Electro-acoustic transducer

Abstract

To provide an electro-acoustic transducer composed of a plurality of piezoelectric vibration parts that is mutually connected via an expandable membrane, in which a decrease in sensitivity due to provision of the expandable membrane is suppressed.SOLUTION: Provided is an electro-acoustic transducer 1, in which an expandable membrane 30 has higher expandability than piezoelectric vibration parts 241, 242, is joined to the piezoelectric vibration parts 241, 242 at respective free ends 24b, and covers a middle hole 25. In a plan view, the middle hole 25 forms a shape expanding from a center position Pc of the middle hole 25 in a first direction D1 rather than a second direction D2. A widened part 252 on one side of the middle hole 25 having a greater hole width extends as far as to a position separate from a narrow width part 251 in the first direction D1, with respect to the center position Pc of the middle hole 25. Thus, free ends 24b of the piezoelectric vibration parts 241, 242 are made easy to move mutually, and a decrease in sensitivity due to provision of the expandable membrane 30 is suppressed.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to electro-acoustic transducers. [Background technology]

[0002] The piezoelectric element described in Patent Document 1 includes a piezoelectric element portion including a piezoelectric film, a support portion supporting the peripheral portion of the piezoelectric element portion, and a stretchable film having higher stretchability than the piezoelectric element portion. The stretchable film is provided in a vibration region inside the peripheral portion of the piezoelectric element portion. The vibration region in the piezoelectric element portion is provided with a through hole penetrating the vibration region in the thickness direction and a plurality of slits, and the plurality of slits extend radially from a through hole disposed at the center of the vibration region. The stretchable film is disposed so as to cover a portion of the through hole and the radial slits in the vibration region and to integrate the vibration region separated by the through hole and the slits.

[0003] However, in this type of piezoelectric element having a cantilever structure in which a through hole and a slit are provided in a piezoelectric element portion including a piezoelectric film, the gap between the beams may increase substantially due to bending of the piezoelectric film, resulting in a decrease in low-frequency sensitivity. In this regard, according to the configuration described in Patent Document 1, the bending of the vibration region is suppressed by providing a stretchable film, and the increase in the gap between the beams is suppressed. In addition, even if the vibration region is curved, the stretchable film is arranged to cover a part of the through hole and the radial slits, so that the decrease in low-frequency sensitivity can be suppressed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 024865 Summary of the Invention [Problem to be solved by the invention]

[0005] According to the configuration of Patent Document 1, it is true that the increase in the gap between the beams of the cantilever structure is suppressed. However, since the multiple piezoelectric vibration parts configured as the beams of the cantilever structure in the piezoelectric element part are connected to each other via the elastic film, it is expected that the rigidity of the multiple piezoelectric vibration parts will increase and each of the multiple piezoelectric vibration parts will become less likely to deform. And, the fact that each of the multiple piezoelectric vibration parts becomes less likely to deform may lead to a decrease in sensitivity in an electroacoustic transducer having multiple piezoelectric vibration parts, such as the piezoelectric element described in Patent Document 1.

[0006] Furthermore, simply enlarging the through hole in the center of the vibration area in order to improve the elasticity of the stretch film leads to a shortening of the beam length of the piezoelectric vibration part that forms a cantilever beam structure, which may also lead to a decrease in the sensitivity of the electro-acoustic transducer.

[0007] In view of the above, the present disclosure aims to provide an electro-acoustic transducer that connects multiple piezoelectric vibration parts to each other via a stretchable film and is capable of suppressing the decrease in sensitivity caused by the provision of the stretchable film. [Means for solving the problem]

[0008] In order to achieve the above object, an electro-acoustic transducer according to one aspect of the present disclosure comprises: A support portion (10); Four or more piezoelectric vibration parts (24, 241, 242) each including a piezoelectric film (21) made of a piezoelectric material, formed in a plate shape having a plate thickness direction along a single axis (Lc), arranged side by side in a circumferential direction (Dc) around the single axis, and configured in a cantilever shape having a fixed end (24a) provided on the outside in the radial direction (Dr) of the single axis and connected to a support part so as not to be displaced, and a free end (24b) provided on the inside in the radial direction and displaceable back and forth in the axial direction (Da) of the single axis relative to the support part; a stretchable film (30) that is laminated in the axial direction on four or more piezoelectric vibration parts and bonded to each free end, and has higher stretchability than the piezoelectric vibration parts; The four or more piezoelectric vibration parts are surrounded by the four or more piezoelectric vibration parts, and a hole periphery (25a) is formed by each free end of each piezoelectric vibration part, forming a central hole (25) penetrating in the axial direction, The central hole is covered with a stretchable film and includes a narrow portion (251) and a one-side widened portion (252) provided on one side of the narrow portion in a first direction (D1) perpendicular to the axial direction, The central hole has a shape expanding from a center position (Pc) of the central hole in both the first direction and the second direction toward the first direction more than in a second direction (D2) perpendicular to the axial direction and the first direction; The one-side widened portion is formed so that the hole width (Wh) in the second direction is larger than that of the narrow portion, and extends to a position farther to one side in the first direction than the narrow portion with respect to the center position of the central hole.

[0009] In this way, since the stretchable membrane is joined to the free end of each piezoelectric vibration part, each piezoelectric vibration part is connected to each other via the stretchable membrane, and the gaps between the piezoelectric vibration parts can be prevented from widening due to bending deformation of the piezoelectric vibration parts, etc.

[0010] Here, the piezoelectric vibration part vibrates so that the free end part reciprocates in the axial direction, and the vibration of the piezoelectric vibration part generates tensile stress in the central hole covering part of the stretch film that covers the central hole. In contrast, according to the electroacoustic transducer described above, the one-side widening part of the central hole, which has a larger hole width, extends to a position farther away from the center position of the central hole in one direction than the narrow width part. Therefore, the stress relaxation region in the central hole covering part, in which the tensile stress is smaller than that around the narrow width part, is formed by expanding to overlap with the one-side widening part of the central hole, and extends to a position farther away from the center position of the central hole in the first direction than the narrow width part. By including such a stress relaxation region in the stretch film, the free ends of the piezoelectric vibration parts connected to each other by the stretch film can easily move relative to each other, so that it is possible to suppress a decrease in sensitivity caused by the provision of the stretch film.

