J. Biomedical Science and Engineering, 2011, 4, 83-89
doi:10.4236/jbise.2011.42011 Published Online February 2011 (http://www.SciRP.org/journal/jbise/
JBiSE
).
Published Online February 2011 in SciRes. http://www.scirp.org/journal/JBiSE
Simultaneous measurements of aeroacoustic sounds and wall
vibration for exploring the contribution of tooth vibration in
the production of sibilant sounds/s/
Masanori Nakamura1, Kazunori Nozaki1, Haruka Takimoto2, K azuya Nagamune2,
Motoharu Fujigaki3, Shigeo Wada2
1The Center for Advanced Medical Engineering and Informatics, Osaka University, Toyonaka, Japan;
2Department of Mechanical Science and Bioengineering, Osaka University, Toyonaka, Japan;
3Department of Opto-Mechatronics, Faculty of Systems Engineering, Wakayama University, Wakayama, Japan.
Email: masanori@me.es.osaka-u.ac.jp
Received 3 December 2010; revised 23 December 2010; accepted 28 December 2010
ABSTRACT
In order to understand the contribution of teeth
vibration to the production of sibilant/s/, the pre-
sent study was designed to develop a method of si-
multaneously measuring aeroacoustic sounds and
the vibration of an obstacle. To measure the vibra-
tion without disturbing flow, the Michelson inter-
ferometer was employed. The flow channel, which
had an obstacle wall inside of it, was fabricated
such that it morphologically mimicked the simpli-
fied geometry of the oral cavity. Given airflows at a
flow rate of 7.5 × 10–4 m
3/s from the inlet, aero-
acoustic sounds were generated. A spectrum analy-
sis of the data demonstrated two prominent peaks
in the sound at 1,300 and 3,500 Hz and one peak in
the wall vibration at 3,500 Hz. The correlation in
peak frequencies between the sound and wall vi-
bration suggests that the sound at 3,500 Hz was
induced by the wall vibration. In fact, the sound
amplitude at 3,500 Hz decreased when the obstacle
wall was thickened, which increased its rigidity (p <
0.05, t-test). The experimental results demonstrate
that the developed techniques are capable of meas-
uring aeroacoustic sound and obstacle wall vibra-
tion simultaneously, and suggest the potential to
pave the way for detailed analysis of the production
of sibilant sounds /s/.
Keywords: Acoustic Measurement; Vibration
Measurement; Michelson Interferometer; Aeroacoustics;
Sibilant/s/
1. INTRODUCTION
Oral therapies for speech disorders are directly linked to
quality of life (QOL). In oral therapies, the modification of
oral morphological features including changes in the spa-
tial position of the jaw is performed surgically for the pur-
pose of maxillofacial orthodontic therapies [1], prosthetic
treatments [2], and the insertion of sports mouth-guards [3].
Alterations in oral morphologies, however, often bring
about vocal disorders, in particular, dental fricative sounds
that are produced within the oral cavity.
Among the dental fricative voices, sibilant /s/ has
gained relatively much attention from dentists and scien-
tists because sibilant sounds /s/ are frequently used in
daily conversations and most languages [4-6]. The sound
source of sibilant /s/ is generally accepted to be the ante-
rior teeth [7,8]. From a fluid mechanical point of view,
when the sibilant /s/ is produced, a jet that develops
through the constriction made by the tongue and maxilla
is speculated to impinge on the anterior teeth, yielding
flow turbulence and causing chaotic formation of vor-
tices of many different length scales. The interaction of
the vortices leads to rapid variation in pressure on the
surface of anterior teeth, thereby inducing pressure fluc-
tuations that cause sibilant /s/.
Sibilant /s/ has been explored experimentally, theo-
retically, and numerically. For example, Stevens [9,10]
applied aerodynamic flow theory to study acoustic
mechanisms of fricative sounds. The study was later
followed by the work of Shadle [7,11,12], who modeled
the oral cavity as a circular duct with constriction and an
obstacle. Shadle’s model is quite simple, but the ob-
tained results were important in the research of sibilant
/s/. The study measured velocities and sound pressures
that radiated from the model by parametrically varying
geometrical features. The authors concluded that acous-
tically, the significant parameters are the length of the
front cavity, the presence of an obstacle, and flow
M. Nakamura et al. / J. Biomedical Science and Engineering 4 (2011) 83-89
84
rate. Theoretical predictions of the far-field sound of
sibilant /s/ were performed by Howe and McGowan [13],
who used a singularity analysis. Similarly, but numeri-
cally, Nozaki et al. [14] predicted far-field sound propa-
gation by implementing large eddy simulation (LES),
along with solving the Lighthill-Curle equation [15,16].
