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Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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Study on Airflow and Sound Characteristics of Straight Type
Silencers with Movable Sound Absorbers
Kaori MIYAUCHI* and Morimasa ITAMOTO**
( Received March 3, 2005 )
Abstract
In this report, pressure loss, insertion loss and aerodynamic noise of straight type silencers with movable
sound absorbers were clarified by experiment. A variable air volume damper which was formed from the
sound absorber was installed inside a straight type silencer. Moreover, airflow and sound characteristics of a
single straight type silencer, a double straight type silencer and a triple straight type silencer with movable
sound absorbers were examined and compared.
Keyword: Air duct systems, Straight type silencers with movable sound absorber, Pressure loss,
Insertion loss, Aerodynamic noise
ISSN 0386-1678
Report of the Research Institute of Industrial Technology, Nihon UniversityNumber 80, 2005
1. Introduction
One of the design requirements for an air duct sys-
tem is to ensure that air generated by a fan is delivered
to a room for air conditioning. The volume of air in the
air duct system must be sufficient but not excessive.
Therefore, a damper which controls the volume of air
is installed in the air duct system. However, the damper
might generate aerodynamic noise which is undesirable.
Meanwhile, the sound level of a room for air condi-
tioning is stipulated by “ Indoor Sound Rating Level ”
which is determined according to the use of the room.
The largest source of noise in an air duct system is the
fan; in order to lower the noise which propagates in an
air duct system, a silencer is installed between the fan
and the room-diffuser as sound design for HVAC (Heat-
ing, Ventilating and Air-Conditioning). However, sound
absorbers which were inside silencer generate a great
deal of pressure loss. If the silencer generates a large
pressure loss, the air moved by the fan may fail to be
delivered to the room. Therefore, a balance must be at-
tained between pressure loss and noise reduction.
In light of these circumstances, a variable air vol-
ume damper made from a sound absorber is installed
in a straight type silencer. For a given blade angle of a
damper, an opposed blade damper provides better air
* Research Assistant, Department of Architecture and Architectural Engineering, College of Industrial Technol-
ogy, Nihon University
** Professor, Department of Architecture and Architectural Engineering, College of Industrial Technology, Nihon
University
Kaori MIYAUCHI and Morimasa ITAMOTO
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volume control than a parallel blade damper. Therefore,
in this paper the damper of test unit is the opposed blade
type.
If a single silencer fails to provide sufficient noise
reduction in an air duct system, two or more silencers
are installed in series. In recently studies1),2),3) have
clarified that pressure loss, insertion loss and aerody-
namic noise generated by multiple silencers are not in-
tegral multiples of those generated by a single silencer,
a single silencer and multiple silencers must be com-
pared in terms of performance.
In this report, the results of two lines of inquiry are
described.
(1) Experiments were conducted to clarify pressure loss,
insertion loss and aerodynamic noise for straight type
silencers which had two kinds of cross sectional area
and were equipped with movable sound absorbers.
(2) A single straight type silencer, a double straight type
silencer and a triple straight type silencer, all equipped
with a movable sound absorber were compared in terms
of pressure loss, insertion loss and aerodynamic noise.
A portion of this research has been reported previously 4), 5).
2. Test unit
2.1 Name of test unit
The name of a test unit consists of “inlet duct size”-
“number of test units connected”-“angle of movable
sound absorbers.” Table 1 shows specifications of test
units.
2.2 Use of single silencer
The test units are shown in Fig. 1 and Fig. 2.
The silencers have two kinds of cross sectional area;
400×400 mm and 1000×600 mm. Square section size
is standard for duct design. Rectangular sectional area
1000×600 mm is almost four times of square sectional
area 400×400 mm. All silencers have cabinets made of
Fig. 1 Test Unit (400×××××400-I, 400×××××400-I’ and 400×××××400-i ) Fig. 2 Test Unit (1000×××××600-I)
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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Table 1 Outline of Test Units
Kaori MIYAUCHI and Morimasa ITAMOTO
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1.2 mm zinc plate and a length of 500 mm.
Silencers have sound absorbers of two types which
differ primarily in density. The first type of sound ab-
sorber called 400×400-I and 1000×600-I have glass fi-
ber of a thickness of 50 mm and a density of 40 kg/m3
and is covered with zinc plate having an open area ra-
tio of 35.4% (ø5-8). The other type of sound absorber
called 400×400-i has glass fiber of a thickness of 50
mm and a density of 24 kg/m3 and is covered with zinc
plate having an open area ratio of 32.6% (ø3-5).
Moreover, 400×400-I was inverted; i.e., the inlet and
outlet were switched and its performance was evalu-
ated. In this case, 400×400-I is renamed 400×400-I’.
