Sound emitting apparatus and blade noise reduction apparatus

According to an embodiment, a sound emitting apparatus includes a helical hollow tube and at least three sound wave sources. The helical hollow tube helically extends in a circumferential direction to form an annular shape as a whole. The first helical hollow tube includes a plurality of openings. The at least three sound wave sources are coupled to the first helical hollow tube and are configured to supply a sound wave to the first helical hollow tube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-145266, filed Sep. 7, 2021, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a sound emitting apparatus and a blade noise reduction apparatus.

BACKGROUND

When an active noise reduction method is used to reduce a blade noise, a blade rotation mode is simulated using a plurality of loudspeakers installed coaxially with the rotation axis of rotational blades. For example, to reduce a noise generated in an Mth-order Lobe mode, 2M+1 or more loudspeakers are discretely arranged. When loudspeakers are used for blade noise reduction, jigs for installing the loudspeakers are required around the rotational blades, and the entire load becomes heavy due to the weights of the loudspeakers. In addition, the loudspeaker installation volume may disturb the sound field and the flow of the blades.

DETAILED DESCRIPTION

According to an embodiment, a sound emitting apparatus includes a helical hollow tube and at least three sound wave sources. The helical hollow tube helically extends in a circumferential direction to form an annular shape as a whole. The first helical hollow tube includes a plurality of openings. The at least three sound wave sources are coupled to the first helical hollow tube and are configured to supply a sound wave to the first helical hollow tube.

Hereinafter, embodiments will be described with reference to the accompanying drawings. In order to avoid redundant description, like reference numerals are given to like components throughout the drawings. In addition, branch numbers are attached to the reference numerals in order to distinguish individual components. In some drawings, one or more components are not shown for simplicity.

An embodiment is directed to a sound emitting apparatus that emits a sound. The sound emitting apparatus according to the embodiment can be used for applications such as blade noise reduction and an alarm buzzer. The blade noise is a noise generated by rotation of one or more rotational blades such as the propeller of a drone or a propeller fan. The blade noise includes noises generated in a plurality of Lobe modes. The following description is made on the assumption that the sound emitting apparatus is used for blade noise reduction.

FIGS.1and2schematically show an example of the configuration of a sound emitting apparatus100according to an embodiment. As shown inFIGS.1and2, the sound emitting apparatus100includes a hollow tube102, three or more sound wave sources106, and an annular member110. In the example shown inFIG.2, eight sound wave sources106-1to106-8are provided. When a noise reduction target is the Mth-order Lobe mode, 2M+1 or more sound wave sources106are provided. The sound emitting apparatus100is annular, and a fan200serving as a noise source is arranged inside the sound emitting apparatus100. The fan200includes one or more rotational blades.

The hollow tube102has an annular or ring shape as a whole. The hollow tube102is a looped helical hollow tube helically extending in the circumferential direction. The circumferential direction corresponds to the rotation direction of the fan200. The section of the hollow tube102may be circular. That is, the hollow tube102may be a hollow circular tube. The hollow tube102has a plurality of openings104that make the internal and external spaces of the hollow tube102communicate with each other. The openings104are formed to face the fan200. In other words, the openings104are formed on the inner side of the hollow tube102.

The hollow tube102has a tube line length dependent on a frequency subjected to noise reduction (specifically, a frequency corresponding to a Lobe mode subjected to noise reduction) such that a natural frequency corresponding to a spatial sound field (Lobe mode) excited in the hollow tube102matches the frequency subjected to noise reduction. The tube line length means a dimension of the hollow tube102along its central axis. In the following description, a Lobe mode subjected to noise reduction is sometimes referred to as a target Lobe mode, and a frequency subjected to noise reduction is sometimes referred to as a target frequency. The natural frequency matching the target frequency means that the natural frequency is within a frequency range having a predetermined width around the target frequency. For example, when the target frequency is f [Hz], the natural frequency is set to a value within a frequency range from (f−100) Hz to (f+100) Hz.

The annular member110is an annular supporting member that supports the hollow tube102. The hollow tube102is helically wound on the annular member110. The annular member110may be a hollow tube. From the viewpoint of space saving, the sound wave sources106are desirably provided in the internal space of the annular member110. Note that the sound wave sources106may be provided outside the annular member110.

In the example shown inFIG.1, the hollow tube102may be fabricated by fabricating a flexible hollow tube, helically winding it on the annular member110, and joining the two ends of the hollow tube. Alternatively, the hollow tube102may be fabricated using a 3D printer. In this case, the annular member110may be omitted, as shown inFIG.3.

In the example shown inFIG.1, the entire shape of the hollow tube102is circular. Alternatively, the entire shape of the hollow tube102may be oval. For example, the supporting member is oval, and the hollow tube102is helically wound on the supporting member.

