing.
According with early published papers about the leading edge with and without serration is known
the leading edge serration contributes to flight stabilization and can prevent the laminar separation of
flow on wing surface (16,17). It appears that the serrations of leading edge behave as a set of closely
streamwise vortex generators that can reduce the flow separation and bound ary layer thickness, and
accordingly stabilize the air flow on the wing surface.
Figure 4. Barn owl wing used in present experiment and the structures on the leading edge of wing this bird
(from left: near alula, in the centre of leading edge, near the top of wing).
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Fig.5 Northern-hawk owl wing used in present experiment and the structures on the leading edge of wing this
bird (from right: near alula, in the centre of leading edge, near the top of wing).
Figure.6 Boreal owl and Little owl wings used in present experiment and the structures in the centre on
leading edge of wings these birds (left - Boreal owl; right – Little owl).
4.2 Aeroacoustic studies
Owls do not fly very fast. According to Neuhaus et al. (7) the maximum speed of the Tawny Owl is
only about 6 to 10 m/s, but other publications mention that owls tend to fly even slower, with speeds in
the range of 2,5 m/s to 7 m/s.
Therefore, in these studies selected air velocity ranges include speed obtained by owls in flight. The
general noise generated by prepared wings increases with increasing flow speed for the Barn owl and
Norther-hawk owl. For the all measurements the background spectrum was took into account and only
differences between spectrum of wings and spectrum of background were presented on the graphs.
The 1/3 acoustic spectra depending on the air velocity are presented on the Fig. 7 and Fig. 8.
Figure 7. Far-field acoustic spectrum 1/3 SPL cumulated spectra depending on the velocity for the Barn owl
(left) and Northern-hawk owl (right)
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Figure 8. Far-field acoustic spectrum 1/3 SPL cumulated spectra depending on the velocity for the Boreal owl
(left) and Little owl (right)
At lower speed the result shows that the nocturnal owl generate a little higher noise than the diurnal
owls, especially in the range 80-315Hz. At the range 160-400 Hz only the Barn owl and Boreal owl
gave small sound signal. Also for all speed the single signals 3 - 3,5 dB in 5kHz and 6,3kHz is observed
for these birds. This may suggest that at lower speeds the arches on the leading edge are a better
solution in noise reduction than serrated especially in lower frequencies and above 5kHz. At the higher
speed the result shows that the wings of nocturnal and diurnal owls generate the same SPL signals in
similar frequencies. At the range 1kHz-10kHz the sound intensity is lower than 1 dB. This is very
important for owls as a hunter, because their prey hear in this range. For Barn owl, nocturnal owl, the
1/3 SPL spectrum at range 80Hz-800Hz is higher than the diurnal owls Northern-hawk owl (especially
at 10 m/s). For the Boreal owl and Little owl the result is different, but it can be due to size of wings,
which are much smaller and the signals could be distorted. However, the 1/3 SPL spectra for studied
wings are different especially for the Boreal owl and Little owl. For the nocturnal - Boreal owl, it is see
on the 1/3 spectrum, that we have got a single signals at the higher frequencies. This is not observe for
the Little owl.
But when the acoustic measurements were made behind the wings we observed differences in the
1/3 spectra of owl wings. The wings diurnal owls generated more noise than nocturnal - Fig.9.
Figure 9. Far-field acoustic spectrum 1/3 octave showing difference between the SPL wings and background
behind the wings at 2m/s (a, b) and 10 m/s (c, d) for studied owls.
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The 1/3 SPL signal for Barn owl is below 10dB, but for the Boreal owl is below 1 dB at the velocity
2m/s (which may also result from the size of a wing or be the result of missing feathers in the win g).
However, it wasn't possible to get additional wings for testing. As we see for Fig. 7, at lower speed (2
m/s) the range of signal is between 16 - 315 Hz, but at higher speed ( 10 m/s), the SPL signals are
extended to the 4kHz for Little owl and to the 3,15kHz for the Northern-hawk owl. Above these
frequencies no SPL signal were observed for all studied wings.
According Cieślak et al. (14) the feathers of owls are more flexible than the feathers the other birds.
The wings of Barn owl and Boreal owl are also more flexible than wings of Northern hawk-owl and
Little owl. The recent studies show that the elastic of the wings is important in their acoustical
parameters. The newest work in this themes indicate that far-field acoustic power of the elastic edge
should be scaled to U7 at low frequencies and also at broader frequency range of human audition (18).
Research about trailing edge noise of the prepared wings require a more detailed approach, due to the
strong elastic effect of the edges. It should be continued in the next phases of work.
5. CONCLUSIONS
The silent species of owls have developed an adapted hunting system that combines a very good
hearing to aurally locate the prey with distinct mechanisms of the wings and feathers that enable the
nearly silent flight in order to not be heard by the prey. This paper presents the microscope studies and
aeroacoustic result obtained from measurements on special stand for prepared wings four species of
owl - noctural: Barn owl (Tyto alba) and Boreal owl (Aegolius funereus) - “good hearing” and diurnal:
Northern hawk-owl (Surnia ulula) and Little owl (Athene noctua) -“good seeing”. These species have
a similar weight and wingspan and can therefore be compared. The differences between the
microstructures presents on the wings were shown. The leading edge of diurnal owls have the small
arcs but the leading edge of the nocturnal owls have the teeth. These structures may be responsible for
the observed acoustical signals. For the diurnal owls who have got the arcs on the leading edge of
wings lower acoustic signals were observed in lower frequencies than for the teeth. Thanks this,
probably the diurnal owls can also silently fly and can not be heard by prey.
The acoustic performance comparisons between the nocturnal owls and diurnal owls show that
there are differences in the 1/3 SPL spectrum generated by wings studied species of owl. These
results prove that the sound suppression characteristics feathers play an important role for their silent
flight. At higher speeds, the presence of teeth is important because it affect on the SPL level. For the
nocturnal owls we not observed the SPL signal at higher velocity behind the wings. This is very
important for owls as a nocturnal hunter, because they are not heard.
This research could give the inspiration for solving the aerodynamic noise of aircraft and flow
machinery. This work can provide a new idea for the design of low-noise devices by using the different
type of cut on the leading edge of the airfoil.
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