As shown in Fig. 3 and Fig. 4, in the silencers hav-
ing 400×400 mm cross sections, the angle of the mov-
able sound absorber was varied in increments of 5 de-
grees between 0 and 45 degrees. In the silencers hav-
ing 1000×600 mm cross sections, the angle of the mov-
able sound absorber was varied in increments of 10
degrees between 0 and 30 degrees.
2.3 Use of double silencers and triple silencers
As shown in Fig. 5, a double straight type silencer
called 400×400-II consists of 400×400-I and 400×400-
I’ connected in series. As shown in Fig. 7, a double
straight type silencer called 1000×600-II consists of
1000×600-I and 1000×600-I’ connected in series. Each
flat plane of fixed sound absorber were connected, thus,
one unit of sound absorber was formed.
As shown in Fig. 6, a triple straight type silencer
called 400×400-III consists of 400×400-I, 400×400-I’
and 400×400-I’ connected in series. As shown in Fig.
8, a triple straight type silencer called 1000×600-III
consists of 1000×600-I, 1000×600-I’ and 1000×600-I’
connected in series. The terminal of test units were
Fig. 4 Angle of Movable Sound Absorber for1000×××××600-I
Fig. 3 Angle of Movable Sound Absorber for 400×××××400-I,400×××××400-I’ and 400×××××400-i
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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Fig. 5 Angle of Movable Sound Absorber for400×××××400-II, e.g. 0, 15 and 30 degrees
Fig. 6 Angle of Movable Sound Absorber for 400×××××400-III,e.g. 0, 30 and 45 degrees
Fig. 7 Angle of Movable Sound Absorber for1000×××××600-II, e.g. 0 and 20 degrees
Fig. 8 Angle of Movable Sound Absorber for 1000×××××600-III,e.g. 10 and 30 degrees
Kaori MIYAUCHI and Morimasa ITAMOTO
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smooth plane of fixed sound absorber.
The double straight type silencer has a length of 1000
mm and the triple straight type silencer has a length of
1500 mm.
In the multiple silencers installed in air duct systems,
all the movable sound absorbers have the same angles.
3. Experimental installation and methods
3.1 Pressure loss and loss coefficient
For the silencers having 400×400 mm and 1000×600
mm cross sections, the pressure loss experiment instal-
lations were shown in Fig. 9 and Fig. 10.
In each case, a quiet source of air operated and quiet
air flowed through the duct. Four static pressure mea-
surement tubes were installed in the duct cross section
arranged in orthogonal directions. Static pressure in the
axial direction of the duct was measured by static tubes
installed in the upstream and downstream steel ducts
of the test unit. Sets of 12 static pressure measurement
tubes were installed at 300 mm intervals along the duct
axis starting at a point 150 mm from the inlet of the test
unit. Moreover, sets of 12 static pressure measurement
tubes were installed at 300 mm intervals along the duct
axis starting at a point 1950 mm from the outlet of the
test unit. Therefore, 96 static tubes were used to obtain
measurements in each test unit.
The static tubes measured static pressure at each in-
let and outlet duct to obtain four or more inlet duct mean
velocities. From static pressure, the inlet and outlet static
pressure differential of the test unit was calculated by a
least square method. The inlet and outlet cross sectional
areas of the test units were equal; therefore, total pres-
sure loss could be calculated from the static pressure
difference.
For each value of mean velocity in the inlet duct,
the total pressure loss coefficient was calculated as the
Fig. 9 Pressure Loss Measurement for 400×××××400-I,400×××××400-I’, 400×××××400-i, 400×××××400-II and 400×××××400-III
Fig. 10 Pressure Loss Measurement for 1000×××××600-I,1000×××××600-II and 1000×××××600-III
Fig. 11 (b) Insertion Loss Measurement for 400×××××400-I,400×××××400-I’, 400×××××400-i, 400×××××400-II and400×××××400-III
Fig. 12 (a) Insertion Loss Measurement for 1000×××××600-I,1000×××××600-II and 1000×××××600-III
Fig. 12 (b) Insertion Loss Measurement for 1000×××××600-I,1000×××××600-II and 1000×××××600-III
Fig. 11 (a) Insertion Loss Measurement for 400×××××400-I,400×××××400-I’, 400×××××400-i, 400×××××400-II and400×××××400-III
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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total pressure loss divided by the velocity pressure cor-
responding to the inlet duct mean velocity. The total
pressure loss coefficient was calculated as an average
for four or more inlet duct mean velocities.
The inlet duct mean velocity was measured by a pitot
tube installed at the duct for measurement of volume
flow rate.
3.2 Insertion loss
For the silencers having 400×400 mm and 1000×600
mm cross sections, the insertion loss experiment instal-
lations were shown in Fig. 11 and Fig. 12.
