JP2012246808A - Fan noise reduction device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To more effectively cancel an acoustic wave transmitted into a duct of a turbo fan engine.SOLUTION: A fan noise reduction device includes: a plasma actuator 22 which is disposed in the duct 11 of the turbo fan engine S1, generates a sound when a voltage is applied thereto; and a control unit 24 that makes the plasma actuator 22 generate the sound using an inverse phase of the acoustic wave in the duct 11.

Description

  The present invention relates to a fan noise reduction device.

  In a turbofan engine, an acoustic wave is generated by the repeated generation of sound generated by the rotation of a fan blade and the presence of a stationary blade, and noise is generated due to the acoustic wave propagating through the duct and leaking outside. It is known.

For example, Non-Patent Document 1 proposes a method of canceling an acoustic wave by installing a speaker in a duct of a turbofan engine.
In this method, an acoustic wave having a phase opposite to that of an acoustic wave is generated from a speaker, so that these acoustic waves cancel each other, thereby reducing the acoustic wave leaking to the outside and reducing noise.

Ulf Tapken and Roland Bauers, German Aerospace Center (DLR), Institute of Propulsion Technology, Berlin, Germany and two others, `` Turbomachinery Exhaust Noise Radiation Experiments-Part 2: In-Duct and Far-Field Mode Analysis '', 14th AIAA / CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference), 5-7 May 2008, Vancouver, British Columbia Canada, AIAA 2008-2858

US Pat. No. 5,478,199

However, if a speaker is installed in the turbofan engine, naturally, only installing the speaker leads to an increase in weight and an increase in size of the turbofan engine.
On the other hand, Patent Document 2 proposes a method for canceling an acoustic wave in the same manner by using a small speaker or piezo device. However, in these small speakers and piezo devices, the output volume is small, The acoustic wave cannot be canceled out sufficiently.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide a fan noise reduction device capable of more effectively canceling an acoustic wave propagating through a duct of a turbofan engine.

  The present invention adopts the following configuration as means for solving the above-described problems.

  A first aspect of the present invention is a fan noise reduction device, which is disposed in a duct of a turbofan engine and generates a sound when a voltage is applied thereto, and has an opposite phase to an acoustic wave in the duct. A configuration is adopted in which the plasma actuator is provided with a control means for generating sound.

  According to a second aspect of the present invention, in the first aspect of the present invention, the plasma actuator is installed on both the upstream side and the downstream side of the fan rotor blade.

  A third invention employs a configuration in which a plurality of the plasma actuators are installed in the circumferential direction of the duct in the first or second invention.

  According to a fourth aspect of the present invention, in any one of the first to third aspects of the present invention, the flow rate measuring device that measures the flow rate in the duct, and the rotational speed measuring device that measures the rotational speed of the fan rotor blade, The means stores in advance a waveform of an acoustic wave in association with the flow velocity in the duct and the rotational speed of the fan rotor blade, and the acoustic wave is based on the measurement result by the flow velocity measuring instrument and the measurement result by the rotational speed measurement instrument. And a configuration in which the plasma actuator is driven based on the acquired waveform. The waveform here refers to the amplitude and period.

  According to a fifth invention, in any one of the first to third inventions, an acoustic wave measuring instrument is disposed between the fan rotor blade and the plasma actuator, and the control means includes the acoustic wave measuring instrument. A configuration is adopted in which the waveform of the acoustic wave is acquired based on the measurement result and the plasma actuator is driven based on the acquired waveform.

  A sixth invention includes any one of the first to third inventions, further comprising an acoustic wave measuring instrument disposed outside the plasma actuator as viewed from the fan rotor blade, wherein the control means includes the acoustic wave measurement. A configuration is adopted in which the waveform of the acoustic wave is acquired based on the measurement result of the vessel and the plasma actuator is driven based on the acquired waveform.

  7th invention employ | adopts the structure that the said acoustic wave measuring device is installed with two or more in the circumferential direction of the said duct in the said 5th or 6th invention.