Van Hirtum et al. [17] studied turbulence using LES and
experiments in the aperture formed by the tooth-shaped
structure. Although these studies have provided valuable
information for understanding the mechanisms underly-
ing the generation and propagation of sibilant /s/, the
obstacle vibration was out of focus in their study. The
possibility exists that the teeth may vibrate and therefore
contribute to the production of sibilant /s/.
When exploring the vibration of an obstacle wall as a
sound source, one must have undisturbed flows. For this
purpose, an optical technique called the Michelson in-
terferometer is suitable. This method has been classically
used in many studies, including optical communications
and astrophysics (e.g., [18]). In addition, as recognized
by pioneering researchers, this measuring system is sen-
sitive to vibrations and the displacement of a target.
Theoretically, a spatial resolution of the Michelson in-
terferometer is smaller than a wavelength of light. In this
sense, the Michelson interferometer is suitable for studying
vibration-induced sounds that necessitate measuring tiny
oscillations of the obstacle wall.
The final goal of our project was to identify the aero-
acoustic sources of sibilant sounds /s/. As stated above,
one must understand the contributions of tooth vibration
to the production of sibilant /s/. As a preliminary study,
here we developed a method of simultaneously measur-
ing aeroacoustic sounds and the vibration of an obstacle.
Using this method, we measured aeroacoustic sounds
and the vibration of an obstacle wall from a flow channel
that morphologically simplified the oral cavity when the
sibilant sound /s/ was produced.
2. METHOD
2.1. Experimental Setup
Figure 1 conceptually illustrates the experimental setup,
which consists of an air compressor (Yaezaki, YC-4RS),
a flow channel with an obstacle wall, a microphone
(Earthworks, 30BX), the Michelson interferometer (Chuo
Precision Industrial Co, Ltd.), and a PC. In this setup,
the air compressor delivers air to the flow channel and
generates the aeroacoustic sound. The aeroacoustic sound
is measured with a microphone. The Michelson inter-
ferometer (elaborated more in later paragraphs) is used
to measure the frequency of the vibration of the obstacle
wall. All of the experimental devices, except the air
compressor and PC, were mounted on a vibration-isolated
table (Chuo Precision Industrial Co, Ltd., ORR-1890)
flow channel
compressor
micro phone
Michelson
interferometer
flow
obstacle wall
vibration
PC (Labvie
w
)
flow channel
compressor
micro phone
Michelson
interferometer
flow
obstacle wall
vibration
PC (Labvie
w
)
Figure 1. Experimental setup for simultaneous measurements
of aeroacoustic sounds and vibration of an obstacle wall.
to minimize the effects of extraneous vibrations on the
system.
2.2. Flow Channel
The flow channel was based on the design of Shadle [11],
who showed that an obstacle in the path of a jet results in
localized sound generation. A cross-sectional image of
the flow channel is presented in Figure 2. The flow
channel mainly consists of a back cavity, a constriction,
a front cavity, and an obstacle wall. Morphologically, the
flow channel simplifies the geometry of the oral cavity
when the sibilant /s/ is produced, including the back cav-
ity, the constriction and the front cavity representing the
pharynges and the oral cavity, the sibilant groove that is
produced by the tip of tongue and the upper maxilla, and
the space among the lips and teeth, respectively [11].
The cross section of the flow channel perpendicular to
the flow direction is circular. The flow channels in the
back cavity, the constriction, and the front cavity are
connected concentrically to the center of the flow chan-
nel. The diameters of the flow channels in the back and
front cavities were both 25.4 mm, and the diameter of
the constriction was 6.4 mm. The thickness of an obsta-
cle wall was 0.5 mm in the standard model, but later
varied to 0.29, 1, 2, and 5 mm to investigate the influ-
ence of thickness. The height of the obstacle wall from
the bottom of the flow channels was 12.7 mm, which
blocks half of the flow channel. In this situation, the
obstacle wall appears to be half-moon-shaped when
viewed from the exit of the flow channel. The flow
channel was made of chalcopyrite and its internal sur-
face was finely polished to minimize the development of
turbulence on the surface such that it satisfies Ra = 1.6,
back cavityfront cavity
obstac le wall
25.4
25.4

6.4
11020 5010
constriction
0.5
flow back cavityfront cavity
obstac le wall
25.4
25.4

6.4
11020 5010
constriction
0.5
flow
Figure 2. Cross-sectional image of a flow channel. The chan-
nel consists of four parts: the back cavity, constriction, obstacle
wall, and front cavity.