[0011] In addition, in each section of the application documents, each element may be given a reference number in parentheses. In this case, the reference number merely indicates an example of the correspondence between the element and the specific configuration described in the embodiment described later. Therefore, the present disclosure is not limited in any way by the description of the reference number. [Brief description of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an electroacoustic transducer according to a first embodiment. [Diagram 2] 2 is a view taken along an arrow II in FIG. 1, and is a bottom view of the electro-acoustic transducer according to the first embodiment. FIG. [Diagram 3] 3 is a view taken along the arrow III in FIG. 1, and is a plan view that diagrammatically illustrates the general configuration of a plurality of piezoelectric vibration parts and elastic films in the first embodiment. FIG. [Figure 4] FIG. 4 is a plan view showing a schematic configuration of an electroacoustic transducer having a conventional structure, and corresponds to FIG. 3. [Diagram 5] FIG. 4 is a stress distribution diagram showing the distribution of tensile stress in a piezoelectric vibration part and a stretchable film in an electroacoustic transducer of a first experimental example corresponding to the first embodiment. [Figure 6] FIG. 6 is a partially enlarged view showing a portion VI in FIG. 5 in an enlarged manner. [Figure 7] 5 is a stress distribution diagram showing the distribution of tensile stress in a piezoelectric vibration part and a stretchable film in an electroacoustic transducer of a second experimental example corresponding to the conventional structure of FIG. [Figure 8] FIG. 8 is a partially enlarged view showing a portion VIII in FIG. 7 in an enlarged manner. [Figure 9] FIG. 11 is a diagram showing the relationship between the resonance frequency of the piezoelectric vibration part and the magnitude of the signal output from the electroacoustic transducer in each of the above first and second experimental examples. [Figure 10] FIG. 11 is a cross-sectional view illustrating a schematic configuration of an electroacoustic transducer according to a second embodiment, the cross-sectional view corresponding to FIG. [Figure 11]FIG. 11 is a plan view showing a schematic configuration of a plurality of piezoelectric vibration parts and elastic films in a third embodiment, the plan view corresponding to FIG. [Figure 12] FIG. 13 is a plan view showing a schematic configuration of a plurality of piezoelectric vibration parts and elastic films in a fourth embodiment, the plan view corresponding to FIG. [Figure 13] FIG. 13 is a plan view showing a schematic configuration of a plurality of piezoelectric vibration parts and elastic films in a fifth embodiment, the plan view corresponding to FIG. [Figure 14] FIG. 13 is a plan view that illustrates a schematic configuration of a plurality of piezoelectric vibration parts and elastic films in a sixth embodiment, and corresponds to FIG. [Figure 15] 15 is a cross-sectional view showing a cross section along the line XV-XV in FIG. 14 in the sixth embodiment, and corresponds to FIG. [Figure 16] FIG. 16 is a cross-sectional view showing a cross section taken along line XVI-XVI of FIG. 14 in the sixth embodiment. [Figure 17] 15. FIG. 16 is a cross-sectional view showing a cross section corresponding to the XV-XV cross section of FIG. 14 in the seventh embodiment, and corresponds to FIG. [Figure 18] FIG. 4 is a plan view diagrammatically illustrating a schematic configuration of an electroacoustic transducer of a first example exemplified in another embodiment, and corresponds to FIG. [Figure 19] FIG. 19 is a plan view diagrammatically illustrating a schematic configuration of an electro-acoustic transducer according to a second example exemplified in another embodiment, and corresponds to FIG. 18. [Figure 20] FIG. 19 is a plan view diagrammatically illustrating a schematic configuration of an electroacoustic transducer according to a third example exemplified in another embodiment, and corresponds to FIG. 18. [Figure 21] FIG. 19 is a plan view diagrammatically illustrating a schematic configuration of an electro-acoustic transducer according to a fourth example of another embodiment, the diagram corresponding to FIG. [Figure 22] FIG. 19 is a plan view diagrammatically illustrating a schematic configuration of an electro-acoustic transducer according to a fifth example of another embodiment, the diagram corresponding to FIG. [Figure 23]FIG. 19 is a plan view diagrammatically illustrating a schematic configuration of an electro-acoustic transducer according to a sixth example of another embodiment, the diagram corresponding to FIG. [Figure 24] FIG. 19 is a plan view diagrammatically illustrating a schematic configuration of an electro-acoustic transducer according to a seventh example of another embodiment, the diagram corresponding to FIG. [Diagram 25] FIG. 19 is a plan view diagrammatically illustrating a schematic configuration of an eighth example of an electro-acoustic transducer exemplified in another embodiment, the diagram corresponding to FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, each embodiment will be described with reference to the drawings. In the following embodiments, the same reference numerals are given to parts that are the same or equivalent to each other.

[0014] (First embodiment) The electroacoustic transducer 1 shown in Figs. 1 to 3 is one of MEMS devices, and has a configuration as a so-called piezoelectric MEMS microphone. Therefore, the electroacoustic transducer 1 of this embodiment converts sound waves or ultrasonic waves into an electric signal. More specifically, the electroacoustic transducer 1 is configured to generate an electric signal corresponding to the reception intensity of the sound waves or ultrasonic waves propagated from the external space. MEMS is an abbreviation for Micro Electro Mechanical System. Note that Fig. 1 shows a cross section II of Fig. 2.

[0015] In this embodiment, each component of the electroacoustic transducer 1 may be described based on a virtual central axis Lc of the electroacoustic transducer 1. The central axis Lc of the electroacoustic transducer 1 is an axis of the present disclosure, and coincides with the directional axis of the electroacoustic transducer 1 in this embodiment.

[0016] The directional axis of the electroacoustic transducer 1 is an imaginary straight line that serves as a reference for the directivity of the electroacoustic transducer 1 that transmits or receives sound waves or ultrasonic waves, and may also be referred to as a "center directional axis." The directional axis typically corresponds to an imaginary straight line that indicates the axial center of a three-dimensional shape, such as an approximately conical or spindle-shaped shape, when the range of directivity, for example, the range in which a predetermined gain or a predetermined sound level can be obtained, is represented by the three-dimensional shape. Specifically, for example, the directional axis is the central axis of the sound pressure half-value angle.

[0017] In addition, in Fig. 1 to Fig. 3, the equipment axial direction Da, which is the axial direction of the central axis Lc, the equipment radial direction Dr, which is the radial direction of the central axis Lc, and the equipment circumferential direction Dc, which is the circumferential direction around the central axis Lc, are each indicated by arrows at both ends. The equipment axial direction Da is a direction parallel to the central axis Lc. The equipment radial direction Dr is a direction perpendicular to the central axis Lc and away from the central axis Lc. That is, the equipment radial direction Dr is the radial direction of a circle drawn in a virtual plane centered on the intersection of a virtual plane perpendicular to the central axis Lc and the central axis Lc. The equipment circumferential direction Dc is the circumferential direction of the circle drawn in the virtual plane.

[0018] Furthermore, in this embodiment, there are cases where the first direction D1 and the second direction D2 shown in Figures 2 and 3 are used for explanation. The first direction D1 is a direction perpendicular to the instrument axis direction Da, and the second direction D2 is a direction perpendicular to both the instrument axis direction Da and the first direction D1.

[0019] 1 to 3, the electroacoustic transducer 1 includes a support section 10, a piezoelectric element section 20, and a stretchable film 30. In Fig. 3 and subsequent figures corresponding to Fig. 3, the stretchable film 30 is hatched in dots to make it easier to understand.

[0020] The support part 10 is formed around the central axis Lc and has a cylindrical or annular shape surrounding the central axis Lc. In this embodiment, the support part 10 has a square cylindrical or annular shape with the central axis Lc as the central axis of symmetry. That is, the support part 10 has a structure in which four flat plate-like wall materials arranged parallel to the central axis Lc are seamlessly joined together to form a square shape in a plan view, for example, as shown in FIG. 2. The plan view is a view along the device axial direction Da.