For each experimental installation, 1/1 octave band
pink noise was generated by a speaker and average
sound pressure level in a reverberation room was mea-
sured. The average sound pressure level in the rever-
beration room Lp (b) was measured by air duct systems
with the test unit. The average sound pressure level in
the reverberation room Lp (a) was measured by air duct
systems without the substitution duct. The insertion loss
was calculated by subtracting Lp (b) from Lp (a).
The input voltage for the speaker was held constant
regardless of the experimental installation.
3.3 Aerodynamic noise
For the silencers having 400×400 mm and 1000×600
mm cross sections, the aerodynamic noise experiment
installations were shown in Fig. 13 and Fig. 14.
A quiet source of air operated and quiet air flowed
through the duct into the reverberation room via the air
supply and exhaust openings. The sound power level
of aerodynamic noise radiated from the duct end open-
ing was calculated from the average sound pressure lev-
els measured in the reverberation room.
The sound power level of each test unit was calcu-
lated from the sound power level at the duct open end
and the sound attenuation of the downstream steel duct
of the test unit. The sound power levels of the upstream
and downstream steel ducts of the test unit were lower
than the sound power level of the test unit. Therefore,
the results confirm that the sound power levels of steel
duct exert no influence on the sound power level of the
test unit.
The inlet duct mean velocity was measured by a pitot
tube installed at the duct for measurement of volume
flow rate.
Fig. 13 (a) Aerodynamic Noise Measurement for400×××××400-I, 400×××××400-I’, 400×××××400-i, 400×××××400-IIand 400×××××400-III
Fig. 13 (b) Aerodynamic Noise Measurement for400×××××400-I, 400×××××400-I’, 400×××××400-i, 400×××××400-IIand 400×××××400-III
Fig. 14 (a) Aerodynamic Noise Measurement for1000×××××600-I, 1000×××××600-II and 1000×××××600-III
Fig. 14 (b) Aerodynamic Noise Measurement for1000×××××600-I, 1000×××××600-II and 1000×××××600-III
Kaori MIYAUCHI and Morimasa ITAMOTO
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4. Experimental results and considerations
4.1 Pressure loss and loss coefficient
Fig. 15 shows pressure loss for the single straight
type silencer 400×400-I. Fig. 16 shows loss coefficient
for the single straight type silencer 400×400-I. Loss
coefficient is related with Reynolds number which is
calculated from the inlet duct mean velocity and the
equivalent diameter. Fig. 17 shows pressure loss for the
single straight type silencer 1000×600-I. Fig. 18 shows
loss coefficient for the single straight type silencer
1000×600-I.
Regardless of angle of the movable sound absorber,
cross sectional area, airflow direction, the density of
sound absorber and the number of test units connected,
pressure loss are proportional to the second power of
the inlet duct mean velocity. Within the given range of
Reynolds number (2.5×105-5.5×105 or 2.5×105-
5.0×105); loss coefficients for all test units are almost
constant.
Table 2 shows loss coefficients for the single straight
type silencers 400×400-I, 400×400-I’ and 400×400-i.
Table 3 shows loss coefficients for the double straight
type silencer 400×400-II and the triple straight type si-
lencer 400×400-III. Table 4 shows loss coefficients for
the single straight type silencer 1000×600-I, the double
straight type silencer 1000×600-II and the triple straight
Fig. 15 Pressure Loss for 400×××××400-I Fig. 16 Loss Coefficient for 400×××××400-I
Fig. 17 Pressure Loss for 1000×××××600-I Fig. 18 Loss Coefficient for 1000×××××600-I
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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type silencer 1000×600-III.
Loss coefficients for all test units increase with in-
creasing angle of the movable sound absorber.
In order to examine the influence of airflow direc-
tion with angle of the movable sound absorber held
constant, loss coefficient for 400×400-I and 400×400-
I’ were compared. The loss coefficient of 400×400-I’-
0~15 are slightly higher than 400×400-I-0~15. The loss
coefficient of 400×400-I’-20~30 are slightly lower than
400×400-I-20~30.
In order to examine the influence of the density of
sound absorber with angle of the movable sound ab-
Table 2 Loss Coefficients for 400×××××400-I , 400×××××400-I’and 400×××××400-i
Table 3 Loss Coefficients for 400×××××400-II and 400×××××400-III
Table 4 Loss Coefficients for 1000×××××600-I , 1000×××××600-II’and 1000×××××600-III
*1: added value for 400×××××400-I and 400×××××400-I*2: added value for 400×××××400-I, 400×××××400-I’ and 400×××××400-I’*3: added value for 400×××××400-II and 400×××××400-I’
Kaori MIYAUCHI and Morimasa ITAMOTO
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sorber held constant, loss coefficient for 400×400-I and
400×400-i were compared. The loss coefficient of
400×400-i-0~5 are slightly lower than 400×400-I-0~5.
