Patent Publication Number: US-2023142560-A1

Title: Speaker

Description:
TECHNICAL FIELD 
     The present disclosure relates to a speaker. 
     BACKGROUND ART 
     For example, Patent Document  1  discloses a speaker including a dome-shaped diaphragm and a phase plug disposed in front of a front surface of the diaphragm. A phase of a sound wave generated from a part of the diaphragm facing the phase plug is matched with a phase of a sound wave generated from another part of the diaphragm. Accordingly, a decrease in sound pressure is suppressed, and sound quality is improved. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document  1 : U.S. Patent No.  5875252   
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     An object of the present disclosure is to further improve sound quality in a speaker including a dome-shaped diaphragm and a phase plug disposed in front of the diaphragm, 
     MEANS FOR SOLVING THE PROBLEMS 
     In order to solve the above problem, according to one aspect of the present disclosure, provided is a speaker including: a diaphragm having a dome shape curved forward in a protruding manner; a tubular member having the diaphragm disposed inside, the tubular member having an inner diameter increasing toward a front; and a phase plug disposed in front of a front surface of the diaphragm. The phase plug includes: a front surface facing forward, a rear surface facing a central portion on a front surface of the diaphragm at a constant interval in parallel, and a side surface connecting the front surface and the rear surface. The front surface of the phase plug is larger than a rear surface of the phase plug as viewed in a front-rear direction of a speaker. At least a part of the side surface of the phase plug has an inclined surface that extends outward while extending forward, faces an inner circumferential surface of the tubular member and a front surface of the diaphragm, and directs a sound wave generated from the diaphragm toward an inner circumferential surface of the tubular member. 
     In addition, according to another aspect of the present disclosure, provided is a speaker including: a diaphragm having a dome shape curved forward in a protruding manner; and a phase plug disposed in front of a front surface of the diaphragm. The phase plug includes: a front surface facing forward, a rear surface facing a central portion on a front surface of the diaphragm at a constant interval in parallel, and a side surface connecting the front surface and the rear surface. The phase plug includes a plurality of ribs on the side surface. Each of the plurality of ribs includes a facing surface that faces a front surface of the diaphragm at the constant interval in parallel. Each of the plurality of ribs protrudes outward beyond an outer circumferential edge of the front surface as viewed in a front-rear direction of a speaker, 
     Effects of the Invention 
     According to the present disclosure, sound quality can be further improved in a speaker including a dome-shaped diaphragm and a phase plug disposed in front of the diaphragm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic perspective view showing the inside of a speaker according to a first embodiment of the present disclosure. 
         FIG.  2    is a cross-sectional view of a sound generation unit of the speaker according to the first embodiment. 
         FIG.  3    is an exploded view of the sound generation unit of the speaker according to the first embodiment. 
         FIG.  4    is a partially enlarged cross-sectional view of he speaker including a phase plug according to the first embodiment. 
         FIG.  5    is a front perspective view of the phase plug according to the first embodiment. 
         FIG.  6    is a rear perspective view of he phase plug according to the first embodiment. 
         FIG.  7    is a &amp;de view of the phase plug according to the first embodiment. 
         FIG.  8    is a rear view of the phase plug according to the first embodiment. 
         FIG.  9    is a schematic diagram of a speaker showing a first problem occurring in a speaker of a comparative example in which a phase plug is not provided. 
         FIG.  10    is a schematic diagram of a speaker showing a second problem occurring in a speaker of a comparative example in which a phase plug is not provided. 
         FIG.  11    is a diagram showing sound pressure-frequency characteristics of a speaker of a comparative example in which a phase plug is not provided. 
         FIG.  12    is a schematic diagram of a speaker showing effects due to the rear surface of the phase plug according to the first embodiment. 
         FIG.  13    is a schematic diagram of a speaker showing effects due to the side surface of the phase plug according to the first embodiment. 
         FIG.  14 A  is a sound pressure distribution diagram in the speaker of the first embodiment. 
         FIG.  14 B  is a sound pressure distribution diagram in a speaker of a comparative example including a dish-shaped phase plug. 
         FIG.  15    is a diagram showing simulation values of the sound pressure-frequency characteristics (solid line) of the speaker according to the first embodiment and the sound pressure-frequency characteristics (one-dot chain line) of the speaker of the comparative example not including the phase plug. 
         FIG.  16 A  is a diagram showing propagation of a wave front in the speaker of the first embodiment. 
         FIG.  16 B  is a diagram showing propagation of a wave front in a speaker of a comparative example including a flat plate-shaped phase plug. 
       FIG,  17  is a diagram showing simulation values of the sound pressure-frequency characteristics (solid line) of a speaker including a phase plug including a plurality of ribs and the sound pressure-frequency characteristics (one-dot chain line) of a speaker including a phase plug not including a plurality of ribs. 
         FIG.  18    is a partially enlarged cross-sectional view of a speaker including a phase plug according to a second embodiment. 
         FIG.  19    is a front perspective view of the phase plug according to the second embodiment. 
         FIG.  20    is a rear perspective view of the phase plug according to the second embodiment. 
         FIG.  21    is a side view of the phase plug according to the second embodiment. 
         FIG.  22    is a rear view of the phase plug according to the second embodiment. 
         FIG.  23    is a partially enlarged cross-sectional view of a speaker including a phase plug according to a third embodiment. 