[0021] Therefore, a support part inner space 10a is formed inside the support part 10. In other words, the support part inner space 10a is surrounded by the support part 10 all around. In a cross section obtained by cutting the support part inner space 10a along a plane perpendicular to the central axis Lc, the support part inner space 10a has a square cross-sectional shape, and the square that forms the cross-sectional shape has two sides parallel to the first direction D1 and two sides parallel to the second direction D2.

[0022] Since this support portion inner space 10a is formed inside the support portion 10, the support portion 10 has an inner wall surface 12 that surrounds the support portion inner space 10a and faces the support portion inner space 10a from the outside in the device radial direction Dr.

[0023] The support part inner space 10a is covered on one side in the device axial direction Da by a plurality of piezoelectric vibration parts 24, and is open on the other side in the device axial direction Da. The support part 10 may be formed of, for example, ceramics such as alumina, a silicon-based semiconductor substrate, or the like.

[0024] The piezoelectric element part 20 is formed in a plate shape having a plate thickness direction along the central axis Lc. That is, the piezoelectric element part 20 has a pair of main surfaces, a one-side surface 20a and an other-side surface 20b, and the one-side surface 20a and the other-side surface 20b are each formed in a plane perpendicular to the central axis Lc. The "main surface" is a surface of a plate-shaped part or member perpendicular to the plate thickness direction. The one-side surface 20a is formed on one side of the piezoelectric element part 20 in the device axial direction Da, and the other-side surface 20b is formed on the other side of the piezoelectric element part 20 in the device axial direction Da.

[0025] The support 10 has one end surface 101 on one side in the device axial direction Da, and the piezoelectric element 20 is joined to the one end surface 101 of the support 10 at the outer edge of the other side surface 20b in the device radial direction Dr. As a result, the piezoelectric element 20 is fixedly supported by the support 10.

[0026] The piezoelectric element section 20 includes a thin-film piezoelectric film 21 made of a piezoelectric material, and an electrode film 22 made of a conductive material, for example, a metal thin film such as copper foil. Examples of the piezoelectric material that makes up the piezoelectric film 21 include AlN (i.e., aluminum nitride), ScAlN (i.e., scandium aluminum nitride), ZnO (i.e., zinc oxide), BaTiO3 (i.e., barium titanate), PZT, KLN, and KNN. PZT stands for zinc zirconate titanate, and KLN stands for K3Li2Nb5O 15 and KNN means (K,Na)NbO3.

[0027] A plurality of piezoelectric films 21 and electrode films 22 are provided, and the piezoelectric element section 20 has a multi-layer structure in which the piezoelectric films 21 and the electrode films 22 are alternately stacked in the equipment axial direction Da. In the multi-layer structure of the piezoelectric element section 20, the outermost layer on one side of the equipment axial direction Da and the outermost layer on the other side are both electrode films 22. Therefore, each of the multiple piezoelectric films 21 is arranged to be sandwiched between the electrode films 22 on both sides in the equipment axial direction Da. Note that although the electrode films 22 are shown in FIG. 1, the electrode films 22 are omitted from illustration in FIGS. 2 and 3.

[0028] As described above, the piezoelectric element part 20 has a multi-layer structure, and when focusing on the role of each part, the piezoelectric element part 20 is configured to include a fixed part 23 and a plurality of piezoelectric vibration parts 24. The fixed part 23 is an outer edge part of the piezoelectric element part 20 in the device radial direction Dr, and is fixed to the support part 10 by being joined to one end face 101 of the support part 10.

[0029] In addition, the fixing portion 23 and the support portion 10 are omitted in Fig. 3. This display method of omitting the fixing portion 23 and the support portion 10 is adopted in the figures described below that correspond to Fig. 3 in the same manner as Fig. 3.

[0030] The multiple piezoelectric vibration parts 24 are provided on the inside of the fixed part 23 in the device radial direction Dr, and are each formed to extend inward in the device radial direction Dr from the fixed part 23. With regard to the positional relationship with the support part 10, the multiple piezoelectric vibration parts 24 are provided on the inside of the inner wall surface 12 of the support part 10 in the device radial direction Dr.

[0031] The above-mentioned multi-layer structure of the piezoelectric element portion 20 extends to the multiple piezoelectric vibration portions 24, and therefore the multiple piezoelectric vibration portions 24 each have the above-mentioned multi-layer structure in which the piezoelectric films 21 and the electrode films 22 are alternately laminated. The number of piezoelectric vibration portions 24 in this embodiment is four.

[0032] The piezoelectric vibration parts 24 are arranged at equal pitches in the device circumferential direction Dc. Each of the piezoelectric vibration parts 24 has a fixed end 24a provided on the outer side of the piezoelectric vibration part 24 in the device radial direction Dr, and a free end 24b provided on the inner side of the piezoelectric vibration part 24 in the device radial direction Dr.

[0033] The fixed end 24a of the piezoelectric vibration part 24 is connected to the fixed part 23 joined to the support part 10, and is therefore connected to the support part 10 so as not to be displaceable. On the other hand, the free end 24b of the piezoelectric vibration part 24 is capable of reciprocating displacement in the device axial direction Da relative to the support part 10. In this manner, each of the multiple piezoelectric vibration parts 24 is configured in a cantilever shape having the fixed end 24a and the free end 24b.

[0034] Furthermore, a central hole 25 is formed in the center of the area occupied by the entirety of the multiple piezoelectric vibration parts 24, i.e., in the center of the piezoelectric element part 20, which penetrates in the device axial direction Da. This central hole 25 penetrates the piezoelectric element part 20 in the device axial direction Da, and is surrounded by the multiple piezoelectric vibration parts 24. In other words, the multiple piezoelectric vibration parts 24 are disposed outside the central hole 25 in the device radial direction Dr. And, since the central hole 25 is bordered by the free ends 24b of the multiple piezoelectric vibration parts 24, the hole periphery 25a of the central hole 25 is formed by the free ends 24b of the respective piezoelectric vibration parts 24.

[0035] As described above, the four piezoelectric vibration parts 24 are arranged at equal pitches in the device circumferential direction Dc. Therefore, the four piezoelectric vibration parts 24 include a pair of piezoelectric vibration parts 24 arranged opposite to each other in the first direction D1 across the central hole 25, and a pair of piezoelectric vibration parts 24 arranged opposite to each other in the second direction D2 across the central hole 25. In the description of this embodiment, the pair of piezoelectric vibration parts 24 arranged opposite to each other in the first direction D1 may be referred to as a first piezoelectric vibration part 241, and the pair of piezoelectric vibration parts 24 arranged opposite to each other in the second direction D2 may be referred to as a second piezoelectric vibration part 242.

[0036] 2 and 3, slits 26 are formed between adjacent piezoelectric vibration parts 24 in the device circumferential direction Dc among the multiple piezoelectric vibration parts 24, and the adjacent piezoelectric vibration parts 24 in the device circumferential direction Dc are separated in the device circumferential direction Dc by the slits 26. In other words, slits 26 are formed between a first piezoelectric vibration part 241 and a second piezoelectric vibration part 242 adjacent in the device circumferential direction Dc, and the first piezoelectric vibration part 241 and the second piezoelectric vibration part 242 are separated in the device circumferential direction Dc by the slits 26. Since four piezoelectric vibration parts 24 are provided, four slits 26 are formed.