The loss coefficient of 400×400-i-10~30 are slightly
higher than 400×400-I-10~30.
Fig. 19 shows the ratio between the measured loss
coefficient for 400×400-II and the calculated loss coef-
ficient for 400×400-I+I’ for the same angle of movable
sound absorber. Fig. 20 shows the ratio between mea-
sured loss coefficient for 400×400-III and calculated
loss coefficient for 400×400-I+I’+I’ or 400×400-II+I’
for the same angle of movable sound absorber.
The ratio is greater than 1.0. The fixed sound ab-
sorber of double straight type silencer integrated, and
terminal of triple straight type silencer were formed by
smooth plane. As a result, the air flowed in test unit
smoothly, and the measured values were lower than the
calculated values.
In the double straight type silencers, the calculated
values were added from each values for two single
straight type silencers 400×400-I and 400×400-I’. In
the triple straight type silencers, the calculated values
were added from each values for three single straight
type silencers 400×400-I, 400×400-I’ and 400×400-I’.
Both the ratio of loss coefficients are approximately 1.2.
Meanwhile, in the triple straight type silencers, the cal-
Fig. 19 Ratio of Loss Coefficient between 400×××××400-II(measured) and 400×××××400-I+I’(calculated)
Fig. 20 Ratio of Loss Coefficient between 400×××××400-III(measured) and 400×××××400-I+I’+I’(calculated) or400×××××400-II+I’(calculated)
Fig. 21 Loss Coefficient for 400×××××400-I, 400×××××400-II,400×××××400-III, 400×××××400-I’ and 400×××××400-i
Fig. 22 Loss Coefficient for 1000×××××600-I, 1000×××××600-IIand 1000×××××600-III
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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culated values were added from each values for
400×400-II and 400×400-I’ and the ratio of loss coeffi-
cient is approximately 1.1.
Fig. 21 and Fig. 22 show loss coefficient which were
related with angle of the movable sound absorber for
the straight type silencer 400×400 and 1000×600.
In the case of the sound absorber has a density of 40
kg/m3, loss coefficient show almost the same tendency
regardless of cross sectional area, airflow direction and
the number of test units connected. Loss coefficient can
be predicted over two ranges of angle of the movable
sound absorber. The first range is 0 to 10 degrees and
the second range is 10 to 30 degrees. Meanwhile, in
the case of the sound absorber having a density of 24
kg/m3, the loss coefficient can be predicted over three
ranges of angle of the movable sound absorber. The first
range is 0 to 5 degrees, the second range is 5 to 10 de-
grees and the last range is 10 to 30 degrees.
4.2 Insertion loss
Fig. 23, Fig. 24 and Fig. 25 show insertion loss for
the single straight type silencer 400×400-I, 400×400-I’
and 400×400-i. Fig. 26 and Fig. 27 show insertion loss
for the double straight type silencer 400×400-II and the
triple straight type silencer 400×400-III.
For all angles of the movable sound absorber, inser-
tion loss for 400×400 are highest in 1kHz band center
frequency.
Fig. 28, Fig. 29 and Fig. 30 show insertion loss for
the single straight type silencer 1000×600-I, the double
straight type silencer 1000×600-II and the triple straight
type silencer 1000×600-III.
Insertion loss for 1000×600-0~35 are highest in 2kHz
band center frequency. Insertion loss for 1000×600-40
is higher in 1kHz to 2kHz band center frequencies than
in other band center frequencies. Insertion loss for
1000×600-45 is highest in 1kHz band center frequency.
Insertion loss for all test units increase with increas-
ing angle of the movable sound absorber. The sound
absorption coefficient of sound absorber, which were
made by glass fiber, were highest in 1kHz to 2kHz band
center frequencies. As a result, insertion loss are higher
in 1kHz and 2kHz band center frequencies than in other
band center frequencies.
Fig. 23 Insertion Loss for 400×××××400-I
Fig. 24 Insertion Loss for 400×××××400-I’
Fig. 25 Insertion Loss for 400×××××400-i
Kaori MIYAUCHI and Morimasa ITAMOTO
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In order to examine the influence of airflow direc-
tion with angle of the movable sound absorber held
constant, insertion loss for 400×400-I and 400×400-I’
were compared. The insertion loss for 400×400-I’ in
63Hz band center frequency are slightly higher than
400×400-I. The insertion loss for 400×400-I’ in 125Hz
and 2kHz band center frequency are slightly lower than
400×400-I.