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M. Nakamura et al. / J. Biomedical Science and Engineering 4 (2011) 83-89 85
Rz = 6.3, and RJIS = 6.3 of the Japanese Industrial Stan-
dards. Note that this is a scale model, about 3 times as
large as an anatomical scale of the oral cavity [19].
2.3. Wall Vibration Measurem ent
Wall vibration frequency was measured using the
Michelson interferometer and photomultiplier. Figure 3
shows a schematic drawing of the experimental appara-
tus for measuring wall vibrations. The experimental ap-
paratus consisted of a He-Ne laser beam oscillator (LO)
(Chuo Precision Industrial Co, Ltd., GL5230, true
wavelength 632.8 nm, maximum optical output power
20 mW), a beam expander unit (BE) (Chuo Precision
Industrial Co, Ltd., C-80, ×20), a slit unit (SU) (Chuo
Precision Industrial Co, Ltd., C-25,
2-30 mm), a beam
splitter unit (BS) (Chuo Precision Industrial Co, Ltd.,
IU-BS1,
20 mm), a mirror unit (MU) (Chuo Precision
Industrial Co, Ltd., IU-M1,
20 mm), and a photomulti-
plier (PM) (Hamamatsu Photonics, light sensor module
H9656, effective wavelength 632.8 nm). Additionally, a
circular mirror with a diameter of 6.3 mm (Edmund Op-
tics, 4-6
) was glued to the upper edge of the obstacle
wall for reflecting the laser beam. The experimental
parts were precisely positioned as follows: The distance
was 330 mm between the OW and BS, 330 mm between
the MU and BS, 200 mm between the PM and BS, 220
mm between the BS and SU, 60 mm between the SU and
BE, and 10 mm between the SE and LO. The Michelson
interferometer produces interference fringes by recom-
bining two beams of light generated from the same beam
source. In brief, the beam of light was generated by the
He-Ne laser beam oscillator. It was then expanded and
made to be parallel by the downstream beam expander
and split in two by the semitransparent mirror. From this
point, two paths of light went to the detector. One re-
flected off the semitransparent mirror, struck the bottom
mirror, and then bounced back, passing through the
LOBESU
BS
PM
MU
flow channel
OW
flow LOBESU
BS
PM
MU
flow channel
OW
flow
photomultiplierPM
mirror unitMU
slit uni tSU
beam expanderBE
laser beam oscillatorLO
obstacle wallOW
photomultiplierPM
mirror unitMU
slit uni tSU
beam expanderBE
laser beam oscillatorLO
obstacle wallOW
Figure 3. Schematic illustration of the Michelson interferome-
ter used to measure the wall vibration.
semitransparent mirror to the photomultiplier. The other
first passed through the semitransparent mirror to the
mirror on the obstacle wall. The reflected light from the
mirror on the obstacle wall continued to the semitrans-
parent mirror and then reflected back into the photomul-
tiplier. Recombination of the two reflected light beams
that followed different paths produced an interference
fringe. Since spacing of the interference fringes was as-
sociated with a difference in the path lengths of two light
beams, vibrations in obstacle wall gave rise to alternat-
ing patterns of interference fringes. A temporal change in
light intensity of interference fringes was detected and
amplified by the photomultiplier. A frequency analysis of
these data provided the vibration frequency of the obsta-
cle wall.
2.4. Experimental Conditions
During the experiments, air was delivered at a steady
flow rate of 7.5 × 10–4 m
3/s (45 L/min) to the flow
channel by the compressor. The Reynolds number cal-
culated from the constriction diameter and material
properties of the air at 15˚C (density of 1.225 kg/m3 and
viscosity of 1.78 × 10–5 kg/(m·s)) was 10,269. Generated
aeroacoustic sounds were measured by a microphone
that was situated 200 mm distal to the center of the flow
channel. The vibration frequency of the obstacle wall
was measured with the Michelson interferometer as de-
scribed above. Measurements of sound and vibration
were synchronized by Labview ver 8.0 (National In-
struments). The sampling frequency was 40,000 Hz.