The sound wave sources106are connected to the hollow tube102and supply sound waves to the hollow tube102. The sound wave sources106are arranged in the hollow tube102at predetermined angular intervals. The angular interval refers to an interval in the circumferential direction and is expressed by an angle with respect to the center. In the example shown inFIG.2, the sound wave sources106-1to106-8are arranged in the hollow tube102at angular intervals of 45°.

FIG.4schematically shows an example of the sound wave source106. In the example shown inFIG.4, the sound wave source106includes an enclosure part107having an internal space, a connecting tube108that makes the internal space of the enclosure part107and that of the hollow tube102communicate with each other, and a loudspeaker109provided in the internal space of the enclosure part107. The loudspeaker109is a transducer that converts an electric signal into a sound. The loudspeaker109may be a compact loudspeaker such as a loudspeaker having a voice coil or a loudspeaker having a piezoelectric element. The enclosure part107may be designed to generate resonance in order to increase the volume. Specifically, the dimensions of the enclosure part107may be designed in accordance with a target frequency.

FIG.5schematically shows a cross section of the sound emitting apparatus100taken along a line V-V inFIG.1when the sound wave source106has the structure shown inFIG.4. As shown inFIG.5, a sound emitted from the loudspeaker109is supplied to the hollow tube102through the connecting tube108, exciting a spatial sound field corresponding to a target Lobe mode in the hollow tube102. The spatial sound field excited in the hollow tube102is output from the hollow tube102to the external space through the openings104.

The structure of the sound wave source106shown inFIG.5is merely an example. As the sound wave source106, a canal type earphone may be used. The earphone is connected to the hollow tube102at the canal part of the earphone.

As shown inFIGS.6and7, the sound emitting apparatus100may further include a cover112covering the hollow tube102.FIG.7schematically shows a cross section of the sound emitting apparatus100taken along a line VII-VII inFIG.6. The cover112has a plurality of openings facing the openings104of the hollow tube102. The hollow tube102disturbs an air flow due to its complicated structure and thus disturbs the flow of rotational blades. The cover112covering the hollow tube102prevents the disturbance of the flow of the rotational blades. The cover112may have the function of a bellmouth to effectively prevent the disturbance of the flow of the rotational blades. Specifically, the cover112may have a shape curved convexly toward the fan200.

The hollow tube102and the cover112may be integrally formed. For example, a combination of the hollow tube102and cover112may be fabricated by fabricating upper and lower members by a 3D printer, attaching the sound wave source106to the upper or lower member, and joining the upper and lower members to each other. The top and bottom are defined along the rotating axis of the fan200.

The sound emitting apparatus100further includes a control circuit that controls the sound wave sources106. For example, the control circuit generates drive signals for driving the sound wave sources106, and sends the drive signals to the sound wave sources106. There is a phase difference between drive signals for the two sound wave sources106separated by the predetermined angular interval, which depends on the order M of the target Lobe mode and the predetermined angular interval. Thus, the sound field excited in the hollow tube102rotates in the circumferential direction, and the Lobe mode characteristics of the blade noise can be simulated. The sound emitting apparatus100is configured to excite a Lobe mode of an order equal to the order of the target Lobe mode. For example, when the target Lobe mode is the fourth-order Lobe mode, the sound emitting apparatus100is configured to excite the fourth-order Lobe mode in the hollow tube102.

As an example, the control circuit includes a processing circuit and a memory. The processing circuit includes, for example, a general-purpose processor such as a CPU (Central Processing Unit). The memory includes a volatile memory and a nonvolatile memory, and stores data such as a control program. At least part of processing to be described below regarding the control circuit can be implemented by executing a control program by the general-purpose processor. The control circuit may include a dedicated processor such as an ASIC (Application Specific Integrated Circuit) or a FPGA (Field Programmable Gate Array) instead of or in addition to the general-purpose processor.

FIG.8schematically shows an example of the configuration of a drive circuit152included in the control circuit. In the example shown inFIG.8, the drive circuit152includes a drive signal generation unit161and phase shifters162-1to162-N. The drive signal generation unit161generates a drive signal. The drive signal is branched into N and supplied to the phase shifters162-1to162-N. The phase shifter162-iapplies a phase shift to the drive signal, where ϕi=2π(i−1)/N, M is the order of the Lobe mode, and N is the number of sound wave sources106. The drive signal to which the phase shift −Mϕiis applied by the phase shifter162-iis sent to the sound wave source106-i.

