Study on Airﬂow and Sound Characteristics of Straight Type ......Study on Airﬂow and Sound...

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)


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Table 1 Outline of Test Units


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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


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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


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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


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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


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


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


— 8 —

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


— 9 —

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’


— 10 —

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


— 11 —

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


— 12 —


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


— 13 —



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)


— 14 —

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


— 15 —

Fig. 37 Aerodynamic Noise for 400×××××400-II

Fig. 38 Aerodynamic Noise for 400×××××400-III


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.



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’).


— 16 —

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


— 17 —

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.


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.



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



— 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


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


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




— 19 —

Fig. 51 Frequency Characteristics of AerodynamicNoise for 400×××××400-I-0, 400×××××400-II-0 and400×××××400-III-0





Fig. 56 Frequency Characteristics of AerodynamicNoise for 1000×××××600-I


— 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.




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.


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.




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





— 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.




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


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]


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]


— 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。


— 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).

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