Fig. 27 Insertion Loss for 400×××××400-III
Fig. 26 Insertion Loss for 400×××××400-II
Fig. 28 Insertion Loss for 1000×××××600-I
Fig. 29 Insertion Loss for 1000×××××600-II
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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In order to examine the influence of the density of
sound absorber with angle of the movable sound ab-
sorber held constant, insertion loss for 400×400-I and
400×400-i were compared. The insertion loss for
400×400-i in 63Hz, 2kHz, 4kHz and 8kHz band center
frequency are slightly higher than 400×400-I.
Fig. 31 shows the difference between measured and
calculated insertion loss for the double straight type si-
lencer 400×400-II. The calculated data were added from
measured values for 400×400-I+I’. The measured val-
ues in 63Hz to 250Hz band center frequencies are lower
than the calculated values. The measured values in
500Hz band center frequency are almost equal to the
calculated values. The measured values in 1kHz to 8kHz
band center frequencies are higher than the calculated
values.
Fig. 32 and Fig. 33 show the difference between
measured and calculated insertion loss for the triple
straight type silencer 400×400-III. The calculated data
were added from measured values for 400×400-I+I’+I’
or 400×400-II+I’. The measured values in 63Hz to 1kHz
band center frequencies and for 400×400-III-0~10 in
Fig. 30 Insertion Loss for 1000×××××600-III
Fig. 31 Difference in Insertion Loss for 400×××××400-II (measured) and 400×××××400-I+I’(calculated)
Fig. 32 Difference in Insertion Loss for 400×××××400-III (measured) and 400×××××400-I+I’+I’(calculated)
Fig. 33 Difference in Insertion Loss for 400×××××400-III (measured) and 400×××××400-II+I’(calculated)
Kaori MIYAUCHI and Morimasa ITAMOTO
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2kHz band center frequency are lower than the calcu-
lated values.The measured values in 4kHz to 8kHz band
center frequencies and for 400×400-III-15~45 in 2kHz
band center frequency are higher than the calculated
values.
According to plane wave theory, the band center fre-
quencies of 63Hz to 1kHz are including in twice the
limit frequency. Thus, 63Hz to 1kHz and 2kHz to 8kHz
differ in characteristics between measured value and
calculated value of insertion loss.
The added value of insertion loss for the straight type
silencer cannot be used as design data for air duct sys-
tems in 63Hz to 500Hz band center frequencies but can
be used in 1kHz to 8kHz band center frequencies.
4.3 Aerodynamic noise
Fig. 34, Fig. 35 and Fig. 36 show over all power
level for the single straight type silencer 400×400-I,
400×400-I’ and 400×400-i. Fig. 37 and Fig. 38 show
over all power level for the double straight type silencer
400×400-II and the triple straight type silencer
400×400-III. In the figure, aerodynamic noise is shown
in the form of a relation between the inlet duct mean
velocity and over all power level.
Over all power level for 400×400-III-0 is propor-
tional to the fourth power of the inlet duct mean veloc-
ity. Regardless of angle of the movable sound absorber,
airflow direction, the density of sound absorber and the
number of test units connected, over all power level
for 400×400 are proportional to the fifth power of the
inlet duct mean velocity.
Fig. 39, Fig. 40 and Fig. 41 show over all power
level for the single straight type silencer 1000×600-I,
the double straight type silencer 1000×600-II and the
triple straight type silencer 1000×600-III.
Over all power level for 1000×600-III-0 is propor-
tional to the fifth power of the inlet duct mean velocity.
Regardless of angle of the movable sound absorber and
the number of test units connected, over all power level
for 1000×600 are proportional to the sixth power of the
inlet duct mean velocity.
Within the measuring range of velocity, over all
power level for all test units increase with increasing
angle of the movable sound absorber.
Fig. 34 Aerodynamic Noise for 400×××××400-I
Fig. 35 Aerodynamic Noise for 400×××××400-I’
Fig. 36 Aerodynamic Noise for 400×××××400- i
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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Fig. 37 Aerodynamic Noise for 400×××××400-II
Fig. 38 Aerodynamic Noise for 400×××××400-III
In order to examine the influence of airflow direc-
tion with angle of the movable sound absorber held con-
stant, overall power level for 400×400-I and 400×400-
I’ were compared. The over all power level for 400×400-
I’-0~15 are higher than 400×400-I-0~15. Meanwhile,
the over all power level for 400×400-I’-20~30 are
slightly lower than 400×400-I-20~30.
In order to examine the influence of the density of
sound absorber with angle of the movable sound ab-
sorber held constant, over all power level for 400×400-
I and 400×400-i were compared. The over all power
level for 400×400-i-0 is slightly lower than 400×400-
I-0. Moreover, the over all power level for 400×400-i-
5~30 are higher than 400×400-I-5~30.
Fig. 42 shows measured and calculated over all
power level for the double straight type silencer
400×400-II. The measured data are plotted as points
and the calculated data which were synthesized from
the measured values for 400×400-I+I’ are plotted as
straight lines.