FIG.9schematically shows another example of the arrangement of the drive circuit152included in the control circuit. In the example shown inFIG.9, the drive circuit152includes the drive signal generation unit161and delay units163-1to163-N. The drive signal output from the drive signal generation unit161is branched into N and supplied to the delay units163-1to163-N. The delay unit163-idelays the drive signal by a time Mϕi/2πf, where ϕi=2π(i−1)/N, and f is the frequency of the drive signal. The drive signal delayed by the time Mϕi/2πf by the delay unit163-iis sent to the sound wave source106-i.

The sound emitting apparatus100having the above-described configuration can emit a sound for reducing the noise of a target Lobe mode. The hollow tube102having an internal space can implement a lightweight apparatus. As described above, the hollow tube102has a tube line length dependent on a target frequency. When the hollow tube102is a simple annular ring, the dimension (Specifically, radius) of the annular ring increases in proportion to the tube line length of the hollow tube102. The dimension of the annular ring can be kept small by helically shaping the hollow tube102. This can implement a compact apparatus.

Next, a method of designing the hollow tube102will be explained.

FIG.10shows the tube line length of a circular tube when a circular tube having a diameter D2is wound on a column having a diameter D1. Letting Nabe the winding number and θ0be the winding angle, a tube line length L of the circular tube is given by:

Assuming that a circular tube having the diameter D2is wound on an annular member having an annular shape of a radius Raand a cross section of the diameter D1, when the winding proceeds

π⁡(2⁢Ra-(D1+D2))Na
per one winding turn, the following relationship is obtained:

From this, the tube line length L is given by:

The tube line length L needs to be corrected to wind the circular tube on the annular member, and a corrected tube line length Lais given by:
Lα=L×α(4)
where α is the correction coefficient and is a function of (D1+D2)/Ra, as shown inFIG.11.

Procedures for designing the hollow tube102will be exemplified.

Step A: The tube line length Lais determined from a target frequency f and the order M of a target Lobe mode:

La=cMf
where c is the speed of sound.

Step B: The radius Raof an annular ring and (D1+D2) are determined.

For example, the radius Rais 1.1 times the radius of the fan200. The radius Rais preferably closer to the radius of the fan200. Although (D1+D2) has no constraint, (D1+D2) is set to, for example, 5% to 20% of the radius Ra(0.05<(D1+D2)/Ra<0.2).

For example, the winding angle θ0is determined by substituting Ladetermined in step A, and Raand (D1+D2) determined in step B into expressions (3) and (4) above.

Step D: The winding number Nais determined.

For example, the winding number Nais determined by substituting Ladetermined in step A, Raand (D1+D2) determined in step B, and the winding angle θ0determined in step C into equation (2) above, and rounding off the obtained Na.

If the winding number Nais smaller than 2M+1, the process returns to step B to change (D1+D2). If the decimal part of the obtained Na(Nabefore round-off) is close to 0.5, the process returns to step B to change (D1+D2).

Step E: The diameter D1of the annular member110and the diameter D2of the hollow tube102are determined.

For example, the diameters D1and D2are determined by distributing (D1+D2) determined in step B. Since the sound wave sources are arranged inside the annular member110, the diameter D1is set to a size enough to install a compact loudspeaker.

Accordingly, the diameter D2of the hollow tube102, the diameter D1of the annular member110, the radius Raof the annular ring, and the winding number Naare determined with respect to the target frequency f and the order M of the target Lobe mode.

To increase the degree of freedom of design, the hollow tube102is helically formed. For example, the tube line length Laof the hollow tube102can be adjusted by the winding number Na.

Next, a method of designing the openings104of the hollow tube102will be explained.

The internal space of each opening104in the hollow tube102and the dimensions of the opening104are optimized in accordance with the target frequency f, generating Helmholtz resonance and increasing the sound emitting efficiency. The opening104functions as a Helmholtz sound hole that amplifies and outputs a sound using the Helmholtz resonance. As a result, compact, lightweight loudspeakers can be used as the sound wave sources106.

Typically, the openings104are formed to face the fan200. The number of openings104is arbitrarily equal to or more than 2M+1, and the openings104are arranged symmetrically. More specifically, the openings104are arranged at equal angular intervals. For example, one opening104may be set every turn of the hollow tube102. In this case, the volume V of the spatial region of one opening104can be given by:
Vh=(La/Na)×π(D2/2)2(5)

For example, when one opening104is set every two turns, the volume V of the spatial region of one opening104is double the volume given by expression (5) above. When one opening104is set every three turns, the volume V of the spatial region of one opening104is three times the volume given by expression (5) above.

A radius ahand height thof the opening104are determined from a general Helmholtz resonance design given by:

fh=ah⁢c2⁢π⁢πVh⁢th′
where t′his the height thafter end correction. The height thcoincides with the thickness (wall thickness) of the hollow tube102. fhis set to or close to the target frequency f.

