PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Outer wall inspection by noncontact acoustic inspection method
using sound source mounted type UAVTsuneyoshi SUGIMOTO1; Kazuko SUGIMOTO2; Itsuki UECHI3;
Noriyuki UTAGAWA4; Chitose KURODA5
1-3

Graduate School of Engineering, Toin University of Yokohama, Japan
4-5

Technical Research Institute SatoKogyo Co. Ltd., Japan

ABSTRACT
By using the noncontact acoustic inspection method that utilizes flexural resonance generated by acoustic
irradiation induced vibration, crack defect existing near the surface of the measurement target can be nondestructively inspected from a long distance. It has been proved that the same test as the hammering test canbe done with tunnels and bridges with a height of over 30 m and shotcrete with a rough surface. This method
has problems such as environmental noise and angle dependency. However, when a sound source itself ismounted on an unmanned aerial vehicle (UAV) which is becoming popular in recent years, it becomes
possible to make the sound source position directly face the measurement target surface, and these problems
are expected to be resolved. Therefore, we investigated outer wall inspection by noncontact acoustic
inspection method using sound source mounted type UAV. The inspection results using the outer wall
specimen were good, and it became clear that peel defects of the tile can be detected even with a small soundsource that can be mounted on UAV.
Keywords: Non-destructive inspection, Noncontact acoustic inspection, Sound source mounted type UAV

1. INTRODUCTION
1.1 BackgroundThe collapse of the Morandi bridge in Genova, which occurred in August 2018, was shocking.
However, visual inspection or hammering test method are still used as inspection methods for theouter wall of a building and a concrete structure. As a quantitative nondestructive inspection method,
an impact echo method (1,2), an ultrasonic inspection method (3,4), an electromagnetic inspection
method (5) and an impact acoustics method (6) have already been developed. However, thesetechniques require contact or nearly contact with the measurement surface. As a result, temporary
scaffolding is necessary, and improvement of workability beyond the hammering test cannot be
expected. Therefore, development of a long distance noncontact inspection method is required.
1.2 Long distance noncontact inspection method
As a long-distance noncontact inspection method, impact exploration method (7) by pressure wave
using gas gun and water hammer exploration method (8) using water gun have been proposed.
However, the former has a problem in continuous measurement, and the latter has a problem that alarge amount of water is required. On the other hand, if an infrared camera is used, it is possible tomeasure the temperature distribution of the object surface without contact (9). However, in a tunnel
where temperature change is small, heating such as a heater is required at the time of detection, so it
is not suitable for high ceiling exploration. In addition, since the measurement results show only the
influence of temperature change near the surface, it seems to be insufficient in principle as an
1