         FIG.  24    is a partially enlarged cross-sectional view of a speaker including a phase plug according to another embodiment. 
         FIG.  25    is a partially enlarged cross-sectional view of a speaker including a phase plug according to still another embodiment. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments will be described in detail with reference to the drawings as appropriate. However, a detailed description more than necessary may be omitted. For example, a detailed description of already well-known matters and a redundant description of substantially the same configuration may be omitted. This is to avoid the unnecessary redundancy of the following description and to facilitate understanding by those skilled in the art. 
     It should be noted that the inventors provide the accompanying drawings and the following description in order for those skilled in the art to fully understand the present disclosure, and do not intend to limit the subject matter described in the claims by the accompanying drawings and the following description. 
     First Embodiment 
       FIG.  1    is a schematic perspective view showing the inside of a speaker according to a first embodiment of the present disclosure.  FIG.  2    is a cross-sectional view of the sound generation unit of the speaker according to the first embodiment.  FIG.  3    is an exploded view of the sound generating unit. 
     As shown in  FIG.  1   , the speaker  10  includes an enclosure  12  and a sound generation unit  14  provided in the enclosure  12 . In the case of the first embodiment, the speaker  10  is a coaxial speaker, and as shown in  FIGS.  2  and  3   , the sound generation unit  14  includes a woofer  16  and a tweeter  18 . In addition, the sound generation unit  14  includes a frame  20  supporting the woofer  16 , and a magnetic circuit  22  supporting the tweeter  18  and the frame  20 . Furthermore, the sound generation unit  14  includes a phase plug  24 . 
     In the case of the first embodiment, the woofer  16  includes a diaphragm  26 , an edge for fixing a front end of the diaphragm  26  to an annular front end portion  20 a of the frame  20 , a cylindrical voice coil bobbin  30  extending from the diaphragm  26  toward the rear of the speaker  10 , a voice coil  32  provided on the voice coil bobbin  30 , a damper  34  supporting the voice coil bobbin  30  so as to be vibrated, and a magnet  36 . 
     The diaphragm  26  of the woofer  16  is a tubular member whose inner diameter increases toward the front of the speaker  10 , for example, a truncated conical member. The diaphragm  26  is supported on the frame  20  through the edge  28  and the damper  34  in a vibratable manner so that the central axis of the shape of the diaphragm  26  coincides with the central axis C of the speaker  10  extending in the front-rear direction of the speaker  10 . Flow of a current through the voice coil  32  disposed adjacent to the magnet  36  for the woofer  16  disposed in the magnetic circuit  22  causes the voice coil bobbin  30  to vibrate in the front-rear direction. Accordingly, the diaphragm  26  of the woofer  16  connected to the voice coil bobbin  30  vibrates to generate a sound wave. 
     The tweeter  18  includes a diaphragm  38 , an edge  40  supporting the diaphragm  38 , a cylindrical voice coil bobbin  42  extending from the diaphragm  38  toward the rear of the speaker  10 , a voice coil  44  provided on the voice coil bobbin  42 , and a magnet  46 . 
     The diaphragm  38  of the tweeter  18  is a dome-shaped member curved forward in a protruding manner. For example, the diaphragm  38  has a hemispherical shape. The diaphragm  38  is provided in a vibratable manner on the top surface of the cylindrical portion  22 a of the magnetic circuit  22  through the edge  40  so that the central axis of the shape of the diaphragm  38  coincides with the central axis C of the speaker  10 . As a result, the diaphragm  26  of the woofer  16  and the diaphragm  38  of the tweeter  18  are disposed coaxially (on the central axis C). It should be noted that the cylindrical portion  22 a of the magnetic circuit  22  is disposed in the voice coil bobbin  30  of the woofer  16 . Accordingly, the diaphragm  38  of the tweeter  18  is disposed in the diaphragm  26  of the woofer  16 . Flow of a current through the voice coil  44  disposed adjacent to the magnet  46  for the tweeter  18  disposed in the magnetic circuit  22  causes the voice coil bobbin  42  to vibrate in the front-rear direction. Accordingly, the diaphragm  38  of the tweeter  18  connected to the voice coil bobbin  42  vibrates to generate a sound wave. 
     The phase plug  24  is disposed in front of the diaphragm  38  of the tweeter  18 . 
       FIG.  4    is a partially enlarged cross-sectional view of the speaker including the phase plug according to the first embodiment. In addition, FIGS.  5  to  8  are a front perspective view, a rear perspective view, a side view, and a rear view of the phase plug according to the first embodiment. 
     As shown in  FIGS.  4  to  8   , the phase plug  24  includes a front surface  24   a  facing forward, a rear surface  24   b  facing the front surface  38   a  of the diaphragm  38  of the tweeter  18 , and a side surface  24   c  connecting the front surface  24   a  and the rear surface  24   b . In addition, the phase plug  24  includes an annular attachment portion  24   d  attached to the top surface of the cylindrical portion  22 a of the magnetic circuit  22 , and a plurality of leg portions  24   e  connecting the side surface  24   c  and the attachment portion  24   d . The attachment portion  24   d  is an annular portion positioned outside the diaphragm  38  of the tweeter  18  and surrounding the diaphragm  38  as viewed in the front-rear direction of speaker  10 . The leg portion  24   e  extends from the attachment portion  24   d  and supports the main portions (front surface  24   a , rear surface  24   b , and side surface  24   c ) of the phase plug  24  in front of the diaphragm  38  of the tweeter  18 . It should be noted that the leg portion  24   e  is spaced apart from the front surface  38   a  of the diaphragm  38  as compared with the rear surface  24   b  and a facing surface of a rib described below so as not to affect sound quality. 