The sound emitting apparatus100is a discrete sound source corresponding to the number of openings104. As the number of openings104is larger, the similarity between the characteristics of a sound emitted from the sound emitting apparatus100and the Lobe mode characteristics of the blade noise increases. The number of openings104depends on the winding number of the hollow tube102and can be increased by increasing the winding number.

[Arrangement of Sound Wave Sources]

Next, the arrangement of the sound wave sources106will be explained.

The arrangement method of the sound wave sources106includes, but is not limited to, an N=4M arrangement and an N=2M+a arrangement.

The N=4M arrangement is a method of arranging the sound wave sources106four times in number the order M of the Lobe mode. In the N=4M arrangement, the number of sound wave sources106is large, but phase shifts to be applied are 0°, 90°, 180°, and 270°, so the Lobe mode can be driven using one 90° phase shifter.

When the Lobe mode is the second-order Lobe mode, the arrangement shown inFIG.2is the N=4M arrangement. A phase shift regarding the sound wave source106-iis −Mϕiwhere ϕi=2π(i−1)/N. That is, a phase shift regarding each of the sound wave sources106-1and106-5is 0°, a phase shift regarding each of the sound wave sources106-2and106-6is −90°, a phase shift regarding each of the sound wave sources106-3and106-7is −180°, and a phase shift regarding each of the sound wave sources106-4and106-8is −270°.

FIG.12schematically shows an example of the drive circuit152used for the N=4M arrangement. In the example shown inFIG.12, the drive circuit152includes a drive signal generation unit161, a 90° phase shifter171, and inverting circuits172and173. A drive signal u from the drive signal generation unit161is branched into three. A first branch drive signal u is output as it is as a drive signal u1. The drive signal u1is sent to the sound wave source106-i, where i satisfies mod(i, 4)=1. Specifically, the drive signal u1is sent to the sound wave sources106-1and106-5. A second branch drive signal u is supplied to the 90° phase shifter171. The 90° phase shifter171applies a phase shift of −90° to the second branch drive signal to generate a drive signal u2. The drive signal u2is branched into two. A first branch drive signal u2is sent to the sound wave source106-i, where i satisfies mod(i, 4)=2. Specifically, the first branch drive signal u2is sent to the sound wave sources106-2and106-6. A second branch drive signal u2is supplied to the inverting circuit173. The inverting circuit173inverts the second branch drive signal u2to generate a drive signal u4. The drive signal u4is sent to the sound wave source106-i, where i satisfies mod(i, 4)=4. Specifically, the drive signal u4is sent to the sound wave sources106-4and106-8. A third branch drive signal u is supplied to the inverting circuit172. The inverting circuit172inverts the third branch drive signal u to generate a drive signal u3. The drive signal u3is sent to the sound wave source106-i, where i satisfies mod(i, 4)=3. Specifically, the drive signal u3is sent to the sound wave sources106-3and106-7.

In the N=4M arrangement, the Lobe mode can be excited using one 90° phase shifter.

The N=2M+α arrangement is a method of arranging the sound wave sources106in number obtained by adding a to double the order M of the Lobe mode, where a is an integer of 1 or more. From the viewpoint of spatial aliasing, a is desirably equal to or larger than 3. In the N=2M+α arrangement, when N is an even number other than 3M, N/2−1 phase shifters are required. For example, when the Lobe mode is the seventh-order Lobe mode and 16 sound wave sources106are arranged in the hollow tube102, seven phase shifters are required. When N is 3M, two phase shifters are sufficient. The value N is determined in consideration of a balance between the number of phase shifters and the number of sound wave sources106.

FIG.13Aschematically shows an example of the N=2M+α arrangement. In the example shown inFIG.13A, the Lobe mode is the fourth-order Lobe mode, and 12 sound wave sources106-1to106-12are arranged in the hollow tube102at intervals of 30°. The arrangement shown inFIG.13Asatisfies N=3M. A phase shift regarding the sound wave source106-iis −Mϕi, where ϕi=2π(i−1)/N. That is, a phase shift regarding each of the sound wave sources106-1,106-4,106-7, and106-10is 0°, a phase shift regarding each of the sound wave sources106-2,106-5,106-8, and106-11is −120°, and a phase shift regarding each of the sound wave sources106-3,106-6,106-9, and106-12is −240°.

FIG.13Bschematically shows an example of the drive circuit152used in the arrangement shown inFIG.13A. As shown inFIG.13B, the drive circuit152includes the drive signal generation unit161, a 120° phase shifter174, and a 240° phase shifter175. A drive signal u from the drive signal generation unit161is branched into three. A first branch drive signal u is output as it is as a drive signal u1. The drive signal u1is sent to the sound wave sources106-1,106-4,106-7, and106-10. A second branch drive signal u is supplied to the 120° phase shifter174. The 120° phase shifter174applies a phase shift of −120° to the second branch drive signal u to generate a drive signal u2. The drive signal u2is sent to the sound wave sources106-2,106-5,106-8, and106-11. A third branch drive signal u is supplied to the 240° phase shifter175. The 240° phase shifter175applies a phase shift of −240° to the third branch drive signal u to generate a drive signal u3. The drive signal u3is sent to the sound wave sources106-3,106-6,106-9, and106-12.