[0037] The plurality of slits 26 penetrate the piezoelectric element portion 20 in the device axial direction Da. The plurality of slits 26 are connected to the central hole 25 on the inside of the device radial direction Dr, and extend radially from the central hole 25 along the device radial direction Dr. In detail, in a plan view, the plurality of slits 26 extend radially from the central axis Lc of the electroacoustic transducer 1 along the device radial direction Dr. More specifically, in a plan view, when all the fixed ends 24a of the plurality of piezoelectric vibration portions 24 are connected together, the fixed ends 24a form a square shape centered on the central axis Lc, and the plurality of slits 26 extend along the diagonals of the square shape. In a plan view, the central axis Lc is located at the center of gravity of the square shape, that is, at the center of gravity of the square-shaped area occupied by the entirety of the plurality of piezoelectric vibration portions 24.

[0038] The slits 26 are arranged at equal pitches in the device circumferential direction Dc. In this embodiment, the number of slits 26 is four, so the mutual pitch of the slits 26 in the device circumferential direction Dc is 90 degrees. In this embodiment, the slits 26 extend in the device radial direction Dr with a constant width.

[0039] Each of the multiple piezoelectric vibration parts 24 separated from each other by such slits 26 has an isosceles trapezoidal shape in a plan view. Therefore, the fixed end 24a and the free end 24b of the pair of first piezoelectric vibration parts 241 each extend along the second direction D2, and the fixed end 24a and the free end 24b of the pair of second piezoelectric vibration parts 242 each extend along the first direction D1.

[0040] 1 and 3, the stretch film 30 is laminated in the device axial direction Da on a plurality of piezoelectric vibration parts 24 and is bonded to the free end 24b of each piezoelectric vibration part 24. Therefore, the central hole 25 of the piezoelectric element part 20 is covered with the stretch film 30, and the entire central hole 25 is blocked by the stretch film 30.

[0041] The stretch film 30 is made of a photosensitive resin, such as a polyimide resin, and has higher stretchability than the piezoelectric vibration part 24. The stretch film 30 is configured so that the Young's modulus of the stretch film 30 is, for example, 1 / 10 or less of that of the piezoelectric vibration part 24. Since the stretch film 30 is made of a photosensitive resin, it can be molded into any outer shape by exposing the stretch film 30 to light. For example, the outer shape of the stretch film 30 in a plan view has two sides parallel to the first direction D1 and two sides parallel to the second direction D2, and forms a square shape centered on the central axis Lc.

[0042] Specifically, the stretchable film 30 has a hole periphery covering portion 31 and a central hole covering portion 32. These hole periphery covering portion 31 and central hole covering portion 32 are integrally formed as a single member.

[0043] The hole-periphery covering portion 31 is disposed so as to surround the entire periphery of the central hole covering portion 32 in a plan view, and the central hole covering portion 32 is connected to the hole-periphery covering portion 31 over its entire periphery. The hole-periphery covering portion 31 is layered on one side surface 20a of the piezoelectric vibration portion 24 at each free end portion 24b of the multiple piezoelectric vibration portions 24 and its peripheral portion, and is joined to the one side surface 20a.

[0044] Central hole covering part 32 is disposed so as to overlap central hole 25 of piezoelectric element part 20 in plan view, and is formed so as to enter central hole 25 while being connected to hole periphery covering part 31. As a result, central hole covering part 32 covers central hole 25 so as to close it.

[0045] Further, a recess 32a recessed from one side in the device axial direction Da is formed in the central hole covering portion 32. This recess 32a is provided to prevent the stretchability of the stretch film 30 from being impaired.

[0046] Specifically, the recess 32a has a shape in which one side in the device axial direction Da is open and the other side in the device axial direction Da is a bottom. In addition, in a plan view, the recess 32a is formed smaller than the central hole 25 so that the entire recess 32a fits within the central hole 25. For example, in a plan view, the peripheral shape of the recess 32a is a shape in which the hole peripheral edge 25a of the central hole 25 is offset toward the inside of the central hole 25.

[0047] Here, an electroacoustic transducer 80 of a conventional structure will be described. In the electroacoustic transducer 80 of the conventional structure, as shown in FIG. 4, the multiple piezoelectric vibration portions 24 are all oriented in different directions in a plan view but have the same shape.

[0048] In contrast, in the electroacoustic transducer 1 of this embodiment, as shown in Figure 3, when viewed in a plan view, the free end portion 24b of at least one of the multiple piezoelectric vibration portions 24 has a different shape compared to the free end portions 24b of the other piezoelectric vibration portions 24.

[0049] Specifically, in this embodiment, in plan view, the shape of the free end portions 24b of the pair of first piezoelectric vibrating parts 241 is different from the shape of the free end portions 24b of the pair of second piezoelectric vibrating parts 242. Note that the different shapes of the free end portions 24b mentioned here also include different lengths or sizes of the free end portions 24b.

[0050] 2 and 3, in plan view, the central hole 25 has a constricted shape in which the hole width Wh of the central hole 25 is narrowed in the second direction D2 at the center of the central hole 25 in the first direction D1. Furthermore, the mutual interval G1 between the free ends 24b of the pair of first piezoelectric vibrating parts 241 is larger than the mutual interval G2 between the free ends 24b of the pair of second piezoelectric vibrating parts 242.

[0051] Therefore, in a plan view, the central hole 25 has a shape different from either a square or a circle. In addition, in a plan view, the central hole 25 has a shape that spreads out from the center position Pc of the central hole 25 in the first direction D1 more than the second direction D2. In other words, in a plan view, the central hole 25 is formed so as to spread out from the center position Pc of the central hole 25 as a base point in the first direction D1 more than the second direction D2. The center position Pc of the central hole 25 is a central position that is the center of the central hole 25 in both the first direction D1 and the second direction D2.

[0052] However, the pair of first piezoelectric vibration parts 241 and the pair of second piezoelectric vibration parts 242 each have a symmetrical shape in the first direction D1 and also in the second direction D2. That is, the entirety of the multiple piezoelectric vibration parts 24 has a symmetrical shape in both the first direction D1 and the second direction D2. Therefore, the central hole 25 of the piezoelectric element part 20 also has a symmetrical shape in the first direction D1 and also in the second direction D2.

[0053] Moreover, the central hole 25 is shaped to be partially narrowed in the second direction D2 by the free ends 24b of the pair of second piezoelectric vibration parts 242 at a middle position in the first direction D1. Therefore, the central hole 25 includes a narrow width part 251, a one-side widened width part 252 provided on one side in the first direction D1 with respect to the narrow width part 251, and an other-side widened width part 253 provided on the other side in the first direction D1 with respect to the narrow width part 251. The narrow width part 251 is provided in the central part of the central hole 25 in the first direction D1, and the one-side widened width part 252 and the other-side widened width part 253 are each formed so that the hole width Wh in the second direction D2 is larger than that of the narrow width part 251.