According to the present invention, an acoustic wave is canceled by emitting a sound in an opposite phase to the acoustic wave propagating through the duct from a plasma actuator disposed in the duct of the turbofan engine.
Further, such a plasma actuator generates a volume force when driven to form an air flow. For this reason, the impedance in a duct is changed and a plasma actuator can be functioned like a sound-absorbing material.
That is, according to the present invention, the acoustic wave can be canceled by both the sound generated by the plasma actuator and the air flow.
Therefore, according to the present invention, the acoustic wave propagating through the duct of the turbofan engine can be canceled out more effectively, and a large silencing effect can be exhibited despite the small size. For this reason, noise can be reduced without increasing the weight and size of the turbofan engine.

It is sectional drawing which shows typically schematic structure of the turbofan engine in which the fan noise reduction apparatus in 1st Embodiment of this invention is installed. It is a schematic diagram of the plasma actuator 22 with which the fan noise reduction apparatus in 1st Embodiment of this invention is provided. FIG. 2 is a cross-sectional view taken along a plane orthogonal to the paper surface of FIG. 1, (a) is a cross-sectional view taken along line AA in FIG. 1, and (b) is a cross-sectional view taken along line BB in FIG. It is a schematic sectional drawing which shows typically a mode that the fan noise reduction apparatus in 2nd Embodiment of this invention was installed in the turbofan engine. It is a schematic diagram which shows the modification of the fan noise reduction apparatus in 2nd Embodiment of this invention.

  Hereinafter, an embodiment of a fan noise reduction device according to the present invention will be described with reference to the drawings. In the following drawings, the scale of each member is appropriately changed in order to make each member a recognizable size.

(First embodiment)
FIG. 1 is a cross-sectional view schematically showing a schematic configuration of a turbofan engine S1 in which the fan noise reduction device 20 of the present embodiment is installed.

  As shown in FIG. 1, the turbofan engine S1 includes an outer cowl 1, an inner cowl 2, a fan 3, a low pressure compressor 4, a high pressure compressor 5, a combustor 6, a high pressure turbine 7, a low pressure A turbine 8, a shaft 9, and a main nozzle 10 are provided.

The outer cowl 1 is a cylindrical member arranged on the most upstream side in the turbofan engine S1, and the upstream end and the downstream end in the air flow direction are open ends, and the upstream end functions as an air intake port. To do.
As shown in FIG. 1, the outer cowl 1 houses the upstream side of the inner cowl 2 and the fan 3 therein.

The inner cowl 2 is a cylindrical member having a smaller diameter than the outer cowl 1, and the upstream end and the downstream end in the air flow direction are open ends, like the outer cowl 1.
The inner cowl 2 includes a low-pressure compressor 4, a high-pressure compressor 5, a combustor 6, a high-pressure turbine 7, a low-pressure turbine 8, a shaft 9, a main nozzle 10, and the like, which are main parts of the turbofan engine S1. Is housed inside.

  In the present embodiment, the inner space of the outer cowl 1 is in a region where the inner cowl 2 is not present in the axial direction (left-right direction in FIG. 1), and the outer cowl 1 and the inner space are in a region where the inner cowl 2 is present in the axial direction. A space sandwiched between the cowls 2 functions as a duct 11 through which an air flow flows.

The inside of the inner cowl 2 is a flow path (hereinafter referred to as a core flow path) through which a part of the air taken into the outer cowl 1 and high-temperature gas generated by the combustor 6 pass.
As shown in FIG. 1, the outer cowl 1 and the inner cowl 2 are arranged concentrically when viewed from the air flow direction, and are arranged with a gap therebetween. And the clearance gap between the outer cowl 1 and the inner cowl 2 is made into the bypass flow path which discharges the remainder which does not flow into a core flow path among the air taken in in the outer cowl 1.
The outer cowl 1 and the inner cowl 2 are attached to the aircraft body by a pylon (not shown).

The fan 3 forms an air flow that flows into the outer cowl 1, and includes a plurality of fan rotor blades 3a fixed to the shaft 9 and a plurality of fan stationary blades 3b arranged in the bypass flow path. Yes.
The shaft 9 described in detail later is divided into two in the radial direction when viewed from the air flow direction. More specifically, the shaft 9 is constituted by a solid first shaft 9a that is a core portion and a hollow second shaft 9b that is disposed outside the first shaft 9a.
The fan rotor blade 3 a is fixed to the first shaft 9 a of the shaft 9.