In the N=3M arrangement, the Lobe mode can be excited using two phase shifters.

As M increases, the number of sound wave sources106increases. In the N=3M arrangement, the sound wave sources106-1and106-4shown inFIG.13Aemit the same sound. The sound wave sources106-1and106-4can be implemented by one loudspeaker109.

FIG.14schematically shows a sound wave source assembly116that may be used for the N=3M arrangement according to the embodiment. As shown inFIG.14, the sound wave source assembly116has a three-layered structure in which layers117-1,117-2, and117-3are stacked. The layer117-1includes the sound wave sources106-1,106-4,106-7, and106-10shown inFIG.13A, the layer117-2includes the sound wave sources106-2,106-5,106-8, and106-11shown inFIG.13A, and the layer117-3includes the sound wave sources106-3,106-6,106-9, and106-12shown inFIG.13A.

FIG.15shows a state in which the sound wave source assembly116shown inFIG.14is disassembled. As shown inFIG.15, the layer117-1includes enclosure parts107-1and107-2, connecting tubes108-1,108-4,108-7, and108-10, and loudspeakers109-1and109-2.

The connecting tubes108-1and108-4are provided at the enclosure part107-1, and the loudspeaker109-1is arranged in the enclosure part107-1. The distance between the loudspeaker109-1and the connecting tube108-1equals that between the loudspeaker109-1and the connecting tube108-4. The sound wave source106-1shown inFIG.13Ais implemented by the enclosure part107-1, the connecting tube108-1, and the loudspeaker109-1. The sound wave source106-4shown inFIG.13Ais implemented by the enclosure part107-1, the connecting tube108-4, and the loudspeaker109-1. The sound wave sources106-1and106-4share the enclosure part107-1and the loudspeaker109-1.

The connecting tubes108-7and108-10are provided at the enclosure part107-2, and the loudspeaker109-2is arranged in the enclosure part107-2. The distance between the loudspeaker109-2and the connecting tube108-7equals that between the loudspeaker109-2and the connecting tube108-10. The sound wave source106-4shown inFIG.13Ais implemented by the enclosure part107-2, the connecting tube108-7, and the loudspeaker109-2. The sound wave source106-10shown inFIG.13Ais implemented by the enclosure part107-2, the connecting tube108-10, and the loudspeaker109-2. The sound wave sources106-4and106-10share the enclosure part107-2and the loudspeaker109-2.

The connecting tubes108-2and108-5are provided at the enclosure part107-3, and the loudspeaker109-3is arranged in the enclosure part107-3. The sound wave source106-2shown inFIG.13Ais implemented by the enclosure part107-3, the connecting tube108-2, and the loudspeaker109-3. The sound wave source106-5shown inFIG.13Ais implemented by the enclosure part107-3, the connecting tube108-5, and the loudspeaker109-3. The sound wave sources106-2and106-5share the enclosure part107-3and the loudspeaker109-3.

The connecting tubes108-8and108-11are provided at the enclosure part107-4, and the loudspeaker109-4, is arranged in the enclosure part107-4. The sound wave source106-8shown inFIG.13Ais implemented by the enclosure part107-4, the connecting tube108-8, and the loudspeaker109-4. The sound wave source106-11shown inFIG.13Ais implemented by the enclosure part107-4, the connecting tube108-11, and the loudspeaker109-4. The sound wave sources106-8and106-11share the enclosure part107-4and the loudspeaker109-4.

The connecting tubes108-3and108-6are provided at the enclosure part107-5, and the loudspeaker109-5is arranged in the enclosure part107-5. The sound wave source106-3shown inFIG.13Ais implemented by the enclosure part107-5, the connecting tube108-3, and the loudspeaker109-5. The sound wave source106-6shown inFIG.13Ais implemented by the enclosure part107-5, the connecting tube108-6, and the loudspeaker109-5. The sound wave sources106-3and106-6share the enclosure part107-5and the loudspeaker109-5.

The connecting tubes108-9and108-12are provided at the enclosure part107-6, and the loudspeaker109-6is arranged in the enclosure part107-6. The sound wave source106-9shown inFIG.13Ais implemented by the enclosure part107-6, the connecting tube108-9, and the loudspeaker109-6. The sound wave source106-12shown inFIG.13Ais implemented by the enclosure part107-6, the connecting tube108-12, and the loudspeaker109-6. The sound wave sources106-9and106-12share the enclosure part107-6and the loudspeaker109-6.