[0054] Furthermore, the one-side widened portion 252 extends to a position farther toward one side in the first direction D1 than the narrow portion 251 from the center position Pc of the central hole 25. In other words, focusing on the first direction D1, the one-side widened portion 252 extends to a position farther toward one side in the first direction D1 than the narrow portion 251 from the first direction center position Phc, which is the center of the central hole 25 in the first direction D1. In other words, as shown in FIG. 2, the distance between the position P1 of the one-side widened portion 252 farthest toward one side in the first direction D1 from the first direction center position Phc of the central hole 25 and the first direction center position Phc is longer than the distance between the boundary position P2 between the narrow portion 251 and the one-side widened portion 252 and the first direction center position Phc. In detail, the one-side widening portion 252 extends to a position farther to one side in the first direction D1 than the narrow width portion 251 from the first direction center position Phc, while maintaining the characteristic that the hole width Wh is larger than that of the narrow width portion 251.

[0055] In contrast, the other-side widened portion 253 extends to a position farther toward the other side in the first direction D1 than the narrow portion 251 from the center position Pc of the central hole 25. In other words, focusing on the first direction D1, the other-side widened portion 253 extends to a position farther toward the other side in the first direction D1 than the narrow portion 251 from the first-direction center position Phc of the central hole 25. In other words, the distance between the position P3 of the other-side widened portion 253 farthest toward the other side in the first direction D1 from the first-direction center position Phc of the central hole 25 and the first-direction center position Phc is longer than the distance between the boundary position P4 between the narrow portion 251 and the other-side widened portion 253 and the first-direction center position Phc. In detail, the other side widening portion 253 extends to a position farther toward the other side of the first direction D1 than the narrow width portion 251 from the first direction center position Phc, while maintaining the characteristic that the hole width Wh is larger than that of the narrow width portion 251.

[0056] In this embodiment, the entire plurality of piezoelectric vibration parts 24 are symmetrical in the first direction D1 and also in the second direction D2, so that the central axis Lc of the electroacoustic transducer 1 passes through the central position Pc of the central hole 25. That is, as shown in Fig. 2 and Fig. 3, in plan view, the central axis Lc of the electroacoustic transducer 1 is displayed so as to coincide with the central position Pc of the central hole 25.

[0057] Further, the narrow width portion 251 of the central hole 25 is formed by the free ends 24b of a pair of the second piezoelectric vibrating parts 242, and the mutual distance G2 between the pair of free ends 24b is the hole width Wh of the narrow width portion 251. The narrow width portion 251 is the narrowest portion of the central hole 25 of the piezoelectric element part 20, where the hole width Wh is the smallest.

[0058] Next, an outline of the operation of the electroacoustic transducer 1 of this embodiment configured as above will be described.

[0059] In the electroacoustic transducer 1 of this embodiment shown in Figs. 1 to 3, each of the multiple piezoelectric vibration parts 24 vibrates flexibly in a manner in which the free end 24b of the piezoelectric vibration part 24 moves back and forth along the device axis direction Da. The electroacoustic transducer 1 has a conversion function for converting between the distortion caused by the flexure and the voltage between a pair of electrode films 22 provided on both sides of the piezoelectric film 21. Therefore, for example, the flexural vibration of the piezoelectric vibration part 24 due to the reception of sound waves or ultrasonic waves is extracted as an inter-electrode voltage. In this way, the electroacoustic transducer 1 generates an electric signal corresponding to the reception intensity of the sound waves or ultrasonic waves propagated from an external space on one side of the device axis direction Da.

[0060] Here, a plurality of cantilever-shaped piezoelectric vibration parts 24 are provided in the piezoelectric element part 20 by forming a central hole 25 and slits 26. In this configuration, if the piezoelectric vibration part 24 is warped due to curvature of the piezoelectric film 21 caused by residual stress, the opening width, i.e., the gap, due to the central hole 25 and slits 26 will increase, reducing the acoustic resistance and deteriorating the characteristics in the low frequency band.

[0061] In this embodiment, the stretch film 30 covering the central hole 25 is bonded to each of the free ends 24b at the tip end of the cantilever-shaped piezoelectric vibration parts 24. This effectively prevents the piezoelectric vibration parts 24 from warping, and reduces the opening area formed between the piezoelectric vibration parts 24.

[0062] The electroacoustic transducer 1 of the present embodiment described above can provide the following effects. As shown in Figs. 1 to 3, according to this embodiment, the elastic film 30 has higher elasticity than the piezoelectric vibration portion 24, and is laminated in the device axial direction Da on the multiple piezoelectric vibration portions 24 while being bonded to the free end portions 24b of the multiple piezoelectric vibration portions 24, covering the central hole 25. Therefore, the piezoelectric vibration portions 24 are mutually connected via the elastic film 30, and the gaps between the piezoelectric vibration portions 24 (in other words, gaps) can be prevented from expanding due to bending deformation of the piezoelectric vibration portions 24. As a result, air is less likely to pass between one side and the other side of the multiple piezoelectric vibration portions 24 in the device axial direction Da, and deterioration of characteristics in the low frequency band is effectively prevented.

[0063] Here, each piezoelectric vibrating part 24 vibrates so as to move the free end 24b back and forth in the device axial direction Da, and with the vibration of the piezoelectric vibrating part 24, tensile forces Fa, Fb are generated by the piezoelectric vibrating part 24 pulling the stretchable film 30. Then, tensile stresses due to the tensile forces Fa, Fb are generated in the central hole covering part 32 of the stretchable film 30.

[0064] In contrast, in the electroacoustic transducer 1 of this embodiment, in a plan view, the central hole 25 has a shape that expands from the center position Pc of the central hole 25 in the first direction D1 more than in the second direction D2. The one-side expanded portion 252 of the central hole 25, which has a larger hole width Wh, extends to a position farther to one side in the first direction D1 than the narrow portion 251 from the center position Pc of the central hole 25.

[0065] Therefore, a stress relaxation region in central hole covering portion 32 where the tensile stress is smaller than that around narrow portion 251 is formed expanding to overlap one side widened portion 252 of central hole 25. The stress relaxation region overlapping one side widened portion 252 expands to a position farther in the first direction D1 than narrow portion 251 with respect to the center position Pc of central hole 25. By including such a stress relaxation region in stretch film 30, free ends 24b of piezoelectric vibration portions 24 connected to each other by stretch film 30 become easier to move relative to each other, so that it is possible to suppress a decrease in sensitivity caused by providing stretch film 30.

[0066] Here, when the tensile forces Fa and Fb in Figure 3 are generated, we will consider the tensile stresses σ1a and σ1b in the central region B1 that overlaps the narrow portion 251 of the central hole covering portion 32 of the stretchable membrane 30, and the tensile stresses σ2a and σ2b in the one side region B2 that overlaps the one side widening portion 252 of the central hole covering portion 32.