As shown in FIG. 1, the low-pressure compressor 4 is disposed on the upstream side of the high-pressure compressor 5, and compresses air sent into the core flow path by the fan 3.
The low-pressure compressor 4 includes a moving blade fixed to the first shaft 9 a of the shaft 9 and a stationary blade fixed to the inner wall of the inner cowl 2.
The moving blades 4a are arranged in a ring shape at equal intervals to form one moving blade row. A plurality of stationary blades 4b are also arranged in a ring at regular intervals to form one stationary blade row. In the low-pressure compressor 4, a plurality of stationary blade rows and moving blade rows are alternately arranged starting from the stationary blade row in the air flow direction.

As shown in FIG. 1, the high-pressure compressor 5 is disposed on the downstream side of the low-pressure compressor 4, and compresses the air fed from the low-pressure compressor 4 to a higher pressure.
The high-pressure compressor 5 includes a moving blade 5 a that is fixed to the second shaft 9 b of the shaft 9 and a stationary blade 5 b that is fixed to the inner wall of the inner cowl 2.
As with the low-pressure compressor 4, the moving blades 5 a are arranged in a ring shape at equal intervals to constitute one moving blade row. A plurality of stationary blades 5b are also arranged in a ring at regular intervals to constitute one stationary blade row. A plurality of stationary blade rows and moving blade rows are alternately arranged in the air flow direction.

  The combustor 6 is disposed on the downstream side of the high-pressure compressor 5, and burns a mixture of compressed air fed from the high-pressure compressor 5 and fuel supplied from an injector (not shown) to generate high-temperature gas. Is to be generated.

The high-pressure turbine 7 is disposed on the downstream side of the combustor 6, and collects rotational power from the high-temperature gas discharged from the combustor 6.
The high-pressure turbine 7 includes a plurality of turbine rotor blades 7a fixed to the second shaft 9b of the shaft 9 and a plurality of turbine stator blades 7b fixed to the core flow path. The generated high temperature gas is received by the turbine rotor blade 7a to rotate the second shaft 9b.

The low-pressure turbine 8 is disposed on the downstream side of the high-pressure turbine 7, and further collects rotational power from the high-temperature gas that has passed through the high-pressure turbine 7.
The low-pressure turbine 8 includes a plurality of turbine blades 8a fixed to the first shaft 9a of the shaft 9, and a plurality of turbine stationary blades 8b fixed to the core flow path, and rectified by the turbine stationary blade 8b. The generated high temperature gas is received by the turbine rotor blade 8a, and the first shaft 9a is rotationally driven.

The shaft 9 is a rod-like member arranged in the air flow direction, and the rotational power recovered by the turbine (the high pressure turbine 7 and the low pressure turbine 8) is converted into the fan 3 and the compressor (the low pressure compressor 4 and the high pressure compression). Machine 5).
As described above, the shaft 9 is divided into two in the radial direction, and is constituted by the first shaft 9a and the second shaft 9b.
The first shaft 9a has the moving blade 4a of the low-pressure compressor 4 and the fan moving blade 3a of the fan 3 attached to the upstream side, and the turbine moving blade 8a of the low-pressure turbine 8 attached to the downstream side.
Further, the second shaft 9b has the moving blade 5a of the high-pressure compressor 5 attached to the upstream side and the turbine moving blade 7a of the high-pressure turbine 7 attached to the downstream side.

The main nozzle 10 is provided further downstream of the low-pressure turbine 8 and injects high-temperature gas that has passed through the low-pressure turbine 8 toward the rear of the turbofan engine S1.
The thrust of the turbofan engine S1 is obtained by the reaction when the high temperature gas is injected from the main nozzle 10.

In the turbofan engine S1 having such a configuration, in a steady state, air is taken into the outer cowl 1 by driving the fan 3, and a part thereof flows into the core flow path.
The air flowing into the core flow path is sequentially compressed by the low pressure compressor 4 and the high pressure compressor 5 and supplied to the combustor 6.
The compressed air supplied to the combustor 6 is mixed with fuel to form an air-fuel mixture. The air-fuel mixture is combusted by the combustor 6 to generate high temperature gas.
The hot gas generated in the combustor 6 passes through the high-pressure turbine 7 and the low-pressure turbine 8 and is injected from the main nozzle 10 to the rear of the turbofan engine S1. This provides a driving force.