In this manner, the 12 sound wave sources106-1to106-12are implemented by the six loudspeakers109-1to109-6.

For M=6 and N=18, 18 sound wave sources106can be implemented by nine loudspeakers109, as shown inFIG.16. For M=8 and N=24, 24 sound wave sources106can be implemented by 12 loudspeakers109, as shown inFIG.17.

The arrangements shown inFIGS.15,16, and17are merely examples. As shown inFIG.18, a pair of sound wave sources106may be implemented by the loudspeaker109and two tubes114. The two tubes114have the same length.

In the example shown inFIG.1, the sound emitting apparatus100includes one hollow tube102. The sound emitting apparatus100may include two hollow tubes102.

FIG.19schematically shows the sound emitting apparatus100according to an embodiment. As shown inFIG.19, the sound emitting apparatus100includes hollow tubes102-1and102-2, and the sound wave sources106(not shown inFIG.19). InFIG.19, to discriminate the hollow tubes102-1and102-2, the hollow tube102-1is indicated by a solid line, and the hollow tube102-2is indicated by a broken line.

The hollow tubes102-1and102-2have a double helical structure. The hollow tube102-1has a plurality of openings104-1, and the openings104-1are formed on the inner side of the hollow tube102-1. The hollow tube102-2has a plurality of openings104-2, and the openings104-2are formed on the outer side of the hollow tube102-2.

The sound wave sources106are provided for each of the hollow tubes102-1and102-2. In the case of a dipole sound source, that is, a case in which the sound wave output (inward sound wave output) of the hollow tube102-1coincides with the sound wave output (outward sound wave output) of the hollow tube102-2, the hollow tubes102-1and102-2may share the sound wave sources106.

Since the hollow tubes102-1and102-2are provided in the double helical structure, the number of rotational sound sources can be increased without increasing the space.

In the example shown inFIG.1, the openings104are formed toward the fan200(on the inner side of the hollow tube102-1). The openings104may be formed in another direction.

FIG.20schematically shows the sound emitting apparatus100according to an embodiment. As shown inFIG.20, the sound emitting apparatus100includes the hollow tube102and the sound wave sources106(not shown inFIG.20). The hollow tube102has a plurality of openings104, and the openings104are formed on the upper side of the hollow tube102.

FIGS.21A and212are top and bottom views schematically showing the sound emitting apparatus100according to an embodiment. As shown inFIGS.21A and21B, the sound emitting apparatus100includes the hollow tubes102-1and102-2, and the sound wave sources106(not shown inFIGS.21A and21B). InFIGS.21A and21B, the hollow tube102-1is indicated by a solid line, and the hollow tube102-2is indicated by a broken line. The hollow tubes102-1and102-2have a double helical structure. The hollow tube102-1has a plurality of openings104-1, and the openings104-1are formed on the upper side of the hollow tube102-1, as shown inFIG.21A. The hollow tube102-2has a plurality of openings104-2, and the openings104-2are formed on the lower side of the hollow tube102-2, as shown inFIG.21B.

[Lobe Mode Separation and Sound Collection]

A method of separating the Lobe mode and collecting a sound will be explained.

FIG.22schematically shows an example of the arrangement of a sound collection device300according to an embodiment. As shown inFIG.22, the sound collection device300includes Nb microphones304each corresponding to a transducer that converts a sound into an electric signal. The microphones304are arranged at predetermined angular intervals. In the example shown inFIG.22, the sound collection device300includes 10 microphones304-1to304-10, and the microphones304-1to304-10are arranged at angular intervals of 36°. The branch numbers are sequentially assigned in a direction opposite to the rotation direction of the Lobe mode indicated by the arrow.

FIG.23schematically shows an example of a processing circuit included in the sound collection device300. In the example shown inFIG.23, the processing circuit is configured to extract a signal related to the Mth-order Lobe mode, and includes Nb phase shifters306-1to306-Nb, an adder307, and an amplifier308having a gain of 1/Nb, where Nb is the number of microphones304. InFIG.23, a signal ei indicates an output signal of the microphone304-i. The phase shifter306-iapplies the phase shift −Mϕito the signal ei, where ϕi=2π(i−1)/Nb. The output signals of the phase shifters306-1to306-Nb are added by the adder307, and the output signal of the adder307is amplified (reduced to 1/Nb) by the amplifier308. An output signal e of the amplifier308is a signal related to the Mth-order Lobe mode.