Fig. 43 and Fig. 44 show measured and calculated
over all power level for the triple straight type silencer
400×400-III. The measured data are plotted as points
and the calculated data which were synthesized from
the measured values for 400×400-I+I’+I’ and 400×400-
II+I’ are plotted as straight lines.
The calculated over all power level were synthesized
by following formula (1 and 2).
Lwj (Cal.) = 10 log 10 (1)
Lw(Cal.) = 10 log 10 (2)
Where,
Lw(Cal.) : The calculated power level for the mul-
tiple straight type silencer [dB]
Lw(n-1) : The measured or calculated power level
for the (n-1)th single straight type silencer*1
[dB]
Lw(n) : The measured power level for the (n)th
single straight type silencer [dB]
IL(n) : The measured insertion loss for the (n)th
single straight type silencer [dB]
Suffix,
n : Number of test units
j : The ( j )th 1/1 octave band center frequency
range of 63Hz to 8kHz
*1 For calculating of over all power level for triple
straight type silencer, value for Lw(n-1) were the mea-
sured value of Lw(II) or the calculated value of Lw(I+I’).
Kaori MIYAUCHI and Morimasa ITAMOTO
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Fig. 42 Aerodynamic Noise for 400×××××400-II (measured)and 400×××××400-I+I’(calculated)
Fig. 43 Aerodynamic Noise for 400×××××400-III (measured)and 400×××××400-I+I’+I’(calculated)
Fig. 44 Aerodynamic Noise for 400×××××400-III (measured)and 400×××××400-II+I’(calculated)
Fig. 39 Aerodynamic Noise for 1000×××××600-I
Fig. 40 Aerodynamic Noise for 1000×××××600-II
Fig. 41 Aerodynamic Noise for 1000×××××600-III
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
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For all angles of the movable sound absorber within
the measuring range of velocity of 20 m/s, the calcu-
lated values are higher than the measured values. There-
fore, the synthesized value of over all power level for
straight type silencer can be used as design data for air
duct systems within the range of velocity of 20m/s.
Fig. 45 and Fig. 46 show relative band power level
for the single straight type silencer 400×400-I-0 and
400×400-I-30. Figure 47 shows relative band power level
for 400×400-I for angles of sound absorber of 0 to 30
degrees, for the inlet duct mean velocity of 18.0 m/s.
Fig. 48, Fig. 49 and Fig. 50 show relative band power
level for 400×400-I, 400×400-I’ and 400×400-i for
angles of sound absorber of 0, 15 and 30 degrees, for
the inlet duct mean velocity of 18.0 m/s. Fig. 51, Fig.
52 and Fig. 53 show relative band power level for
400×400-I, 400×400-II and 400×400-III with angles of
movable sound absorber of 0, 15 and 30 degrees, for
the inlet duct mean velocity of 22.0 m/s.
For all angles of the movable sound absorber, the
main component of aerodynamic noise for 400×400 fall
within the range of 63Hz to 250Hz band center frequen-
cies. When angle of the movable sound absorber is
small, relative band power level are slightly high in
1kHz to 8kHz band center frequencies. However, rela-
tive band power level in 1kHz to 8kHz band center fre-
quencies decrease with increasing angle of the mov-
able sound absorber.
In order to examine the influence of airflow direc-
tion with angle of the movable sound absorber held con-
stant, relative band power level for 400×400-I and
400×400-I’ were compared. The difference in relative
band power level between 400×400-I and 400×400-I’
decrease with increasing angle of the movable sound
absorber in 500Hz to 8kHz band center frequencies.
In order to examine the influence of the density of
sound absorber with angle of the movable sound ab-
sorber held constant, relative band power level for
400×400-I and 400×400-i were compared. The relative
band power level for 400×400-I and 400×400-i show
dissimilar tendencies in 1kHz to 8kHz band center fre-
quencies.
Fig. 47 Frequency Characteristics of AerodynamicNoise for 400×××××400-I
Fig. 46 Frequency Characteristics of AerodynamicNoise for 400×××××400-I-30
Fig. 45 Frequency Characteristics of AerodynamicNoise for 400×××××400-I-0
Kaori MIYAUCHI and Morimasa ITAMOTO
— 18 —
Fig. 54 and Fig. 55 show relative band power level
for the single straight type silencer 1000×600-I-0 and
1000×600-I-30. Fig. 56 shows relative band power level
for 1000×600-I for angles of sound absorber of 0 to 30
degrees, for the inlet duct mean velocity of 9.0 m/s.
Fig. 57, Fig. 58 and Fig. 59 show relative band power
level for 1000×600-I, 1000×600-II and 1000×600-III
with angles of movable sound absorber of 0, 10 and 30
degrees, for the inlet duct mean velocity of 9.0 m/s.