tsugimot@toin.ac.jp
kazukosu@toin.ac.jp
3iuechi84@toin.ac.jp
4utagawa@satokogyo.co.jp
5kuroda@satokogyo.co.jp
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alternative to the hammer method. Furthermore, laser ultrasonic method (10) using laser irradiationinduced vibration is also a method that can remotely detect defects, and it seems to be particularly
suitable for inspection of metals having a high melting point. However, in the case of outer wall tiles
and concrete, since the melting point is low, only a very short time of laser irradiation can be
performed so as not to damage the surface. Therefore, there is a disadvantage that it is not possible to
add effective vibrational energy commensurate with high power. In addition, problems with safety
due to using multiple high-power lasers have been pointed out. In recent years, a method using
noncontact airborne ultrasound has also been proposed. Extremely high sound pressure can begenerated by focusing on ultrasound (11). However, due to the large propagation attenuation in theair, it seems difficult to be practically used for the actual structure.
1.3 Noncontact acoustic inspection method using acoustic irradiation induced vibration
Therefore, we proposed a noncontact acoustic inspection (NCAI) method to detect the flexural
resonance due to the acoustic irradiation induced vibration with a laser Doppler vibrometer (LDV).
And it has been shown that void defects in concrete specimens can be detected even from a distanceof 5m or more (12). Then, in order to improve practical defect detection performance, single tone
burst wave (13), vibration energy ratio (14) and defect detection algorithm using spectral entropy(15,16) were devised. We also devised multitone burst wave (17) that enable faster measurement andspatial spectral entropy (18) for separating and detecting defects and head resonance of LDV. In
addition, it has been shown that defect detection equivalent to hammering test can be performed byan actual railway tunnel, a high bridge over 30 m (19) and a shotcrete with rough surface (20,21).
However, the problems of this method were angular dependence of sound source and environmental
noise. Here, the angle dependence means that the defect detection capability becomes lower as theirradiation angle of the acoustic wave becomes larger. The reason for this is that it becomes difficult
to generate flexural vibration when it is impossible to apply a uniform surface excitation force.
1.4 NCAI method using sound source mounted type UAV
On the other hand, an unmanned aerial vehicle (UAV), so-called drone with multiple rotors has
become available for aerial photography in recent years (22). It is already used for crack inspection
etc. which can be confirmed from the surface. In the NCAI method, angle dependence andenvironmental noise were problems, but in reality these are problems caused by the sound source.
However, if the sound source itself is mounted on the UAV and it is possible to irradiate the acoustic
wave from near the measured object, it is assumed that these problems will be solved. In other words,
since the sound source mounted type UAV can make the sound source face directly on themeasurement target surface, it can be predicted that the problem of the angle dependency is solved.
In addition, since the sound source itself can be brought close to the surface to be measured,
measurement with a small sound source becomes possible, and environmental noise can also be
expected to resolve spontaneously. Furthermore, the application range of this method can be expanded
to the distance up to the measurable limit of LDV, and as the position of the LDV and the sound source
are away from each other, the resonance problem of the LDV head is also solved, high speed
measurement can be expected. Therefore, in order to solve the problem of the NCAI method andexpand the range of application, we examined the outer wall inspection by the NCAI method using
sound source mounted type UAV.

2. Sound source mounted type UAV and outer wall specimen
2.1 Sound source mounted type UAV (prototype)Figure 1 shows an external view of the sound source mounted type UAV (prototype). The basebody is DJI's Matrice 600 Pro, which is equipped with a flat speaker (FPS Corp., 1030M3F1R), asighting laser pointer and a laser rangefinder on the underside of the UAV body. The weight of the
UAV itself is about 10 kg, and it can fly for about 20 minutes with a sound source and an amplifier.
In addition, the waveform for acoustic irradiation induced vibration can be transmitted by wirelesscommunication, and it can be synchronized with the measurement on the LDV sid e.
2.2 Tiled outer wall specimen
A tiled outer wall specimen (2 × 1.0 × 0.2 m3 +2 × 0.6 × 0.6 m3 ) was made for verification
experiments on outer wall inspection using UAV (The bottom part is thickened because it simulates

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the pillar part). The overview photograph is shown in Fig. 2 (a). Because the weight is about 2.6 tons,
wheels are attached to the bottom of the specimen to facilitate movement. The layout of the simulated
defective sheets embedded in the specimen are shown in Fig. 2 (b). Various sizes of styrene sheets ofthickness 0.5 mm and foamed sheets of 1 mm thickness were used as simulated cracks. The burial
depth of the sheet is about 9 mm from the tile surface, assuming that the thickness from the concretebase material to the top surface of the tile is 10 mm. The size of one tile is about 45 × 95 mm 2 . The
same simulated defects are attached to the upper and lower two stages, but the difference is the
thickness of the concrete wall. The top two rows are placed on concrete walls about 20 cm thick andthe bottom two rows are placed on concrete walls about 60 cm thick simulating pillars. However, thisdifference in thickness had no particular effect on the experimental results. From the visual appearance
it is completely unknown where the defects are.
(a)

(b)

Figure 1 –Photographs of the sound source mounted type UAV (prototype).
(a) At the time of the ground, (b) At the time of flight.
(a)

(b)

Figure 2 – A tiled outer wall specimen (2×1.0×0.2 m 3 +2×0.6×0.6 m3 ). (a) Overview picture, (b)Arrangement of simulated defect sheet (depth is about 9 mm). Various sizes of styrene sheets ofthickness 0.5 mm and foamed sheets of 1 mm thickness were used as simulated crack s.