     The front surface  24   a  of the phase plug  24  is a portion of the surface viewed from the front of the speaker  10 , and has a circular shape centered on the central axis C as viewed in the front-rear direction of the speaker  10 . In addition, in the case of the present first embodiment, the front surface  24   a  is a conical protruding surface as viewed in a direction orthogonal to the central axis C. The conical front surface  24   a  has a conical surface curved inward in a protruding manner. In addition, as viewed in the front-rear direction, the size of the front surface  24   a  is substantially equal to the size of the diaphragm  38  of the tweeter  18 . The role of the front surface  24   a  of the phase plug  24  including this conical protruding surface will be described below. 
     The rear surface  24   b  of the phase plug  24  is a portion of a surface of the phase plug  24  facing a central portion of the front surface  28   a  of the diaphragm  38  of the tweeter  18  at a constant interval din parallel, The interval d is, for example, 0.5 mm, The role of the rear surface  24   b  of the phase plug  24  will be described below. 
     The side surface  24   c  of the phase plug  24  is a portion of a surface of the phase plug  24  that connects the front surface  24   a  and the rear surface  24   b . In the case of the present first embodiment, the front surface  24   a  of the phase plug  24  is larger than the rear surface  24   b  thereof as viewed in the front-rear direction of the speaker  10 . Therefore, the side surface  24   a  of the phase plug  24  at least partially includes an inclined surface  241  extending outward while extending forward. As a result, the inclined surface  24   f  faces a portion of the front surface  38   a  of the diaphragm  38  not facing the rear surface  24   b  in the front-rear direction of the speaker  10 , and faces the inner circumferential surface  26   a  of the diaphragm  26  of the woofer  16  in a direction orthogonal to the front-rear direction. It should be noted that in the case of the first embodiment, the inclined surface  24   f  is a part of the side surface  24   c , and is connected to the front surface  24   a . In addition, in the case of the present first embodiment, the inclined surface  24   f  is a curved surface curved outward in a protruding manner as viewed in a direction orthogonal to the front-rear direction. The role of the side surface  24   c  of the phase plug  24  including this inclined surface  24   f  will be described below. 
     Furthermore, in the case of the present first embodiment, the phase plug  24  includes a plurality of ribs  24   g  protruding rearward from the side surface  24   c  toward the front surface  38   a  of the diaphragm  38 . In addition, the plurality of ribs  24   g  are plate-shaped protrusions supported by the side surface  24   c  in a cantilever manner, and extend in the radiation direction with respect to the central axis C as viewed in the front-rear direction of the speaker  10 . Furthermore, each of the plurality of ribs  24   g  includes a facing surface  24   h  that faces the front surface  38   a  of the diaphragm  38  at a constant interval d in parallel. The facing surface  24   h  and the rear surface  24   b  are continuous curved surfaces. It should be noted that in the case of the present first embodiment, the plurality of ribs  24   g  do not protrude outward beyond the outer circumferential edge of the front surface  24   a  as viewed in the front-rear direction of the speaker  10 . 
     In the case of the present first embodiment, each of the plurality of ribs  24   g  has a shape whose thickness increases toward the front of the speaker  10 . That is, as shown in FIG,  7 , the wall thickness t 1  on the front side of the plurality of ribs  24   g  is larger than the wall thickness t 2  on the rear side (diaphragm  38  side). 
     In addition, in the case of the present first embodiment, the facing surface  24   h  of each of the plurality of ribs  24   g  has a shape in which the width decreases toward the outside as viewed in the front-rear direction of the speaker  10 . That , as shown in  FIG.  8   , as viewed in the front-rear direction of the speaker  10 , the width w 1  on the center side is larger than the width w 2  on the outer side. 
     The role of these plurality of ribs  24   g  will be described below. 
     Hereinafter, the role and effects of the phase plug  24  having the above-described features will be described. It should be noted that before describing the role and effects of the phase plug  24 , a problem that occurs when the phase plug  24  is not present will be described as a reference. 
       FIG.  9    is a schematic diagram of a speaker showing a first problem occurring in a speaker of a comparative example in which a phase plug is not provided. In addition,  FIG.  10    is a schematic diagram of a speaker showing a second problem occurring in the speaker of the comparative example. 
     As shown in  FIG.  9   , in the case of the speaker of the comparative example without the phase plug, the sound wave propagating forward from the dome-shaped diaphragm of the tweeter has a reaching distance different depending on the position where the sound wave is generated. For example, a reaching distance difference ΔL 1  is generated between the sound wave SW 1  generated from the central portion of the tweeter diaphragm and the sound wave SW 2  generated from the outer circumferential portion of the tweeter diaphragm. The ΔL 1  corresponds to the height H of the dome-shaped diaphragm. 