Three experiments were performed under the same
flow conditions as described above. In the first experi-
ment, we examined the effects of the presence of an ob-
stacle wall and front cavity on sounds. For this experi-
ment, we prepared three models as summarized in Table
1. In the second experiment, we performed simultaneous
measurements of sound pressure and vibration of the
obstacle wall to identify vibration-induced sound. In the
third experiment, the thickness of the obstacle wall was
varied to be 0.29, 0.5, 1, 2, and 5 mm to examine the
effects of wall rigidity on the sounds.
2.5. Data Analysis
A Fourier transformation with a Hanning window was
used to attain a frequency spectrum. For smoothening,
the amplitude data were averaged in each frequency bin,
Table 1. Models used for the first experiment.
back cavityconstriction front cavity obstacle wall
model 1+ + + +
model 2+ + + -
model 3+ + - -
+: Present, –: Absent.
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86
the center of which was 100 + 200N Hz (N = 0, 1, 2 …);
the bandwidth was 200 Hz. For example, a result at the
frequency of 500 Hz represents an average of the data
from 400 to 600 Hz. All results in this paper are pre-
sented in this fashion.
Measurements were repeated 5 times in each condi-
tion. Although Figure 4 and Figure 5 presented below
represent the data of one measurement (not average),
reproducibility of the measurements was good enough to
give the quantitatively same results.
3. RESULTS
3.1. First Experiment
Aeroacoustic sounds were measured for three models
that differ in assembly as summarized in Table 1. In
brief, model 1 was a completely assembled flow channel
that had all components, namely, the back cavity, the
constriction, the front cavity, and the obstacle wall. The
obstacle wall was not present in model 2, but the rest
was the same as model 1. Model 3 lacked both the front
cavity and the obstacle wall, and therefore consisted of
Frequency [Hz]
Sound Amplitude [V]
model 1
model 2
model 3
020004000 60008000
×10-5
5
10
0
Frequency [Hz]
Sound Amplitude [V]
model 1
model 2
model 3
020004000 60008000
×10-5
5
10
0
Figure 4. Frequency spectrum of the sound amplitude for
model 1 (), model 2 (), and model 3 (). The models differ
in their assembly as summarized in Table 1.
0
5
10
200040006000 80000
0
1
2
3
4
5
×10-3
Vibration Amplitude [V]
×10-5
Frequency [Hz]
Sound Amplitude [V]
sound
vibration
0
5
10
200040006000 80000
0
1
2
3
4
5
×10-3
Vibration Amplitude [V]
×10-5
Frequency [Hz]
Sound Amplitude [V]
sound
vibration
Figure 5. Frequency spectrum of the sound amplitude () and
vibration of the obstacle wall (). The thickness of obstacle
wall was 0.5 mm and the flow rate was 45 L/min.
the back cavity and the constriction. Figure 4 illustrates
the frequency spectrum of the sound pressure amplitudes
obtained from the models. The symbols represent model
1 (), model 2 (), and model 3 (). As shown, model
1, which has all parts including the front cavity, constric-
tion, back cavity, and obstacle wall, exhibited several
peaks over the entire frequency range. In particular,
prominent peaks were found at the frequencies of 1,300
Hz and 3,500 Hz. In contrast, model 2, which does not
have the obstacle wall, showed a peak at the frequency
of 1,300 Hz, but no peak at 3,500 Hz. No peaks were
found at these frequencies in model 3, which only had
the front cavity and constriction. A comparison of these
results clearly indicates that the sound at 1,300 Hz is
associated with the presence of a front cavity while that
at 3,500 Hz is associated with the presence of an obsta-
cle wall.
3.2. Second Experiment
Sound and wall vibrations were simultaneously meas-
ured for the model that had all elements. Here, the
thickness of the obstacle wall was 0.5 mm and the flow
rate at the inlet was 7.5 × 10–4 m
3/s. Figure 5 plots the
frequency spectrum of the sound pressure amplitudes
and vibration of the obstacle wall. In this graph, circles
and triangles represent sound pressure and vibration of
the obstacle wall, respectively. Prominent peaks in sound
are observed at the frequencies of 1,300 Hz and 3,500
Hz, as described in Section 3.1. In contrast, vibration of
the obstacle wall showed a sharp peak at the frequency
of 3,500 Hz. These results confirmed that sounds at the
frequency of 3,500 Hz are generated by the vibration of
the obstacle wall.