[0067] First, when considering the stretching of the stretch film 30 in the second direction D2 due to the vibration of the pair of second piezoelectric vibrating parts 242 with the tensile force Fb in Fig. 3, the relationship between the tensile stress σ1b in the second direction D2 in the central region B1 and the strain ε1b in the second direction D2 is expressed by the following formula F1. The relationship between the tensile stress σ2b in the second direction D2 in the one side region B2 and the strain ε2b in the second direction D2 is expressed by the following formula F2. In the following formulas F1 and F2, E is the Young's modulus of the stretch film 30. σ1b = E ε1b (F1) σ2b = E ε2b (F2)

[0068] When the pair of second piezoelectric vibrating parts 242 vibrates, the amount of elongation δb in the second direction D2 of the stretchable film 30 due to the tensile force Fb does not change whether it is in the central region B1 or the one-side region B2. The unit of the amount of elongation δb is, for example, "μm". Further, the strain ε1b in the second direction D2 in the central region B1 is obtained from the following formula F3 using the hole width W1h which is the hole width Wh of the narrow part 251. Similarly, the strain ε2b in the second direction D2 in the one-side region B2 is obtained from the following formula F4 using the hole width W2h which is the hole width Wh of the one-side widened part 252. ε1b = δb / W1h ···(F3) ε2b = δb / W2h ···(F4)

[0069] As shown in FIG. 3, since the hole width W1h of the narrow part 251 and the hole width W2h of the one-side widened part 252 are "W1h < W2h", from the above formulas F3 and F4, the strains ε1b and ε2b are "ε1b > ε2b". Then, from the above formulas F1 and F2, the magnitude relationship between the tensile stress σ1b in the second direction D2 in the central region B1 and the tensile stress σ2b in the second direction D2 in the one-side region B2 is "σ1b > σ2b".

[0070] Subsequently, the expansion and contraction of the stretchable film 30 in the first direction D1 due to the vibration of the pair of first piezoelectric vibrating parts 241 accompanied by the tensile force Fa in FIG. 3 will be examined. In the case of the expansion and contraction of the stretchable film 30, since the central region B1 and the one-side region B2 are arranged in series and pulled in the first direction D1, the strain ε1a in the first direction D1 in the central region B1 and the strain ε2a in the first direction D1 in the one-side region B2 are the same as each other. Therefore, the tensile stress σ1a in the first direction D1 in the central region B1 and the tensile stress σ2a in the first direction D1 in the one-side region B2 are also the same as each other. Note that the tensile stress in the other-side region B3 of the central hole covering part 32 that overlaps with the other-side widened part 253 is the same as or substantially the same as the tensile stresses σ2a and σ2b in the one-side region B2.

[0071] From the above considerations, the composite tensile stress caused by the tensile forces Fa and Fb is concentrated in the central region B1 among the above-mentioned regions B1, B2, and B3, and the composite tensile stress in the one-side region B2 and the other-side region B3 is smaller than that in the central region B1. Since the central hole covering portion 32 of the stretch film 30 has a portion with low tensile stress such as the one-side region B2 or the other-side region B3, each of the multiple piezoelectric vibration portions 24 is easily movable as a cantilever beam. Therefore, as described above, it is possible to suppress the decrease in sensitivity caused by the provision of the stretch film 30.

[0072] Furthermore, the distribution of tensile stress in the above-mentioned multiple piezoelectric vibration parts 24 and stretch film 30 was also verified by computer simulation. The results of the computer simulation are shown in Figures 5 to 8. Figures 5 to 8 all show the stress distribution that occurs in the piezoelectric element part 20 when the piezoelectric vibration part 24 vibrates at a predetermined frequency as the experimental frequency, and in Figures 5 to 8, the stress is displayed on the same scale in each figure so that the greater the stress, the darker the display.

[0073] 5 and 6 show stress distribution in an electroacoustic transducer of a first experimental example corresponding to the electroacoustic transducer 1 of this embodiment. For example, in the electroacoustic transducer of the first experimental example, similar to the electroacoustic transducer 1 of this embodiment, the central hole 25 has a shape that is wider from the center position Pc of the central hole 25 in the first direction D1 than in the second direction D2, and has a one-side widened portion 252, a narrowed portion 251, and an other-side widened portion 253.

[0074] 7 and 8 show stress distribution in an electroacoustic transducer of a second experimental example, which corresponds to the electroacoustic transducer 80 of the conventional structure in Fig. 4. For example, in the electroacoustic transducer of the second experimental example, similar to the electroacoustic transducer 80 of the conventional structure, the four piezoelectric vibration parts 24 are all oriented in different directions in a plan view but have the same shape.

[0075] 5 and 6, it was confirmed that stress concentration occurs in the region of the stretch film 30 that overlaps with the narrow portion 251 of the central hole 25. It was also confirmed that stress is alleviated in the region of the stretch film 30 that overlaps with the widened portion 252 on one side and the region that overlaps with the widened portion 253 on the other side.

[0076] In addition, the relationship between the resonance frequency of the piezoelectric vibration part 24 and the magnitude of the signal, which is the voltage signal output from the electroacoustic transducer during the computer simulation, was also verified in each of the first experimental example in Figures 5 and 6 and the second experimental example in Figures 7 and 8. The verification results are shown in Figure 9.

[0077] The relationship points R0a, R0b, R1a, R1b, R2a, and R2b in Fig. 9 respectively indicate the resonant frequency of the piezoelectric vibration part 24 and the magnitude of the signal. The relationship point R0a is for the electroacoustic transducer of the first experimental example in which the residual stress of the piezoelectric vibration part 24 is set to zero, and the relationship point R0b is for the electroacoustic transducer of the second experimental example in which the residual stress of the piezoelectric vibration part 24 is set to zero. Furthermore, the relationship point R1a is for the electroacoustic transducer of the first experimental example in which the residual stress of the piezoelectric vibration part 24 is set to a first magnitude larger than zero, and the relationship point R1b is for the electroacoustic transducer of the second experimental example in which the residual stress of the piezoelectric vibration part 24 is set to the first magnitude. Furthermore, relationship point R2a is for the electro-acoustic transducer of the first experimental example in which the residual stress of the piezoelectric vibration part 24 is a second magnitude greater than the above-mentioned first magnitude, and relationship point R2b is for the electro-acoustic transducer of the second experimental example in which the residual stress of the piezoelectric vibration part 24 is the above-mentioned second magnitude.

[0078] As can be seen from Fig. 9, regardless of the magnitude of the residual stress in the piezoelectric vibration part 24, the first experimental example has a smaller resonant frequency and a larger signal as indicated by arrows A0, A1, and A2, compared to the second experimental example. That is, the piezoelectric vibration part 24 is easier to vibrate and has improved sensitivity in the first experimental example, compared to the second experimental example. The above was confirmed in Fig. 9.

[0079] (1) According to this embodiment, as shown in Fig. 1 to Fig. 3, the mutual interval G1 between the free ends 24b of the pair of first piezoelectric vibrating parts 241 is larger than the mutual interval G2 between the free ends 24b of the pair of second piezoelectric vibrating parts 242. Therefore, it becomes easier to expand the one-side widened portion 252 and the other-side widened portion 253 in the central hole 25 while suppressing the area expansion of the central hole 25. This makes it possible to improve the sensitivity of the electroacoustic transducer 1.

[0080] (2) Furthermore, according to this embodiment, central hole 25 includes, in addition to one-side widened portion 252, other-side widened portion 253, and other-side widened portion 253 is formed so that the hole width Wh in the second direction D2 is larger than that of narrow portion 251. Further, other-side widened portion 253 extends to a position farther away from the center position Pc of central hole 25 on the other side in the first direction D1 than narrow portion 251.