     Due to the reaching distance difference ΔL 1  a phase shift close to 180 degrees occurs at a certain frequency at the measurement position in front of the speaker, and the sound pressure level at the frequency decreases. As a result, a user in front of the speaker may feel insufficient sound pressure in a certain sound range. 
     In addition, as shown in  FIG.  10   , the sound wave generated from the diaphragm of the tweeter has a reaching distance different depending on whether to be reflected by the diaphragm of the woofer. For example, a reaching distance difference ΔL 2  is generated between the sound wave SW 1  generated from the central portion of the diaphragm of the tweeter and the sound wave SW 3  propagated from the diaphragm of the tweeter toward the diaphragm of the woofer and reflected by the diaphragm of the woofer, The reaching distance difference ΔL 2  is determined by the inclination angle of the diaphragm of the woofer with respect to the front-rear direction of the speaker. 
     Due to the reaching distance difference ΔL 2 , a phase shift close to 180 degrees occurs at a certain frequency at the measurement position in front of the speaker, and the sound pressure level at the frequency decreases. As a result, a user in front of the speaker may feel insufficient sound pressure in a certain sound range. 
     FIG,  11  is a diagram showing sound pressure-frequency characteristics of a speaker of a comparative example in which a phase plug is not provided. It should be noted that in  FIG.  11   , the solid line indicates a measured value, and the one-clot chain line indicates a simulation value. 
     As shown in  FIG.  11   , in the case of the speaker of the comparative example without the phase plug, a dip occurs in the sound range A of 8000 to 9000 Hz, and a decrease in the sound pressure level occurs in the high sound range of 16000 Hz or more. It should be noted that “dip” means that the sound pressure level in a certain sound range is lower than the sound pressure level in the surrounding sound range. Thus, when there is no phase plug, a decrease in the sound pressure level occurs in some frequency bands, and as a result, the frequency characteristics of the speaker deteriorate. The dip in the sound range A is caused by the second problem shown in  FIG.  10   , and the decrease in the sound pressure level in the sound range B is caused by the first problem shown in  FIG.  9   . It should be noted that a general human audible range is a range of 20 to 20000 Hz. 
     In order to suppress such deterioration in sound quality, the phase plug  24  in the speaker  10  of the present first embodiment has a plurality of features as described above and as shown in  FIG.  4   . 
     First, effects of the rear surface  24   b  of the phase plug  24  will be described with reference to  FIG.  12   . 
       FIG.  12    is a schematic diagram of a speaker showing effects due to the rear surface of the phase plug according to the first embodiment. 
     As described above, and as shown in  FIG.  4   , the rear surface  24   b  of the phase plug  24  faces a central portion of the front surface  38   a  of the diaphragm  38  of the tweeter  18  at a constant interval d in parallel. As a result, as shown in FIG,  12 , the sound wave SW 1  generated from the central portion on the front surface of the diaphragm  38  of the tweeter  18  facing the rear surface  24   b  of the phase plug  24  propagates outward through the gap between the phase plug  24  and the diaphragm  38 , and propagates forward after exiting from the gap. As a result, the first problem shown in  FIG.  9    can be solved, that is, a reaching distance difference between the sound wave SW 1  generated from the central portion facing the phase plug and the sound wave SW 2  generated from the outer portion not facing the phase plug becomes small. 
     Next, effects due to the side surface  24   c  of the phase plug  24  will be described with reference to  FIGS.  13 ,  14 A, and  14 B . 
       FIG.  13    is a schematic diagram of a speaker showing effects due to the side surface of the phase plug according to the first embodiment. In addition,  FIG.  14 A  is a sound pressure distribution diagram in the speaker of the first embodiment. Furthermore,  FIG.  14 B  is a sound pressure distribution diagram in a speaker of a comparative example including a dish-shaped phase plug. It should be noted that in  FIGS.  14 A and  14 B , broken lines indicate boundaries between different sound pressure levels. 
     As shown in  FIG.  4   , the front surface  24   a  of the phase plug  24  is larger than the rear surface  24   b  thereof as viewed in the front-rear direction of the speaker  10 . In addition, therefore, the side surface  24   c  of the phase plug  24  at least partially includes an inclined surface  24   f  extending outward while extending forward. In addition, in the case of the present first embodiment, the inclined surface  24   f  is a curved surface curved outward in a protruding manner as viewed in a direction orthogonal to the front-rear direction of the speaker  10 . 
     With the side surface  24   c  of this phase plug  24 , as shown in  FIG.  13   , the sound waves generated from different positions of the diaphragm  38  of the tweeter  18  are directed not forward but toward the inner circumferential surface  26   a  of the diaphragm  26  of the woofer  16 . Accordingly, the sound waves from the diaphragm  38  of the tweeter  18  propagate toward the inner circumferential surface  26   a  of the diaphragm  26  of the woofer  16 , and then propagate forward along the inner circumferential surface  26   a . That is, both the sound wave generated from the central portion of the diaphragm  38  of the tweeter  18  and the sound wave generated from the outer portion propagate along substantially the same path. 
     In addition, as can be seen from the sound pressure distribution diagram shown in  FIG.  14 A , due to the side surface  24   c  of the phase plug  24 , the sound wave is suppressed (delayed) in diffraction toward the front of the phase plug. As a result, the sound wave propagates along the diaphragm of the woofer while maintaining the sound pressure level. On the other hand, in the case of the comparative example shown in  FIG.  14 B , since the phase plug has a thin dish shape, the sound wave is diffracted toward the front of the phase plug. As a result, the sound wave propagates along the inner circumferential surface of the diaphragm of the woofer while being decreased in the sound pressure level. 