When the high-temperature gas passes through the high-pressure turbine 7, the rotational power is recovered by the high-pressure turbine 7, and the rotor blade 5a of the high-pressure compressor 5 is rotationally driven through the second shaft 9b.
Further, when the high-temperature gas passes through the low-pressure turbine 8, the rotational power is recovered by the low-pressure turbine 8, and the rotor blade 4a of the low-pressure compressor 4 and the fan rotor blade 3a of the fan 3 are rotationally driven through the first shaft 9a. Is done.

In such a turbofan engine S1, when the fan rotor blade 3a is rotationally driven, an air flow is formed. Then, due to the presence of the fan rotor blade 3a and the stationary blade 3b in the air flow, sound is repeatedly emitted and an acoustic wave is generated.
This acoustic wave is a close-packed wave that is repeated with a period that depends on the rotational speed of the fan rotor blade and the air flow velocity inside the duct 11, and becomes noise by propagating through the duct 11 and leaking outside.

And the fan noise reduction apparatus 20 of this embodiment reduces the noise (fan noise) caused by the drive of the fan by canceling the acoustic wave as described above.
As shown in FIG. 1, the fan noise reduction device 20 includes a flow velocity measuring device 21, a plasma actuator 22, a rotation number measuring device 23, and a controller 24 (control means).

  The flow velocity measuring device 21 measures the flow velocity of the air flow in the duct 11 and is arranged on the upstream side of the air flow with respect to the plasma actuator 22 and the rotation speed measuring device 23. In the present embodiment, It is installed in the duct 11 and in the vicinity of the inlet end of the duct 11.

The plasma actuator 22 is arranged in the duct 11 and emits a sound when a voltage is applied thereto.
FIG. 2 is a schematic diagram of the plasma actuator 22. As shown in this figure, the plasma actuator 22 includes an insulating layer 30, a first electrode plate 32 disposed on the upper surface of the insulating layer 30, and a second electrode plate 31 disposed on the lower surface of the insulating layer 30. ing.
As shown in FIG. 2, the first electrode plate 32 and the second electrode plate 31 are arranged so as to be displaced as viewed from the thickness direction of the insulating layer 30.
When power is supplied to the first electrode plate 32 and the second electrode plate 31 of the plasma actuator 22 under the control of the controller 24, the thickness of the insulating layer 30 is on the side of the second electrode plate 32. Plasma is generated in a region overlapping the second electrode plate 31 when viewed from the direction. At this time, a sound is emitted from the plasma actuator 22. Further, the air in the region where the plasma is generated is ionized to generate a body force, and an air flow indicated by an arrow is formed.

  And as shown in FIG. 1, the plasma actuator 22 is arrange | positioned at both the upstream and downstream of the fan rotor blade 3a. In the following description, the plasma actuator 22 disposed on the upstream side of the fan rotor blade 3a is referred to as an upstream plasma actuator 22a, and the plasma actuator 22 disposed on the downstream side of the fan rotor blade 3a is referred to as a downstream plasma actuator. 22b.

3 is a cross-sectional view taken along an orthogonal plane orthogonal to the paper surface of FIG. 1, (a) is a cross-sectional view taken along line AA in FIG. 1, and (b) is a cross-sectional view taken along line BB in FIG. is there.
As shown in these drawings, a plurality of upstream plasma actuators 22 a and downstream plasma actuators 22 b are installed at equal intervals in the circumferential direction of the duct 11.

Returning to FIG. 1, the rotational speed measuring device 23 measures the rotational speed of the fan rotor blade 3 a and outputs the measurement result.
And in this embodiment, the rotation speed measuring device 23 is arrange | positioned at the chip | tip side of the fan rotor blade 3a, as shown in FIG.

The controller 24 controls the entire operation of the fan noise reduction device 20 of the present embodiment, and is electrically connected to the flow velocity measuring device 21, the plasma actuator 22, and the rotation number measuring device 23.
In the present embodiment, the controller 24 stores a database for acquiring the waveform of the acoustic wave propagating through the duct 11 in advance.
The waveform of the acoustic wave propagating in the duct 11 changes depending on the flow velocity in the duct 11 and the rotational speed of the fan rotor blade 3a. For this reason, the database stored in advance in the controller 24 is such that the acoustic wave waveform is associated with the flow velocity in the duct 11 and the rotational speed of the fan rotor blade.