3.3. Third Exper iment
The thickness of the obstacle wall was varied at 0.29, 0.5,
1, 2, and 5 mm in the model used in Section 3.2. Figure
6 depicts changes in the sound pressure amplitude at the
frequencies of (a) 1,300 Hz and (b) 3,500 Hz against the
thickness of the obstacle wall. Here, the sound amplitude
was normalized with the mean amplitude obtained at the
thickness of 0.29 mm. The mean and standard deviations
are a result of five repeated experiments. We found that
the sound at 1,300 Hz remained almost the same regard-
less of the thickness of the obstacle wall. Furthermore,
the slope of the linear regression analysis was not statis-
tically different from zero in 1,300 Hz. In contrast, the
sound at 3,500 Hz decreased with an increase in wall
thickness. In this case, the slope obtained using a linear
regression analysis showed a significant difference from
zero (p < 0.05), indicating a significant decrease in the
amplitude with an increase in the wall thickness. Fur-
thermore, a statistical analysis (t-test) demonstrated a
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M. Nakamura et al. / J. Biomedical Science and Engineering 4 (2011) 83-89 87
012
3
4
5
0
0.5
1
1.5
N.S.
Thickness [mm]
Normalized sound amplitude
y
=0.041
x
+0.974
r
2
= 0.134
(a) 1,300 Hz
012345
0
0.5
1
1.5
Thickness [mm]
Normalized sound amplitude
y= -0.107x + 0.899
r2= 0.704
p<0.001
p<0.01
012345
0
0.5
1
1.5
Thickness [mm]
Normalized sound amplitude
y= -0.107x + 0.899
r2= 0.704
p<0.001
p<0.01
(b) 3,500 Hz
Figure 6. Changes in the sound amplitude at the frequency of
(a) 1,300 Hz and (b) 3,500 Hz against the thickness of the ob-
stacle wall. All data are normalized with the mean amplitude at
a thickness of 0.29 mm. The data are presented as the mean ±
SD over five randomly implemented experiments.
significant difference in the amplitude between the two
thicknesses of 0.29 mm and 2 mm (p < 0.01) and be-
tween thicknesses of 0.29 mm and 5 mm (p < 0.001).
Concomitantly, we measured wall vibrations for all
cases. However, wall vibrations were detectable only at
the wall thickness of 0.29 and 0.5 mm, probably because
the oscillation was too tiny for walls thicker than 0.5 mm.
The wall vibration results showed that, for both 0.29 and
0.5 mm, a sharp peak was present at 3,500 Hz and no
other remarkable peaks were observed. The amplitude of
vibration was larger for the 0.29 mm thickness than for
0.5 mm.
4. DISCUSSION
Simultaneous measurements of sound pressure and vi-
bration provide deep insights into the generation mecha-
nisms of vibration-induced sound. Although it is intui-
tively obvious that the frequency of vibration is equal to
that of sound, it is important to confirm that the obstacle
vibrates at the frequency of sound. Here, we used a
Michelson interferometer for measuring the vibration of
an obstacle. One of the most important features of this
method is noninvasiveness to the flow. For measure-
ments, the laser intensity was not too high as to cause
thermo-fluid interactions that may alter flows. Another
advantage was the capability of measuring the vibration
of a target from a distance. Because the laser beam trav-
els in straight lines, it is able to measure the vibration at
a distance as long as we do not interrupt or shade the
laser beam that strikes and reflects off of the obstacle
wall. In addition, theoretically, a spatial resolution of the
Michelson interferometer is much smaller than a wave-
length of light. Therefore, the Michelson interferometer
is suitable for studying vibration-induced sound.
The association between the sound and wall vibration
amplitude peaks suggests that the wall vibration would
induce sound at 3,500 Hz. We speculate that the airflow
injected from the inlet of the flow channel became a jet
when passing through the constriction and impinged on
the upper edge of the obstacle wall, causing the wall to
vibrate and generate the sound. This hypothesis is sup-
ported by experiment 3, which demonstrated that the
sound amplitude at 3,500 Hz decreased with increased
wall thickness, which actually increased the rigidity of
the wall.
To date, it is widely accepted that sibilant sounds /s/
are produced as a result of flow turbulence provoked by
teeth positioned in the path of a jet that develops through
constriction [7,9,20]. In contrast, the tooth vibration has
not attained much attention from scientists as a sound
source of sibilant /s/, probably because it had been ex-
perimentally challenging to assess the wall vibration
without disturbing flows. Here, we suggest that tooth
vibration can also be a sound source of sibilant /s/. In
reality, an anterior tooth is not as thin as the obstacle
wall used in this experiment, which means the teeth are
stiff enough to bare fluid dynamic forces induced by the
impingement of a jet and turbulence. However, it is
speculated that the anterior tooth swings or vibrates in a
labio-lingual direction from its root, which is only sup-
ported by soft periodontal ligaments. At this moment,
beyond demonstrating that sound was generated when
the obstacle was present in the path of a jet, we are not
able to conclude which source is more dominant for the
production of sibilant /s/. Future models of the oral cav-
ity will mimic more realistic oral cavity conditions and
examine the effects of the elasticity of obstacles.