FIG.24schematically shows another example of the processing circuit included in the sound collection device300. In the example shown inFIG.24, the processing circuit is configured to extract a signal related to the Mth-order Lobe mode, and includes Nb delay units309-1to309-Nb, the adder307, and the amplifier308having a gain of 1/Nb. The delay unit309-idelays the signal ei by a time Mϕi/2πf, where ϕi=2π(i−1)/Nb. The output signals of the delay units309-1to309-Nb are added by the adder307, and the output signal of the adder307is amplified (reduced to 1/Nb) by the amplifier308. The output signal e of the amplifier308is a signal related to the Mth-order Lobe mode.

By preparing a plurality of processing circuits as shown inFIG.23or24, a plurality of Lobe modes can be separated.

When extracting a signal related to the Mth-order Lobe mode, the number of phase shifters can be reduced using 3M or 4M microphones304. For example, a processing circuit including two phase shifters can extract a signal related to the Mth-order Lobe mode from a signal obtained by the 3M microphones304arranged at regular intervals. Also, a processing circuit including one phase shifter can extract a signal related to the Mth-order Lobe mode from a signal obtained by the 4M microphones304arranged at regular intervals.

Next, an example in which the sound emitting apparatus according to the embodiment is applied to blade noise reduction will be described.

FIG.25schematically shows the outer appearance of a blade noise reduction apparatus400according to an embodiment. As shown inFIG.25, the blade noise reduction apparatus400includes a sound emitting apparatus402and a sound collection device404. The sound emitting apparatus402may be the sound emitting apparatus100shown inFIGS.1and2. The sound emitting apparatus402includes a hollow tube102and sound wave sources106. The sound collection device404may be the sound collection device300shown inFIG.22. The sound collection device404includes microphones304.

A fan200corresponding to a noise source is arranged inside the sound emitting apparatus402, and the microphones304are arranged outside the sound emitting apparatus402. When the noise source is only the rotational blades and the influence of environmental reflection is low, one microphone is sufficient. In other cases, it is desirable to use 2M+1 or more microphones.

FIG.26schematically shows an example of a control circuit of the blade noise reduction apparatus400. In the example shown inFIG.26, the control circuit is based on feedforward active noise control (ANC). In the feedforward ANC, a blade-passing pulse signal or a blade drive current signal is used as a reference signal. The blade-passing pulse signal is a signal in which the timing when a rotational blade passes through a certain point is recorded, and is, for example, a signal in which the presence or absence of the blade is output by 0/1 using an optical sensor. The blade drive current signal is a current signal for driving the fan200. For example, the blade drive current signal is a current signal applied to a motor that rotates the fan200.

InFIG.26, a signal r is a reference signal. A signal u is a drive signal for driving the sound wave sources106to emit a control sound for reducing the noise generated in the target Lobe mode. A control filter K is an adaptive filter that converts the reference signal r into the drive signal u. The drive signal u is sent to the sound wave sources106through the drive circuit152as shown in, for example,FIG.8or9. A signal e is an error signal obtained by the sound collection device404. Specifically, the error signal e is obtained by combining the output signals of the microphones304by the processing circuit as shown inFIG.23or24.

A signal x is an auxiliary signal and is obtained by converting the reference signal r by a filter having a secondary path characteristic C. The secondary path characteristic C is a transmission characteristic from the drive signal u to the error signal e when no noise is generated. A signal udis an auxiliary signal, and is obtained by subtracting, from a signal obtained by converting the auxiliary signal x by the control filter K, a signal obtained by converting the drive signal u by a filter having the secondary path characteristic C.

A control circuit452generates the drive signal u based on the error signal e and the reference signal r. As an ANC algorithm, a known ANC algorithm such as normal Filtered-X or input constraint can be used. Therefore, a detailed description of generation of the drive signal u will be omitted.

In the normal Filtered-X, the control filter K is updated to minimize evaluation function J(t):
J(t)=e2(t)
where e(t) is the error signal at time t.

In this case, the update rule of the control filter K is derived into:

K⁡(t+1)=K⁡(t)-2⁢μ⁢e⁡(t)⁢ϕx❘"\[LeftBracketingBar]"ϕx❘"\[RightBracketingBar]"2+β(6)
where μ is the step size in the gradient descent, β is an arbitrary numerical value (>0), for example, 0.01, K(t) is the control filter K at the time t, and ϕxis time-series data of the auxiliary signal x. The control circuit452updates the control filter K based on the update rule of equation (6).

In the input constraint, the control filter K is updated to minimize the evaluation function J(t):
J(t)=e2(t)+αud2(t)
where α is a variable from 0 to 1 that determines the degree of input constraint (no constraint for α=0 and the input constraint becomes larger as a approaches 1) (no constraint for α=0 and the input constraint becomes larger as a approaches 1), and ud(t) is the auxiliary signal udat the time t.

In this case, the update rule of the control filter K is derived into:

The control circuit452updates the control filter K based on the update rule of equation (7).