For all angles of the movable sound absorber, the
main component of the aerodynamic noise for
1000×600 fall within the range of 63Hz to 125Hz band
center frequencies. When angle of the movable sound
absorber is small, relative band power level are highest
in 63Hz band center frequency and relative band power
level are slightly high in 1kHz to 4kHz band center fre-
quencies. However, relative band power level in 125Hz
band center frequency increase and relative band power
level in 1kHz to 4kHz band center frequencies decrease
with increasing angle of the movable sound absorber.
Fig. 48 Frequency Charactaristics of AerodynamicNoise for 400×××××400-I-0, 400×××××400-I’-0 and400×××××400-i-0
Fig. 49 Frequency Charactaristics of AerodynamicNoise for 400×××××400-I-15, 400×××××400-I’-15 and400×××××400-i-15
Fig. 50 Frequency Charactaristics of AerodynamicNoise for 400×××××400-I-30, 400×××××400-I’-30 and400×××××400-i-30
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
— 19 —
Fig. 51 Frequency Characteristics of AerodynamicNoise for 400×××××400-I-0, 400×××××400-II-0 and400×××××400-III-0
Fig. 54 Frequency Characteristics of AerodynamicNoise for 1000×××××600-I-0
Fig. 52 Frequency Characteristics of AerodynamicNoise for 400×××××400-I-15, 400×××××400-II-15 and400×××××400-III-15
Fig. 55 Frequency Characteristics of AerodynamicNoise for 1000×××××600-I-30
Fig. 53 Frequency Characteristics of AerodynamicNoise for 400×××××400-I-30, 400×××××400-II-30 and400×××××400-III-30
Fig. 56 Frequency Characteristics of AerodynamicNoise for 1000×××××600-I
Kaori MIYAUCHI and Morimasa ITAMOTO
— 20 —
5. Conclusions
The measured airflow and sound characteristics for
straight type silencer show the following:
[ 1 ] Regardless of angle of the movable sound ab-
sorber, cross sectional area, airflow direction, the
density of sound absorber and the number of test
units connected, pressure loss for a straight type
silencer are proportional to the second power of
the inlet duct mean velocity. Within the given range
of Reynolds number, loss coefficients are almost
constant.
[ 2 ] Regardless of angle of the movable sound ab-
sorber, cross sectional area, airflow direction, the
density of sound absorber and the number of test
units connected, pressure loss and loss coefficient
increase with increasing angle of the movable
sound absorber.
[ 3 ] The calculated pressure loss and loss coefficient
for the double straight type silencer and the triple
straight type silencer were compared with the mea-
sured pressure loss and loss coefficient. For all
angles of the movable sound absorber, the calcu-
lated pressure loss and loss coefficient are slightly
higher than the measured values.
[ 4 ] Loss coefficient can be predicted from angle of
the movable sound absorber.
[ 5 ] Regardless of angle of the movable sound ab-
sorber, cross sectional area, airflow direction, den-
sity of the sound absorber and the number of test
units connected, insertion loss are highest in 1kHz
to 2kHz band center frequencies.
[ 6 ] Regardless of angle of the movable sound ab-
sorber, cross sectional area, airflow direction, the
density of sound absorber and the number of test
units connected, insertion loss increase with in-
creasing angle of the movable sound absorber.
[ 7 ] The calculated insertion loss for the double straight
type silencer and the triple straight type silencer
were compared with the measured insertion loss.
In the 63Hz to 500Hz band center frequencies, the
measured insertion loss are lower than the calcu-
lated values. In the 4kHz to 8kHz band center fre-
quencies, the measured insertion loss are higher
than the calculated values. The calculated value
of insertion loss for straight type silencers cannot
Fig. 57 Frequency Characteristics of AerodynamicNoise for 1000×××××600-I-0, 1000×××××600-II-0 and1000×××××600-III-0
Fig. 58 Frequency Characteristics of AerodynamicNoise for 1000×××××600-I-10, 1000×××××600-II-10 and1000×××××600-III-10
Fig. 59 Frequency Characteristics of AerodynamicNoise for 1000×××××600-I-30, 1000×××××600-II-30 and1000×××××600-III-30
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
— 21 —
be used as design data for air duct systems in 63Hz
to 500Hz band center frequencies, but can be used
in 1kHz to 8kHz band center frequencies.
[ 8 ] Over all power level for 400×400-III-0 is propor-
tional to the fourth power of the inlet duct mean
velocity. Regardless of angle of the movable sound
absorber, airflow direction, the density of sound
absorber and the number of test units connected,
over all power level for 400×400 are proportional
to the fifth power of the inlet duct mean velocity.
Over all power level for 1000×600-III-0 is pro-
portional to the sixth power of the inlet duct mean
velocity. Regardless of angle of the movable sound
absorber and the number of test units connected,
over all power level for 1000×600 are proportional
to the fifth power of the inlet duct mean velocity.