[0081] Therefore, since the above-mentioned stress relaxation regions, in which the tensile stress is relatively small, are formed symmetrically on the stretchable film 30 in an arrangement that sandwiches the narrow portion 251 of the central hole 25, it is possible to vibrate the multiple piezoelectric vibration parts 24 in a balanced manner.

[0082] (3) Furthermore, according to this embodiment, the number of piezoelectric vibration parts 24 is four. That is, the number of piezoelectric vibration parts 24 is an even number, and more specifically, the number of piezoelectric vibration parts 24 is a multiple of four. Therefore, it is easy to form the multiple piezoelectric vibration parts 24, the central hole 25, and the multiple slits 26 symmetrically in the first direction D1 and the second direction D2, and it is possible to vibrate the multiple piezoelectric vibration parts 24 in a well-balanced manner.

[0083] (4) Furthermore, according to this embodiment, the entire piezoelectric vibration parts 24 are symmetrical in the first direction D1 and also in the second direction D2. At the same time, the central hole 25 is symmetrical in the first direction D1 and also in the second direction D2. Therefore, each of the piezoelectric vibration parts 24 can be vibrated in a well-balanced manner.

[0084] Furthermore, according to this embodiment, central hole 25 of piezoelectric element part 20 has a shape that is wider from center position Pc of central hole 25 in first direction D1 than in second direction D2. Therefore, it is possible to obtain a hole shape of central hole 25 that makes it easy to form one-side widened portion 252 and other-side widened portion 253 on both sides of narrow portion 251 while suppressing area expansion of central hole 25.

[0085] Second embodiment Next, a second embodiment will be described. In this embodiment, differences from the first embodiment will be mainly described. Also, parts that are the same as or equivalent to the above-mentioned embodiment will be omitted or simplified. This also applies to the following embodiments.

[0086] In the first embodiment described above, as shown in Fig. 1, a recess 32a is formed in the central hole covering portion 32 of the stretch film 30. In contrast, in this embodiment, as shown in Fig. 10, the recess 32a is not formed in the stretch film 30. Therefore, the central hole covering portion 32 of the stretch film 30 is thicker in the device axial direction Da than the hole peripheral covering portion 31.

[0087] Except for the points described above, this embodiment is similar to the first embodiment. In this embodiment, the same effects as those of the first embodiment can be obtained from the configuration common to the first embodiment.

[0088] Third embodiment Next, a third embodiment will be described. In this embodiment, differences from the second embodiment will be mainly described.

[0089] 11, in this embodiment, the hole shape of the central hole 25 of the piezoelectric element part 20 is different from that of the second embodiment. Specifically, in a plan view, the central hole 25 of this embodiment has a shape in which two identical circular shapes having centers shifted in the first direction D1 partially overlap each other.

[0090] In this embodiment, as in the second embodiment, for example, the central hole 25 of the piezoelectric element part 20 has a shape that is wider from the center position Pc of the central hole 25 in the first direction D1 than in the second direction D2, and has a shape that is symmetrical in the first direction D1 and also in the second direction D2. The entire plurality of piezoelectric vibration parts 24 also has a shape that is symmetrical in the first direction D1 and also in the second direction D2.

[0091] Except for the points described above, this embodiment is similar to the second embodiment. In this embodiment, the same effects as those of the second embodiment can be obtained from the configuration common to the second embodiment.

[0092] (Fourth embodiment) Next, a fourth embodiment will be described. In this embodiment, differences from the second embodiment will be mainly described.

[0093] 12, in this embodiment, the hole shape of the central hole 25 of the piezoelectric element part 20 is different from that of the second embodiment. Specifically, in a plan view, the central hole 25 of this embodiment has a shape in which two identical isosceles trapezoidal shapes symmetrically arranged in the first direction D1 partially overlap each other on the upper base side with the upper base and the lower base parallel to the second direction D2. The upper base of the isosceles trapezoid referred to here is the shorter base of the two bases of the isosceles trapezoid, and the lower base is the longer base of the two bases.

[0094] In this embodiment, as in the second embodiment, for example, the central hole 25 of the piezoelectric element part 20 has a shape that is wider from the center position Pc of the central hole 25 in the first direction D1 than in the second direction D2, and has a shape that is symmetrical in the first direction D1 and also in the second direction D2. The entire plurality of piezoelectric vibration parts 24 also has a shape that is symmetrical in the first direction D1 and also in the second direction D2.

[0095] Except for the points described above, this embodiment is similar to the second embodiment. In this embodiment, the same effects as those of the second embodiment can be obtained from the configuration common to the second embodiment.

[0096] Fifth embodiment Next, a fifth embodiment will be described. In this embodiment, differences from the second embodiment will be mainly described.

[0097] 13, in this embodiment, the hole shape of the central hole 25 of the piezoelectric element part 20 is different from that of the second embodiment. Specifically, in a plan view, the central hole 25 of this embodiment has a shape in which two identical diamond shapes arranged symmetrically in the first direction D1 partially overlap with their diagonals parallel to the first direction D1.

[0098] In this embodiment, as in the second embodiment, for example, the central hole 25 of the piezoelectric element part 20 has a shape that is wider from the center position Pc of the central hole 25 in the first direction D1 than in the second direction D2, and has a shape that is symmetrical in the first direction D1 and also in the second direction D2. The entire plurality of piezoelectric vibration parts 24 also has a shape that is symmetrical in the first direction D1 and also in the second direction D2.

[0099] Except for the points described above, this embodiment is similar to the second embodiment. In this embodiment, the same effects as those of the second embodiment can be obtained from the configuration common to the second embodiment.

[0100] Sixth embodiment Next, a sixth embodiment will be described. In this embodiment, differences from the first embodiment will be mainly described.

[0101] 14, in this embodiment, the hole shape of the central hole 25 of the piezoelectric element part 20 is different from that of the first embodiment. Specifically, the central hole 25 of this embodiment has a shape in which four identical rectangular shapes are partially overlapped on the inside in the device radial direction Dr in a plan view. In detail, the four identical rectangular shapes are each formed such that the four slits 26 are connected in a direction perpendicular to the short side of the rectangular shape and the rectangular shape extends in the device radial direction Dr.

[0102] In this embodiment, as in the first embodiment, for example, the central hole 25 of the piezoelectric element part 20 has a shape that is wider from the center position Pc of the central hole 25 in the first direction D1 than in the second direction D2, and has a symmetrical shape in the first direction D1 and also in the second direction D2. The entire plurality of piezoelectric vibration parts 24 also has a symmetrical shape in the first direction D1 and also in the second direction D2.

[0103] 14 to 16, in this embodiment, unlike the first embodiment, the central hole covering portion 32 of the stretchable film 30 has a thick portion 321, a one-side thin portion 322, and an other-side thin portion 323. The thick portion 321 overlaps the narrow portion 251 of the central hole 25 and covers the narrow portion 251.