Then, the controller 24 compares the measurement result of the flow velocity measuring device 21 and the measurement result of the rotation speed measuring device 23 with the database, acquires the waveform of the acoustic wave propagating in the duct 11, and acquires the acquired acoustic wave waveform. Based on the above, the plasma actuator 22 is driven.
Specifically, the controller 24 drives the plasma actuator 22 so that a sound having an opposite phase to the acoustic wave propagating in the duct 11 is emitted.

It should be noted that the controller 24 can store an arithmetic expression of an acoustic wave waveform using the flow velocity in the duct 11 and the rotational speed of the fan rotor blade as parameters, instead of the database.
In this case, the controller 24 calculates (acquires) the waveform of the acoustic wave by substituting the measurement result of the flow velocity measuring device 21 and the measurement result of the rotation speed measuring device 23 into an arithmetic expression, and the calculated acoustic wave The plasma actuator 22 is driven based on the waveform.

According to the fan noise reduction device 20 of the present embodiment having such a configuration, sound is emitted from the plasma actuator 22 disposed in the duct 11 of the turbofan engine S1 with a phase opposite to that of the acoustic wave propagating through the duct 11. To cancel the acoustic wave.
Further, such a plasma actuator 22 generates a volume force when driven to form an air flow. For this reason, the impedance in the duct 11 can be changed and the plasma actuator 22 can be functioned in the same manner as the sound absorbing material.
That is, according to the present invention, the acoustic wave can be canceled by both the sound generated by the plasma actuator and the air flow.
Therefore, according to the fan noise reduction device 20 of the present embodiment, the acoustic wave propagating through the duct 11 of the turbofan engine S1 can be canceled out more effectively, and a large noise reduction effect can be exhibited despite its small size. For this reason, noise can be reduced without increasing the weight and size of the turbofan engine S1.

Further, in the fan noise reduction device 20 of the present embodiment, the plasma actuators 22 are installed on both the upstream side and the downstream side of the fan rotor blade 3a.
For this reason, the acoustic wave propagating to the upstream side of the fan rotor blade 3a can be canceled by the upstream plasma actuator 22a, which is the plasma actuator 22 arranged on the upstream side, and the acoustic wave propagating to the downstream side of the fan rotor blade 3a. The wave can be canceled by the downstream plasma actuator 22b, which is the plasma actuator 22 arranged on the downstream side.
Therefore, the acoustic wave can be prevented from leaking to the outside from both the inlet side and the outlet side of the duct 11, and the fan noise can be further reduced.

In the fan noise reduction device 20 of the present embodiment, a plurality of plasma actuators 22 are installed in the circumferential direction of the duct 11.
For this reason, an acoustic wave can be canceled over the perimeter of the duct 11, and a fan noise can be reduced more.

  Further, in the fan noise reduction device 20 of the present embodiment, the controller 24 stores a database in advance, and acquires an acoustic wave waveform based on the measurement result of the flow velocity measurement device 21 and the measurement result of the rotation speed measurement device 23. did. For this reason, the waveform of the acoustic wave can be obtained with a simple configuration.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. In the description of the present embodiment, the description of the same parts as those of the first embodiment is omitted or simplified.
FIG. 4 is a schematic cross-sectional view schematically showing a state where the fan noise reduction device 20A of the present embodiment is installed in the turbofan engine S1.

  As shown in this figure, the fan noise reduction device 20 includes an acoustic wave measuring instrument 25 disposed between the fan rotor blade 3 a and the plasma actuator 22, and the controller 24 measures the measurement result of the acoustic wave measuring instrument 25. The waveform of the acoustic wave is acquired based on the above and the plasma actuator 22 is driven based on the acquired waveform.

More specifically, the acoustic wave measuring instrument 25 is arranged on both the upstream side and the downstream side of the fan rotor blade 3a.
Then, the controller 24 transmits acoustic waves propagating upstream of the fan rotor blade 3a by an acoustic wave measuring instrument 25 (hereinafter referred to as upstream acoustic wave measuring instrument 25a) disposed on the upstream side of the fan rotor blade 3a. The waveform is acquired and the upstream plasma actuator 22a is driven.
Further, the controller 24 transmits acoustic waves propagating to the downstream side of the fan rotor blade 3a by an acoustic wave measuring instrument 25 (hereinafter referred to as a downstream acoustic wave measuring instrument 25b) disposed on the downstream side of the fan rotor blade 3a. The waveform is acquired and the downstream plasma actuator 22b is driven.