       FIG.  15    shows simulation values of the sound pressure-frequency characteristics (solid line) of the speaker of the first embodiment and the sound pressure-frequency characteristics (one-dot chain line) of the speaker of the comparative example not including the phase plug. 
     As shown in  FIG.  15   , due to the side surface  24   c  of the phase plug  24 , occurrence of a dip in the sound range A of 8000 to 9000 Hz is suppressed. As a result, the sound quality is improved, and in particular, the color of the sound is improved. 
     It should be noted that in the case of the present first embodiment, as shown in  FIG.  4   , the inclined surface  24   f  on the side surface  24   c  of the phase plug  24  is a curved surface curved outward in a protruding manner as viewed in a direction orthogonal to the front-rear direction of the speaker  10 , but the present invention is not limited thereto. The inclined surface  24   f  may be linear as viewed in a direction orthogonal to the front-rear direction of the speaker  10 . It should be noted that in order to suppress the diffraction of the sound wave toward the front of the phase plug (to delay the occurrence of the diffraction), a curved surface is preferable. In addition, the entire side surface  24   c  may be the inclined surface  24   f.    
     In addition, in the case of the present first embodiment, as shown in  FIG.  4   , the side surface  24   c  of the phase plug  24  includes a wall surface  24   i  extending at least in the front-rear direction of the speaker  10  from the rear surface  24   b , and an eaves-soffit-shaped wall surface  24   j  extending at least outward from the wall surface  24   i  and connected to the inclined surface  24   f . In the case of the present first embodiment, the wall surface  24   i  extends outward while extending forward from the rear surface  24   b  toward the wall surface  24   j , and the wall surface  24   j  extends rearward while extending outward from the wall surface  24   i  toward the inclined surface  24   f . The sound wave exiting from between the rear surface  24   b  and the diaphragm  38  travels along the wall surface  24   i  from the outer circumferential end of the rear surface  24   b , and then travels along the wall surface  24   j  from the front end of the wall surface  24   i.    
     The sound wave traveling along the wall surface  24   i  precedes the sound wave traveling along the diaphragm  26  of the woofer  16 . However, the sound wave traveling along the wall surface  24   i  then travels outward along the wall surface  24   i.  Meanwhile, the sound wave traveling along the diaphragm  26  of the woofer  16  catches up. Specifically, at the timing when the sound wave traveling along the wall surface  24   j  reaches the inclined surface  24   f , the sound wave traveling along the diaphragm  26  of the woofer  16  reaches the substantially same front-rear direction position as the connection place between the wall surface  24   j  and the inclined surface  24   f . Thereafter, the sound wave traveling along the phase plug  24  and the sound wave traveling along the diaphragm  26  of the woofer  16  travel forward in a state of being in phase. The path length of the sound wave traveling along the phase plug  24  can be adjusted by appropriately adjusting the shapes and lengths of the wall surfaces  24   i  and  24   j  positioned between the rear surface  24   b  and the inclined surface  24   f . As a result, the entire sound wave can travel toward the front of the speaker  10  with the phases in phase. 
     Next, effects due to the front surface  24   a  of the phase plug  24  will be described with reference to  FIGS.  16 A and  16 B . 
       FIG.  16 A  is a diagram showing propagation of a wave front in the speaker of the first embodiment. In addition,  FIG.  16 B  is a diagram showing propagation of a wave front in a speaker of a comparative example including a flat plate-shaped phase plug. It should be noted that, in  FIGS.  16 A and  16 B , two different wave fronts W 1  and W 2  are indicated by a one-dot chain line and a two-dot chain line, respectively, and a wave front W of a composite wave thereof is indicated by a broken line. 
     As shown in  FIG.  4   , in the case of the present first embodiment, the front surface  24   a  of the phase plug  24  is a conical protruding surface protruding forward as viewed in a direction orthogonal to the central axis C. The conical front surface  24   a  has a conical surface curved inward in a protruding manner, 
     According to the front surface of this phase plug, and according to the property that the sound wave propagates along the surface of the object so that the wave front is orthogonal to the surface of the object, when two different wave fronts W 1  and W 2  merge in front of the phase plug as shown in  FIG.  16 A , the wave front W of the composite wave becomes a wave front that uniformly curves forward in a protruding manner over the entire area. On the other hand, as shown in  FIG.  16 B , when the phase plug is plate-shaped, two different wave fronts W 1  and W 2  interfere on the front surface of the phase plug, and as a result, the wave front W of the composite wave has a concave wave front in a portion propagating in front of the phase plug. 
     The front surface  24   a  of this phase plug  24  suppresses a decrease in the sound pressure level in the high sound range B of  15000  Hz or more as shown in  FIG.  15   . As a result, sound quality is improved, and in particular, sound extension is improved. 
     Next, effects of the plurality of ribs  24   g  of the phase plug  24  will be described. 
     As described above, and as shown in  FIG.  4   , each of the plurality of ribs  24   g  extends in the radiation direction with respect to the center line C as viewed in the front-rear direction of the speaker  10 . Furthermore, each of the plurality of ribs  24   g  includes a facing surface  24   h  that faces the front surface  38   a  of the diaphragm  38  at a constant interval d in parallel. 