[ 9 ] Regardless of angle of the movable sound ab-
sorber, cross sectional area, airflow direction, the
density of sound absorber and the number of test
units connected, over all power level increase with
increasing angle of the movable sound absorber.
[10] The main component of aerodynamic noise for
400×400 fall within the range of 63Hz to 250Hz
band center frequencies. The main component of
the aerodynamic noise for 1000×600 fall within
the range of 63Hz to 125Hz band center frequen-
cies.
[11] The calculated over all power level for the double
straight type silencer and triple straight type si-
lencer were compared with measured over all
power level. The calculated over all power level
are higher than the measured value. The calculated
values of over all power level for the straight type
silencer can be used as design data for air duct
systems.
Acknowledgments
Test units were proffered from Mr. Konno of Tohoku-
Kogyo. Graduate students and senior students in Nihon
University assisted us with this experiment. The authors
would like to express their sincere gratitude to all of
these contributors.
References
1) Morimasa ITAMOTO, Hiroyoshi SHIOKAWA and
Kaori MIYAUCHI: Study on Airflow and Sound
Characteristics of Double Lined Elbows, Report of
the Institute of Industrial Technology Nihon Univer-
sity, No. 64, 2002
2) Morimasa ITAMOTO, Hiroyoshi SHIOKAWA and
Kaori MIYAUCHI: On Aerodynamic Noise for
Double Lined Elbows, Journal of Architecture, Plan-
ning and Environmental Engineering (Transactions
of AIJ), No. 556, pp.1-8, 2002 [in Japanese]
3) Morimasa ITAMOTO, Hiroyoshi SHIOKAWA and
Kaori “URATA” MIYAUCHI: On Insertion Loss of
Double Lined Elbows, Journal of Architecture, Plan-
ning and Environmental Engineering (Transactions
of AIJ), No. 536, pp.7-12, 2000 [in Japanese]
4) Morimasa ITAMOTO, Kaori MIYAUCHI et al.: On
Airflow and Sound Characteristics of Silencers with
Movable Sound Absorbers –Part 1- Pressure Loss
and Insertion Loss, Summaries of Technical Papers
of Annual Meeting Architectural Institute of Japan,
D-1, pp.87-88, 2002 [in Japanese]
5) Kaori MIYAUCHI, Morimasa ITAMOTO et al.: On
Airflow and Sound Characteristics of Silencers with
Movable Sound Absorbers –Part 2- Aerodynamic
Noise, Summaries of Technical Papers of Annual
Meeting Architectural Institute of Japan, D-1, pp.89-
90, 2002 [in Japanese]
Kaori MIYAUCHI and Morimasa ITAMOTO
— 22 —
可動羽根吸音体を用いた直管型消音器の気流および音響特性に関する研究
宮内 香織 , 板本 守正
概 要
本論文は,可動羽根吸音体を用いた直管型消音器の損失圧力,挿入損失および気流による発生騒音について実験的に明らかにしたものである。研究対象となる消音器は,直管型消音器の内部に,角度を変化させることが可能な羽根型吸音体を挿入したものである。また,可動羽根吸音体を用いた直管型消音器をダクト系に単体,2連結および3連結に設置した場合の気流および音響特性についても,あわせて言及している。
Study on Airflow and Sound Characteristics of Straight Type Silencers with Movable Sound Absorbers
— 23 —
Biographical Sketches of the Authors
Miyauchi Kaori was born in November 30, 1975 in Saitama Prefecture, Japan.
She received her Bachelor of Engineering Degree from Nihon University in 1998.
She is a research assistant of the department of architecture and architectural engineering,
college of industrial technology, Nihon University.
She is member of the Architectural Institute of Japan (AIJ), the Society of Heating, Air-
Conditioning and Sanitary Engineers of Japan (SHASE) and the Acoustical Society of Japan
(ASJ).
Itamoto Morimasa was born in March 13, 1937 in Okayama Prefecture, Japan.
He received his Bachelor of Engineering Degree from Nihon University in 1961, his
Masters of Engineering Degree from University of Tokyo in 1963 and Doctorate of En-
gineering Degree from University of Tokyo in 1966.
Dr. Itamoto is a professor of the department of architecture and architectural engi-
neering, college of industrial technology, Nihon University.
He is a member of the Architectural Institute of Japan (AIJ), the Society of Heating,
Air-Conditioning and Sanitary Engineers of Japan (SHASE), the Acoustical Society of
Japan (ASJ), the Institute of Noise Control Engineering/Japan (INCE/J), the Japan Soci-
ety of Mechanical Engineers (JSME) and Center for Environmental Information Sci-
ence (CEIS).