FIG.27schematically shows another example of the control circuit452of the blade noise reduction apparatus400. In the example shown inFIG.27, the control circuit452is based on feedback ANC. A detailed description of parts similar to those of the feedforward ANC will be omitted.

InFIG.27, the error signal e is obtained by processing the error signal obtained by the sound collection device404by a bandpass filter. The bandpass filter is configured to extract a signal of a frequency band including a target frequency. The signal r is obtained by subtracting, from the error signal e, a signal obtained by converting the drive signal u by a filter having the secondary path characteristic C, and delaying the obtained signal by a predetermined time. The drive signal u is obtained by converting the signal r by the control filter K. The signal x is an auxiliary signal and is obtained by converting the signal r by a filter having the secondary path characteristic C. The signal udis an auxiliary signal, and is obtained by subtracting, from a signal obtained by converting the auxiliary signal x by the control filter K, a signal obtained by converting the drive signal u by a filter having the secondary path characteristic C.

The control circuit452updates the control filter K based on the update rule of equation (6) or (7) described above.

When a phase difference between signals is obtained by delaying the signals by the delay units in the sound emitting apparatus402and/or the sound collection device404, the delay time needs to be set again every time the target frequency changes. If the delay time changes, the secondary path characteristic changes, so the change in secondary path characteristic needs to be estimated. The estimation can be performed by calculation, database extraction, or online estimation.

To the contrary, when a phase difference between signals is obtained by applying the phase shift to the signals by the phase shifter in each of the sound emitting apparatus402and the sound collection device404, no phase shift amount need be set again even upon a change in target frequency. Hence, no complicated processing is required. Since no secondary path characteristic changes, the use of a complicated ANC algorithm can be avoided. This is a great advantage in feedback ANC in which it is difficult to apply online estimation.

A frequency fi of the blade noise can be expressed by:

fi=Bx⁢Ω2⁢π
where B is the number of blades, Ω is the blade rotation speed [rad/s], and x is the order of the Lobe mode.

When the blades include only rotational blades, there is one Lobe mode for one frequency fi. When the blades include rotational and stationary blades, there are M0Lobe modes for one frequency fi, where M0=Bx−pV, V is the number of stationary blades, and p is an integer.

The blade noise includes noises generated in many Lobe modes.

Since the mode separation is executed by frequency separation, the number of microphones may be one when the blades include only rotational blades. However, in an actual environment, there is the influence of environmental reflection. Therefore, mode separation processing using 2M+1 or more microphones is required.

When L Lobe modes (fi, Mi) are driven, the blade noise reduction apparatus400includes L sound emitting apparatuses402and L control circuits, where L is an integer of 2 or more. The Lobe mode (fi, Mi) represents a Mith-order Lobe mode having the frequency fi. For example, each sound emitting apparatus402includes the hollow tube102having a tube line length dependent on the corresponding target frequency fi. Each control circuit may be the control circuit as shown inFIG.26or27. An error signal input to each control circuit is a signal related to the corresponding Lobe mode (fi, Mi).

The blade noise reduction apparatus according to the embodiment may use a passive sound absorber together with the sound emitting apparatus.

FIG.28schematically shows a blade noise reduction apparatus410according to an embodiment. As shown inFIG.28, the blade noise reduction apparatus410includes sound emitting apparatuses411-1,411-2,411-3, and411-4, a passive sound absorber412, and a cover414. The cover414covers the sound emitting apparatuses411-1,411-2,411-3, and411-4and the passive sound absorber412to prevent the disturbance of the flow of the rotational blades included in the fan200.

Each of the sound emitting apparatuses411-1,411-2,411-3, and411-4can have, for example, a configuration similar to that of the sound emitting apparatus100shown inFIGS.1and2. The sound emitting apparatuses411-1and411-4are configured to drive a Lobe mode (fa, Ma), and the sound emitting apparatuses411-2and411-3are configured to drive a Lobe mode (fb, Mb).

The passive sound absorber412includes a plurality of sound absorbers413arranged to surround the fan200. Each sound absorber413includes a Helmholtz resonator. The passive sound absorber412is configured to reduce noise in, for example, the 0th-order Lobe mode.

A slit sound absorber may be used as the sound absorber413. When the sound absorber413is the slit type, the slit of the sound absorber413may be curved for space saving, as shown inFIG.29.

A combination of the sound emitting apparatuses and the passive sound absorber according to the embodiment can more effectively reduce noise generated by the fan200.

The hollow tube102is not limited to a helical hollow tube.FIG.30schematically shows part of the sound emitting apparatus100when viewed from the center (fan200). As shown inFIG.30, the hollow tube102may be a hollow tube zigzagged at turns. Since a sound wave is reflected at the turns, the curvature of the curve is minimized.