Further, a plurality of upstream acoustic wave measuring instruments 25a are arranged at equal intervals in the circumferential direction of the duct 11 as in the upstream plasma actuator 22a.
Further, a plurality of downstream acoustic wave measuring instruments 25b are arranged at equal intervals in the circumferential direction of the duct 11, like the downstream plasma actuator 22b.

According to the fan noise reduction device 20A of the present embodiment having such a configuration, the waveform of the acoustic wave is acquired from the measurement result of the acoustic wave measuring instrument 25, and a more accurate acoustic wave is obtained using the waveform of the acoustic wave. Get the waveform.
Therefore, according to the fan noise reduction device 20A of the present embodiment, noise can be more reliably reduced.

Note that the fan noise reduction device 20A of the present embodiment employs a configuration in which the acoustic actuator 25 is driven by so-called feedforward control by disposing the acoustic wave measuring instrument 25 on the acoustic wave sound source side of the plasma actuator 22. did.
However, as shown in FIG. 5, the acoustic wave measuring instrument 25 may be disposed outside the plasma actuator 22 when viewed from the fan rotor blade 3a. In such a case, feedback control is performed to control the plasma actuator 22 so that the acoustic wave measured by the acoustic wave measuring instrument 25 becomes small.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to the said embodiment. Various shapes, combinations, and the like of the constituent members shown in the above-described embodiments are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

For example, in the above embodiment, the configuration in which a plurality of plasma actuators 22 are arranged at equal intervals in the circumferential direction of the duct 11 has been described.
However, the present invention is not limited to this, and a plurality of plasma actuators 22 may be arranged at unequal intervals in the circumferential direction of the duct 11. Further, the plasma actuators 22 arranged in the circumferential direction of the duct 11 may be arranged shifted in the air flow direction. Further, the number of plasma actuators 22 arranged in the circumferential direction of the duct 11 is arbitrary.

  20, 20A: Fan noise reduction device, 21 ... Flow velocity measuring device, 22 ... Plasma actuator, 23 ... Rotation measuring device, 24 ... Controller (control means), 25 ... Acoustic wave measuring device, S1 ... ... Turbofan engine, 11 ... duct, 3a ... fan blade, 3b ... fan stationary blade

Claims (7)

A plasma actuator which is arranged in a duct of a turbofan engine and emits sound when a voltage is applied;
And a control means for causing the plasma actuator to emit sound in a phase opposite to that of the acoustic wave in the duct.   2. The fan noise reduction device according to claim 1, wherein the plasma actuator is installed on both the upstream side and the downstream side of the fan rotor blade.   The fan noise reduction device according to claim 1 or 2, wherein a plurality of the plasma actuators are installed in a circumferential direction of the duct. A flow velocity measuring device for measuring the flow velocity in the duct, and a rotational speed measuring device for measuring the rotational speed of the fan blade,
The control means stores in advance a waveform of an acoustic wave in association with the flow velocity in the duct and the rotation speed of the fan rotor blade, and based on the measurement result in the flow velocity measuring instrument and the measurement result in the rotation speed measurement instrument, The fan noise reduction device according to any one of claims 1 to 3, wherein the plasma actuator is driven based on an acquired waveform of an acoustic wave. An acoustic wave measuring instrument disposed between the fan rotor blade and the plasma actuator;
The said control means drives the said plasma actuator based on the acquired waveform while acquiring the waveform of the said acoustic wave based on the measurement result of the said acoustic wave measuring device. The described fan noise reduction device. An acoustic wave measuring instrument disposed outside the plasma actuator as seen from the fan blade,
The said control means drives the said plasma actuator based on the acquired waveform while acquiring the waveform of the said acoustic wave based on the measurement result of the said acoustic wave measuring device. The described fan noise reduction device.   The fan noise reduction device according to claim 5 or 6, wherein a plurality of the acoustic wave measuring devices are installed in a circumferential direction of the duct.

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