3. Acoustic irradiation experiment
The following three experiments were conducted to confirm the effectiveness of the NCAI methodby acoustic irradiation induced vibration from sound source mounted type UAV to an outer wall
specimen.
࣭Experiment I : When the acoustic wave is irradiated in front of a simulated defect.
࣭Experiment II : When the acoustic wave is irradiated to the center of the specimen .
࣭Experiment III: When the acoustic wave is irradiated during UAV flight at very low attitudes.
࣭Experiment IV: When the acoustic wave is irradiated during UAV flight at about 4m height.

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3.1

Experiment I

3.1.1 Experimental setup
Firstly, verification experiment was carried out with the sound source facing simulated defects.
Therefore, the UAV was mounted on the rear part of the transportation vehicle (actually there is areason that the strong wind was blowing and it was not able to fly). Acoustic irradiation experiment
was carried out on eight topside simulated defects (defect size 50 to 200 mm 2 ) of the outer wall
specimen. An experimental setup in Experiment I is shown in Fig. 3(a). The distance between thesound source and the outer wall specimen was about 1.6 to 1.7 m. The vibration velocity distribution
around the defect positions were measured using a scanning laser Doppler vibrometer (SLDV: Polytec
Corp., PSV-500 Xtra) which can measure the surface vibration velocity distribution. Because the rearpart of the transportation vehicle was near the specimen, the SLDV was moved to a position that waseasy to measure due to the defect position. Therefore, the distance between the SLDV and thespecimen is about 2.4 to 3.4 m, the laser incident angle was about 38° to 55° when vertical incidence
was 0°. The waveform used for acoustic irradiation was a single tone burst (STNB) wave (17), andtwo kinds of waveforms were created and used to match the flexural resonance frequency of the target
defect. The first waveform was used when the defect size was 100 to 200 mm 2, and the frequency
range was 0.5 to 4 kHz. The second waveform was used when the defect size was 50 mm 2 , and thefrequency range was 9 to 13 kHz. Both waveforms have a pulse width of 5 ms (modulation frequency
was 100 Hz), and the emission interval of each pulse was 15 ms. The sound pressure at the time of
excitation was set to about 95 dB (the maximum value of the Z characteristic) near the measurementtarget surface.
3.1.2 Experimental result
An example of the result based on the vibration energy ratio in the frequency band used is shownin Fig. 3(b). The frequency range for calculating the vibration energy ratio (14) is 0.5 to 4 kHz when
the size of the defect is 100 to 200 mm 2 , 9 to 13 kHz when it is 50 mm 2 according to the emission
waveform. The number of measurement points was 121 (11 × 11), and the measurement time wasabout 9 minutes 11 seconds when the addition average was 5 times. It can be seen from this figurethat all defect sizes of 50 to 200 mm 2 are detected. Among them, 50 mm 2 defects have a high flexural
resonance frequency as high as 10 kHz or more, so it is a difficult defect to be found by normal
hammering test.
(a)

(b)

Figure 3 –Experiment when the acoustic wave is irradiated in front of a simulated defect .
(a) Experimental setup, (b) Experimental result using vibrational energy ratio. The integration range
is 500-4000 Hz for 100 - 200 mm2 , 9 - 13 kHz for 50 mm 2 . All simulated defects can be detected.
3.2

Experiment II

3.2.1 Experimental setup
When the UAV actually fly, it is expected that the UAV itself will swing due to the influence ofwind etc. However, in the case of acoustic irradiation induced vibration, even if the sound source