     Due to the plurality of ribs  24   g , sound waves generated from various places of the diaphragm  38  have a ratio of directly traveling forward increasing as generation positions thereof move away from the center, That is, as the generation position moves away from the center, the number of sound waves immediately propagating forward through the space between the plurality of ribs  24   g  increases, and the number of sound waves traveling forward after propagating through the gap between the facing surface  24   h  of the rib  24   g  and the diaphragm  38  decreases. In short, as the generation position is farther from the center, the number of short-circuiting sound waves increases and the number of detouring sound waves decreases. Specifically, the propagation path of the sound wave generated from the portion of the diaphragm  38  facing the facing surface  24   h  of the rib  24   g  is longer than the propagation path of the sound wave generated from the portion of the diaphragm  38  not facing the facing surface  24   h  and propagating directly forward because the sound wave travels forward after exiting the gap between the facing surface  24   h  and the diaphragm  38 . Since the length of the propagation path of the sound wave is variously different as described above, the sound pressure level at each frequency reaching the measurement position in front of the speaker is flattened. 
       FIG.  17    shows simulation values of the sound pressure-frequency characteristics (solid line) of a speaker including a phase plug including a plurality of ribs and the sound pressure-frequency characteristics (one-dot chain line) of a speaker including a phase plug not including a plurality of ribs. It should be noted that a phase plug not including a plurality of ribs indicated by the one-dot chain line is shown in  FIG.  24    described below. 
     As shown in  FIG.  17   , since the phase plug includes a plurality of ribs, the sound pressure level at each frequency reaching the measurement position in front of the speaker is flattened in the sound range of 30,000 Hz or less, and the sound quality is improved. 
     It should be noted that in the case of the present first embodiment, in order to finely adjust the degree of flattening of the sound pressure level and the sound pressure level at a specific frequency, as shown in FIGS,  7  and  8 , the wall thickness of each of the plurality of ribs  24   g  and the width of the facing surface  24   h  of each of the plurality of ribs  24   g  are finely adjusted. Since the wall thickness increases toward the front, the length of the propagation path of the sound wave propagating between the plurality of ribs  24   g , specifically, the sound wave propagating near the rib  24   g  becomes slightly larger than the sound wave propagating through the center between the ribs. In addition, since the width of the facing surface  24   h  decreases toward the outside, a part of the sound wave propagating through the gap between the facing surface  24   h  and the diaphragm  38  escapes to between the plurality of ribs before reaching the outer end of the facing surface  24   h.    
     According to the first embodiment as described above, the sound quality can be further improved in the speaker including the dome-shaped diaphragm and the phase plug disposed in front thereof. 
     Second Embodiment 
     In the case of the first embodiment described above, as shown in  FIG.  4   , the plurality of ribs  24   g  do not protrude outward beyond the outer circumferential edge of the front surface  24   a  of the phase plug  24  as viewed in the front-rear direction of the speaker  10 . Therefore, the sound wave propagating forward between the plurality of ribs  24   g  is reflected by the side surface  24   c  and propagates toward the inner circumferential surface  26   a  of the diaphragm  26  of the woofer  16 . Unlike this, in the speaker according to the present second embodiment, the sound wave propagating forward between the plurality of ribs  24   g  is output forward from the speaker as it is. Since being substantially the same except for this different point, the present second embodiment will be described focusing on the different point. 
       FIG.  18    is a partially enlarged cross-sectional view of the speaker including the phase plug according to the second embodiment. In addition,  FIGS.  19  to  22    are a front perspective view, a rear perspective view, a side view, and a rear view of the phase plug according to the second embodiment. 
     As shown in  FIGS.  18  to  22   , the phase plug  124  in the speaker according to the present second embodiment includes a front surface  124   a  facing forward, a rear surface  124   b  facing the front surface  138   a  of the diaphragm  138  of the tweeter, a side surface  124   c  connecting the front surface  124   a  and the rear surface  124   b , an annular attachment portion  124   d  attached to the magnetic circuit, and a plurality of leg portions  124   e  connecting the side surface  124   c  and the attachment portion  124   d.    
     In the phase plug  124  according to the present second embodiment, the front surface  124   a  and the rear surface  124   b  have substantially the same size as viewed in the front-rear direction of the speaker. Therefore, the side surface  124   c  extends outward while extending forward, and includes a flat inclined surface as viewed in a direction orthogonal to the front-rear direction. However, the inclined surface has a small inclination angle with respect to the central axis C of the speaker, and is substantially parallel to the central axis. Therefore, the side surface  124   c  of the phase plug  124  according to the present second embodiment hardly plays a role of directing the sound wave generated from the diaphragm  138  of the tweeter toward the inner circumferential surface  126   a  of the diaphragm  126  of the woofer, unlike the outward protruding curved-surface-shaped side surface  24   c  of the phase plug  24  according to the first embodiment described above. 