The results show that the sound at 1,300 Hz was not
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M. Nakamura et al. / J. Biomedical Science and Engineering 4 (2011) 83-89
88
the vibration-induced sound. In fact, this sound was gen-
erated when the front cavity was present. Since the front
cavity of the flow channel has a rigid surface at the inlet
(constriction) and is open at the outlet, it can be regarded
as a cylinder with one closed end and one open end. In
such a cylinder, the acoustic resonance frequency f is
calculated by

4
nv
fLl

where n is an odd number, L is the length of the front
cavity, v is the speed of sound, and l is the length of the
end correction. The end correction is practically given by
lr
 
where r is the radius of the flow channel (= 12.7 mm)
and
is a correction coefficient. According to Levine
and Schwinger [21], the correction coefficient
varies
from around 0.15 to 0.6133 depending on the ratio of the
wavelength to the radius of the flow channel. Given the
length of the front cavity (= 60 mm), the length of the
end correction l is 1.905-7.789 mm. Consequently, as-
suming that the speed of sound v is 343 m/s, we predict
that acoustic resonance occurs at the frequency of 1,264-
1,385 Hz or its integral multiple in this flow channel.
This acoustic resonance frequency falls in the frequency
band of 1,200-1,400 Hz represented by its central fre-
quency of 1,300 Hz. Therefore, we speculate that the
sound at 1,300 Hz was a result of the acoustic resonance
in the front cavity.
The present experiment has some limitations. First,
the thickness of the mirror (0.5 mm) attached to the ob-
stacle wall to ensure the reflection of the light beam for
vibration measurements is comparable to that of obstacle
wall. This may have resulted in losing intrinsic nature of
the wall vibration. To overcome this problem, we may
need to use a negligibly thin mirror or polish the surface
of the obstacle wall like a mirror. Second, the present
setup of the Michelson interferometer is not capable of
measuring the displacement amplitude in length metrics
like a “micrometer” and analyzing spatial variations over
the wall vibration. This information would be essential
to gain a deeper insight into the generation mechanisms
of vibration-induced sound. Third, due to this averaging
procedure, we might have lost information regarding the
frequency shift of sound pressure and vibration of the
obstacle wall with changing wall thicknesses. Although
we have examined various bandwidths of the frequency
to the averaged data to determine if such a shift in the
frequency with changes in the wall thickness occurred, it
was not observed. Future studies will employ more so-
phisticated means to minimize measurement errors such
that we can explore the relationship between wall vibra-
tion and sound pressure in detail. Notwithstanding these
limitations, the results in this paper suggest the potential
of the presented methodology to pave the way for de-
tailed analysis of vibration-induced sound.
5. SUMMARY AND CONCLUSION
In the present study, we performed simultaneous meas-
urements of aeroacoustic sounds and obstacle wall vi-
bration utilizing an optical technique called the Michel-
son interferometer. The flow channel, which simplified
the geometry of the mouth cavity, generated aeroacoustic
sounds, given airflows from the inlet. The results dem-
onstrate two prominent peaks in sound at 1,300 and
3,500 Hz, and one peak in the wall vibration at 3,500 Hz.
The association between peak frequencies of sound and
wall vibration suggested that the sound at 3,500 Hz is
induced by wall vibration. In fact, the amplitude of
sound at 3,500 Hz decreased with thickening of the ob-
stacle wall, which also increased its rigidity. The ex-
periments demonstrate that the developed techniques are
capable of measuring aeroacoustic sound and obstacle
wall vibration simultaneously. Future studies will adopt
a more complex geometry of the oral cavity and explore
the effects of obstacle wall rigidity on generated sounds,
while ameliorating fixation of the obstacle to the model
and measurements of wall vibration with the Michelson
interferometer. These results will provide us with addi-
tional insights into the production of vibration-induced
sounds and sibilant sounds /s/.
6. ACKNOWLEDGEMENTS
This work was supported in part by the Global COE Program “In
Silico Medicine-oriented Worldwide Open Platform” at Osaka Univer-
sity.
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