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cannot be accurately confronted with the defect position, the measurement itself is c onsidered to bepossible if it is within the directivity range of the sound source. The flat speaker mounted on the UAV
has a directivity of about 30 degrees at 4 kHz according to the manufacturer's published datasheet.
Therefore, if the distance is about 3 m, it is possible to irradiate acoustic waves with frequencies of 4
kHz or less for all the front surface of the outer wall specimen. Therefore, a verification experiment
was conducted in the case where the acoustic wave was irradiated to the central part of the outer wall
specimen next. Figure 4(a) is a photograph showing the arrangement of the equipment during theexperiment. For indoor experiment, an UAV was placed on a table, and the distance between a soundsource and an outer wall specimen was about 3 m. The distance between a SLDV (Polytec Corp., PSV500 Xtra) and an outer wall specimen was about 4.3 m and it was placed somewhat diagonally behindthe sound source. As the irradiation acoustic wave, a multitone burst (MTNB) wave (17) having afrequency range of 0.3 to 4 kHz was used. In this waveform, 38 frequencies of pulse width 5 ms
(modulation frequency was 100 Hz) were included, and the total time length was about 197 ms. In
order to scan the front of the specimen at high speed, by setting the emission interval (200ms) to beslightly longer than the whole length of the waveform, all frequencies were sent with one emission
and the average processing was not performed.
3.2.2 Experimental result
The scan area size was set at about 4.5 cm intervals in the area of about 130 × 153 cm 2, and thenumber of measurement points was set to 1015 (29 × 35) points. The sound pressure was set to beabout 95 dB (the maximum value of the Z characteristic) near the measurement target surface, and themeasurement time was about 7 minutes (about 0.4 seconds per point). Figure 4(b) shows the
distribution of vibration energy ratio integrated over the range of 0.3 to 4 kHz. The white frames show
the size and position of the defect parts. It can be seen that 50 mm2 with high resonance frequency is
not detected due to the emission frequency range, but other defects are almost detected. The reason
that the noise is slightly conspicuous is that, in addition to the condition that there was no average,
the position of the laser head and the sound source were not apart from each other in the closed
environment, it seems that it was influenced by the reflected sound from the specimen and the multiple
reflected sounds from the surroundings.
(a)

(b)

Figure 4 –Experiment when the acoustic wave is irradiated at the center of the outer wall specimen.
(a) A photograph of the experimental setup, (b) An example of the experimental result using
vibrational energy ratio (300-4000Hz).
3.3

Experiment III

3.3.1 Experimental setup
An experiment was conducted to make the UAV with sound source actually fly outdoors andexplore the outer wall specimen. An experiment setup is shown in Fig. 5(a). As shown in this figure,
the UAV flew so that it faced the specimen. The flight height was set to about 2 m according to theheight including the truck of the specimen so that the distance between the sound source and thespecimen was about 2 to 3 m. The reason why this distance is ambiguous is that the UAV itself swings
during flight. In other words, since the height is as low as about 2 m, the wind taken up by the multiple
rotors returns to the UAV itself from the ground. Also, SLDV (Polytec Corp., PSV-500 Xtra) was

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placed at a position about 3.8 m diagonally backward to avoid the influence of reflected wave. The
resonance phenomenon of the laser head due to the reflected wave does not occur in principle when
the SLDV is away from the sound axis with strong sound pressure. Furthermore, it can be expectedthat the signal to noise ratio also improves if there is no influence of multiple reflection sound etc.
from the surroundings by outdoor experiments. For this reason, continuous irradiation of the MTNB
wave with a shorter pulse width was carried out in order to check whether measurement faster thanthe previous section is possible (Frequency range; 0.5 to 4 kHz, pulse width; 3 ms (modulation
frequency; 200 Hz), waveform overall length; about 59 ms, and emission interval; 60 ms). In addition,
the sound pressure was about 90 to 95 dB (maximum value of Z characteristic) near the measurementtarget surface.
3.3.2 Experimental result
The measured object was a 200 mm 2 styrene sheet placed on the upper left of the specimen. Figure5(b) shows an example of the image result by vibration energy ratio (0.5-2.4 kHz). The white frameshows the size and position of the measured object. From this figure, it was confirmed that although
the UAV itself swayed a little during the experiment, even if the irradiation of the acoustic wave wasin a short time, the defect could be detected if it was in the directivity range of the sound source. Since
the number of measurement points was 81 (scan area size; 32 × 32 cm2 , scan pitch; 4 cm), themeasurement time was only 21 seconds. This means that only about 0.26 seconds were needed per
point for this measurement. In addition, since the frequency band of the wind noise by the multiple
rotors was low frequency of 100-200 Hz, it was easily removed by the low cut filter.
(a)

(b)