     In addition, as viewed in the front-rear direction of the speaker, the size of the front surface  124   a  of the phase plug  124  is smaller than that of the diaphragm  138 . Furthermore, the plurality of ribs  124   g  provided on the side surface  124   c  of the phase plug  124  protrude outward beyond the outer circumferential edge of the front surface  124   a  as viewed in the front-rear direction of the speaker, and do not substantially protrude in the direction from the side surface  124   c  toward the diaphragm  138 . Then, each of the plurality of ribs  124   g  includes a facing surface  124   h  that faces the front surface  138   a  of the diaphragm  138  at a constant interval in parallel. It should be noted that in the plurality of ribs  124   g , some ribs (three ribs in the case of the present second embodiment) are integrated with the plurality of leg portions  124   e  in an overlapping manner as viewed in the front-rear direction of the speaker. 
     According to the plurality of ribs  124   g , sound waves propagating forward between the plurality of ribs  124   g  (that is, a sound wave generated from a portion of the diaphragm  138  not facing the facing surface  124   h  of the rib  124   g  and a sound wave after generated from a portion of the diaphragm  138  facing the facing surface  124   h  and exiting the gap between the facing surface  124   h  and the diaphragm  138 ) propagate forward as they are while maintaining the sound pressure level without being substantially reflected toward the diaphragm  126  of the woofer by the side surface  124   c  of the phase plug  124 . Accordingly, a decrease in the sound pressure level is suppressed at the measurement position in front of the speaker. As a result, it is possible to complement the role of the side surface  24   c  of the phase plug  24  in the first embodiment described above, that is, the role of suppressing the occurrence of a dip in the sound range of 8000 to 9000 Hz, Therefore, according to the plurality of ribs  124   g , it is also possible to provide the side surface  124   c  of the phase plug  124  parallel to the central axis C of the speaker. 
     It should be noted that the configuration of the phase plug  124  in the second embodiment is effective when the diaphragm  126  of the woofer has a flat plate or a shape with a loose inclination angle, that is, when the diaphragm of the tweeter is not disposed in the diaphragm of the woofer whose inner diameter increases toward the front. 
     Similarly to the first embodiment described above, also in the second embodiment as described above, sound quality can be further improved in a speaker including a dome-shaped diaphragm and a phase plug disposed in front of the diaphragm. 
     Third Embodiment 
     The present third embodiment is an improved form of the second embodiment described above. Therefore, the present third embodiment will be described focusing on this different point. 
       FIG.  23    is a partially enlarged cross-sectional view of the speaker including the phase plug according to the third embodiment. 
     As shown in  FIG.  23   , the phase plug  224  in a speaker according to the present third embodiment includes a front surface  224   a  facing forward, a rear surface  224   b  facing the front surface  238   a  of the diaphragm  238  of the tweeter, and a side surface  224   c  connecting the front surface  224   a  and the rear surface  224   b.    
     In the phase plug  224  according to the present third embodiment, at least a part of the side surface  224   c  thereof includes an inclined surface  224   f  extending outward while extending forward, similarly to the side surface  24   c  in the phase plug  24  according to the first embodiment described above. The inclined surface  224   f  is a curved surface curved outward in a protruding manner as viewed in a direction orthogonal to the front-rear direction of the speaker. 
     In addition, as viewed in the front-rear direction of the speaker, the size of the front surface  224   a  of the phase plug  224  is smaller than that of the diaphragm  238 . In addition, the plurality of ribs  224   g  provided on the side surface  224   c  of the phase plug  224  protrude outward beyond the outer circumferential edge of the front surface  224   a  as viewed in the front-rear direction of the speaker. In addition thereto, the plurality of ribs  224   g  protrude toward the diaphragm  238 . 
     According to the phase plug  224 , an effect by the side surface  24   c  in the phase plug  24  according to the first embodiment and an effect by the plurality of ribs  124   g  in the phase plug  124  according to the second embodiment can be obtained. That is, the sound wave generated from the portion of the diaphragm  238  overlapping the front surface  224   a  of the phase plug  224  can be directed to the diaphragm  226  of the woofer as viewed in the front-rear direction of the speaker. In addition, as viewed in the front-rear direction of the speaker, on the outer side of the front surface  224   a  of the phase plug  224 , the sound wave generated from the portion of the diaphragm  238  not facing the facing surface  224   h  of the rib  224   g  passes forward between the plurality of ribs  224   g  and propagates forward as it is. Similarly thereto, the sound wave after generated is from the portion of the diaphragm  238  facing the facing surface  224   h  of the plurality of ribs  224   g  and exiting from the gap between the facing surface  224   h  and the diaphragm  238  also passes forward between the plurality of ribs  224   g  and propagates forward. 
     Similarly to the first embodiment described above, also in the third embodiment as described above, sound quality can be further improved in a speaker including a dome-shaped diaphragm and a phase plug disposed in front of the diaphragm. 
     As described above, although the present disclosure has been described with reference to three embodiments of the first to third embodiments, the embodiment of the present disclosure is not limited thereto. 
     For example, each of the phase plugs of the speakers according to the above-described first to third embodiments includes a plurality of ribs. 
     However, the embodiment of the present disclosure is not limited thereto. 
       FIG.  24    is a partially enlarged cross-sectional view of the speaker including the phase plug according to another embodiment. 
     As shown in  FIG.  24   , a phase plug  324  in a speaker according to another embodiment is substantially the same as the phase plug  24  according to the first embodiment except that a plurality of ribs are not provided. That is, as viewed in the front-rear direction of the speaker, the front surface  324   a  is larger than the rear surface  324   b , whereby the side surface  324   c  includes the inclined surface  324   f  extending outward while extending at least partially forward. In addition, the front surface  324   a  is a conical protruding surface as viewed in a direction orthogonal to the central axis C. The conical front surface  324   a  has a conical surface curved inward in a protruding manner. 
     According to the phase plug  324  according to the other embodiment, although it cannot be expected to flatten the sound pressure level due to the plurality of ribs being not provided, it is possible to suppress the occurrence of dips in the sound range of 8000 to 9000 Hz with the side surface  324   c . In addition thereto, it is possible to suppress a decrease in the sound pressure level in the high sound range of 15000 Hz or more with the front surface  324   a , That is, it is possible to partially enjoy the sound quality improving effect by the phase plug  24  according to the first embodiment. 
     It should be noted that regarding the plurality of ribs, in the case of the phase plugs  24  and  124  of the first and second embodiments described above, as shown in  FIGS.  5  and  19   , the number of the plurality of ribs  24   g  and  124   g  is larger than the number of the plurality of leg portions  24   e  and  124   e . However, the embodiment of the present disclosure is not limited thereto. The number of the plurality of ribs and the number of the plurality of leg portions may be the same. In this case, the ribs and the leg portions may be integrated to overlap each other as viewed in he front-rear direction of the speaker. 
     In addition, in the case of the above-described first embodiment. as shown in  FIG.  4   , the front surface  24   a  of the phase plug  24  is a conical protruding surface as viewed in a direction orthogonal to the central axis C, and the conical front surface  24   a  has a conical surface curved inward in a protruding manner. However, the embodiment of the present disclosure is not limited thereto. 
       FIG.  25    is a partially enlarged cross-sectional view of a speaker including a phase plug according to still another embodiment. 
     As shown in  FIG.  25   , a front surface  424   a  of the phase plug  424  in the speaker according to still another embodiment is a conical protruding surface as viewed in a direction orthogonal to the central axis C, similarly to the front surface  24   a  of the phase plug  24  according to the first embodiment shown in  FIG.  4   . However, the conical front surface  424   a  of the phase plug  424  has a conical surface curved outward in a protruding manner. In addition, the diaphragm  426  of the woofer has a shape corresponding to the front surface  424   a  of the phase plug  424 , that is, a shape curved in a protruding manner toward the center side. 
     That is, as shown in  FIG.  16 A , in front of the phase plug, in order to generate a wave front of a composite wave uniformly protruding forward over the entire area, the front surface shape of the phase plug and the diaphragm shape of the woofer need to cooperate with each other, Therefore, as shown in  FIG.  25   , the front surface shape of the phase plug is changed based on the shape of the diaphragm of the woofer. 
     Furthermore, in the case of the first embodiment described above, the speaker  10  is a coaxial speaker including the woofer  16  and the tweeter  18  as shown in  FIG.  4   , However, the embodiment of the present disclosure is not limited thereto, That is, the tubular member in which the dome-shaped diaphragm is disposed inside and the inner diameter increases toward the front is not limited to the diaphragm of the woofer, and may be, for example, a horn of the enclosure. 
     That is, an embodiment according to the present disclosure is, in a broad sense, a speaker including: a diaphragm having a dome shape curved forward in a protruding manner; a tubular member having the diaphragm disposed inside, the tubular member having an inner diameter increasing toward a front; and a phase plug disposed in front of a front surface of the diaphragm. The phase plug includes: a front surface facing forward, a rear surface facing a central portion on a front surface of the diaphragm at a constant interval in parallel, and a side surface connecting the front surface and the rear surface. The front surface of the phase plug is larger than a rear surface of the phase plug as viewed in a front-rear direction of a speaker. At least a part of the side surface of the phase plug has an inclined surface that extends outward while extending forward, faces an inner circumferential surface of the tubular member and a front surface of the diaphragm, and directs a sound wave generated from the diaphragm toward an inner circumferential surface of the tubular member. 
     In addition, another embodiment of the present disclosure is, in a broad sense, a speaker including: a diaphragm having a dome shape curved forward in a protruding manner; and a phase plug disposed in front of a front surface of the diaphragm. The phase plug includes: a front surface facing forward, a rear surface facing a central portion on a front surface of the diaphragm at a constant interval in parallel, and a side surface connecting the front surface and the rear surface. The phase plug includes a plurality of ribs on the side surface. Each of the plurality of ribs includes a facing surface that faces a front surface of the diaphragm at the constant interval in parallel. Each of the plurality of ribs protrudes outward beyond an outer circumferential edge of the front surface as viewed in a front-rear direction of a speaker. 
     As described above, the embodiments are described as the exemplification of the technique in the present disclosure. To that end, accompanying drawings and detailed description are provided. Therefore, among the components described in the accompanying drawings and the detailed description, not only the components essential for solving the problem, but also the components not essential for solving the problem may be included in order to exemplify the above technique. Therefore, it should not be recognized that these non-essential components are essential immediately because these non-essential components are described in the accompanying drawings and the detailed description. 
     In addition, since the above preferred embodiments are for exemplifying the technique in the present disclosure, various changes, substitutions, additions, omissions, and the like can be made within the scope of the claims or the equivalent thereof. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to a speaker including a dome-shaped diaphragm and a phase plug disposed in front of the diaphragm.