Figure 5 –Experiment when the acoustic wave is irradiated during UAV flight at very low attitudes.
(a) Experimental setup, (b) An example of the experimental result using vibrational energy ratio (5002400Hz). The measured object was a 200 mm 2 styrene sheet placed on the upper left of the specimen.
3.4

Experiment IV

3.4.1 Experimental setup
The large UAV was somewhat unstable at very low altitudes about the height of the specimen due
to the wind generated by its own propeller. Then, the experiment at the time of UAV flight was carriedout by lifting and arranging the outer wall specimen itself on another concrete specimen (3 m inheight) using a large crane vehicle. Figure 6 (a) shows the experimental setup for UAV flight. At thetime of the experiment, the sound source mounted UAV was manually operated so as to make it facesubstantially at the center of the specimen. Although the distance to the tile surface was co ntrolled to
be in the range of about 3 to 5 m, this distance is not constant because the position of the UAV swings
occasionally due to the influence of the wind blowing. As seen from the SLDV (Polytec Corp., PSV500 Xtra), the specimens were obliquely angled upward at an angle of about 26.7° and at a distanceof about 11.2 m. The waveform used for acoustic irradiation induced vibration was a MTNB wave,
and a waveform with an overall waveform length of 60 ms was used with a frequency range of 0.5 to
4 kHz and a pulse length of 3 ms (modulation frequency of 100 Hz). Also, the sound pressure is set

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to about 90 dB (maximum value of Z characteristic) at 5 m distance.
3.4.2 Experimental result
The size of the measurement area was about 1.4 × 1.7 m 2 , and measurements of 525 points (21 ×25) were performed at intervals of about 70 mm. Figure 6 (b) shows an example of the image resultsby vibration energy ratio. The frequency range for calculating the vibrational energy ratio was 0.9 to
4 kHz in accordance with the flexural resonance frequency of the size 100 to 200 mm 2 of the simulated
defect. From the figure, it can be seen that 200 mm 2 for the styrene sheet and 150 mm 2 and 200 mm2
for the foamed sheet are clearly detected. The following three points are considered to be the reason
why 100 mm2 and 150 mm2 of the styrene sheet and 100 mm 2 of the foamed sheet were not detected.
㺃The sound pressure at the time of measurement was a little lower compared to indoor experiments.
㺃The number of measurement points was about half that of the indoor experiment (1015 points)
㺃The position of the UAV was not stable due to the influence of the wind, and accurate acoustic
irradiation could not be performed on the defect part (The smaller the defect, the higher the resonant
frequency, so the UAV position becomes important because of the directivity of the sound source).
Since averaging was not performed, the measurement time was about 137 seconds (This means thatthe measurement is 0.26 seconds per point).In addition, since the 50 mm2 defect has a high flexural
resonance frequency of 10 kHz or more, it was excluded from measurement this time. 
(a)

(b)

Figure 6 –Experiment when the acoustic wave is irradiated during UAV flight at about 4m height.
(a) Experimental setup, (b) An example of the experimental result using vibrational energy ratio (9004000Hz).

4. CONCLUSIONSAn UAV equipped with a flat speaker and a tiled outer wall specimen buried with thin sheets
simulating crack defects were fabricated and fundamental experiments were conducted to check
whether outer wall tiles can be inspected using the acoustic irradiation induced vibration from UAV.
Experimental results proved that it is possible to detect crack defect of outer wall tile by the NCAI
method using flexural resonance even for small sound source that can be mounted on UAV. Moreover,even if the UAV itself sways due to the influence of the wind, it was con firmed that the defect can bedetected if the defect is within the directivity range of the sound source. Especially, since the positionof the sound source and the high-sensitivity LDV are separated from each other, the influence ofresonance of the laser head can be avoided, and it was also found that extremely high -speed
measurement is possible. However, in order to use this method for actual outer wall inspection, there
are many problems to be overcome, such as autonomous flight independent of GPS and a utomatic
measurement in cooperation with LDV. Therefore, we will continue to consider practical applicationof this method in the future.

ACKNOWLEDGEMENTS
This work was supported by MLIT (Japan’s Ministry of Land, Infrastructure, Transport andTourism) Grant of Construction Technology R & D (2017-2018) and JSPS KAKENHI Grant Number
JP17K12991.

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