[0104] Moreover, one-side thin portion 322 is disposed on one side in first direction D1 with respect to narrow portion 251 and thick portion 321, and overlaps with one-side widened portion 252 of central hole 25 to cover said one-side widened portion 252. Moreover, the other-side thin portion 323 is disposed on the other side in first direction D1 with respect to narrow portion 251 and thick portion 321, and overlaps with other-side widened portion 253 of central hole 25 to cover said other-side widened portion 253.

[0105] Furthermore, the recess 32a of the central hole covering portion 32 is formed so that the depth of the recess 32a in the device axial direction Da varies depending on the position in the first direction D1. As a result, the one-side thin portion 322 and the other-side thin portion 323 are each formed to have a thickness Ts in the device axial direction Da that is thinner than the thick portion 321.

[0106] (1) Therefore, it is possible to increase the overall stretchability of the central hole covering portion 32 of the stretch film 30 while ensuring sufficient strength of the stretch film 30 in the thick portion 321 where stress concentrates due to vibration of the piezoelectric vibration portion 24. In other words, it is possible to make the free end portion 24b of the piezoelectric vibration portion 24 easier to move while appropriately ensuring the strength of the stretch film 30.

[0107] Except for the points described above, this embodiment is similar to the first embodiment. In this embodiment, the same effects as those of the first embodiment can be obtained from the configuration common to the first embodiment.

[0108] It should be noted that although this embodiment is a modification based on the first embodiment, this embodiment can also be combined with any of the third to fifth embodiments described above.

[0109] Seventh embodiment Next, a seventh embodiment will be described. In this embodiment, differences from the sixth embodiment will be mainly described.

[0110] As shown in Fig. 17, in this embodiment, unlike the sixth embodiment, the depth of the recess 32a of the central hole covering portion 32 in the device axis direction Da is the same at any position in the first direction D1. However, in this embodiment, a first other-side recess 32b and a second other-side recess 32c are formed in the central hole covering portion 32. Note that in this embodiment, as in the sixth embodiment, the central hole covering portion 32 of the stretchable membrane 30 has a thick portion 321, a one-side thin portion 322, and an other-side thin portion 323.

[0111] The first other-side recess 32b is disposed so as to overlap with one-side widened portion 252 of central hole 25 in plan view, and the other-side widened portion 253 is disposed so as to overlap with the other-side widened portion 253 of central hole 25 in plan view. The first other-side recess 32b and the second other-side recess 32c each have a shape recessed from the other side in the device axial direction Da.

[0112] In this embodiment, by forming the first other-side recess 32b, the one-side thin portion 322 is formed to have a thickness Ts in the device axial direction Da that is thinner than the thick portion 321. In addition, by forming the second other-side recess 32c, the other-side thin portion 323 is formed to have a thickness Ts in the device axial direction Da that is thinner than the thick portion 321.

[0113] Except for the points described above, this embodiment is similar to the sixth embodiment. In this embodiment, the same effects as those of the sixth embodiment can be obtained from the configuration common to the sixth embodiment.

[0114] (Other embodiments) (1) In each of the above-described embodiments, the electroacoustic transducer 1 functions as a microphone that converts sound waves or ultrasonic waves into an electric signal, but this is just one example. Conversely, the electroacoustic transducer 1 may function as a speaker that converts an electric signal into sound waves or ultrasonic waves.

[0115] (2) In each of the above-described embodiments, the central axis Lc of the electroacoustic transducer 1 shown in Fig. 1 as one axis coincides with the directional axis of the electroacoustic transducer 1, but this is just an example. For example, the central axis Lc of the electroacoustic transducer 1 may not coincide with the directional axis, but may be a straight line parallel to the directional axis and deviated from the directional axis.

[0116] (3) In the above-mentioned third embodiment, as shown in Fig. 11, the recess 32a (see Figs. 1 and 3) is not formed in the central hole covering portion 32 of the stretch film 30, but as in the first embodiment, the recess 32a may be formed in the central hole covering portion 32 of the stretch film 30. This also applies to the fourth and fifth embodiments.

[0117] (4) In each of the above-described embodiments, the number of piezoelectric vibration portions 24 is four as shown in FIG. 3, for example, but it may be five or more.

[0118] (5) In each of the above-described embodiments, as shown in Fig. 1, the piezoelectric element section 20 has a multi-layer structure composed of a plurality of piezoelectric films 21 and a plurality of electrode films 22, but the piezoelectric film 21 does not necessarily have to be provided in a plurality of layers. For example, the piezoelectric element section 20 may be composed of a single piezoelectric film 21 and a pair of electrode films 22 laminated on both sides of the piezoelectric film 21.

[0119] (6) In each of the above-described embodiments, the piezoelectric element portion 20 has a plurality of piezoelectric vibration portions 24 which are symmetrical in both the first direction D1 and the second direction D2 as a whole, as shown in Fig. 3. The plurality of piezoelectric vibration portions 24 may not be symmetrical in both the first direction D1 and the second direction D2 as a whole.

[0120] For example, FIGS. 18 to 25 each show a specific example in which the plurality of piezoelectric vibration parts 24 as a whole are symmetrical in the second direction D2 but are not symmetrical in the first direction D1.

[0121] For example, in the specific example of Figures 18 to 20, there are four piezoelectric vibration parts 24, in the specific example of Figures 21 to 23, there are five piezoelectric vibration parts 24, and in the specific example of Figures 24 and 25, there are six piezoelectric vibration parts 24. In the specific example of Figure 18, central hole 25 has narrow portion 251 and one-side widened portion 252, but does not have the other-side widened portion 253 (see Figure 3).

[0122] In the specific example of Fig. 18 to Fig. 25, when all the fixed ends 24a of the piezoelectric vibration parts 24 are connected together in a plan view, the fixed ends 24a form a circle centered on the central axis Lc. In the specific example of Fig. 18 to Fig. 25, the central hole 25 has a shape that is wider from the central position Pc of the central hole 25 in the first direction D1 than in the second direction D2, which is the same as, for example, the first embodiment described above. In the specific example of Fig. 19 to Fig. 25, the free end 24b of only one piezoelectric vibration part 24 out of all the piezoelectric vibration parts 24 of the electroacoustic transducer 1 has a different shape compared to the other piezoelectric vibration parts 24.

[0123] (7) The present disclosure is not limited to the above-described embodiment, and can be implemented in various modified forms. The above-described embodiments are not unrelated to each other, and can be appropriately combined, except in cases where the combination is clearly impossible.

[0124] In each of the above embodiments, the elements constituting the embodiment are not necessarily essential, unless otherwise specified as essential or considered to be obviously essential in principle. In each of the above embodiments, when the numbers, values, amounts, ranges, etc. of the components of the embodiment are mentioned, they are not limited to the specific numbers, except when specified as essential or when they are obviously limited to a specific number in principle. In each of the above embodiments, when the materials, shapes, positional relationships, etc. of the components are mentioned, they are not limited to the materials, shapes, positional relationships, etc., except when specified as essential or when they are obviously limited to a specific material, shape, positional relationship, etc. in principle. [Explanation of symbols]

[0125] 1 Electroacoustic transducer 10 Support part 21 Piezoelectric film 24 Piezoelectric vibration section 24a Fixed end 24b Free end 25 Central hole 30 Stretch membrane 251 Narrow section 252 One side widening

Claims

1. An electroacoustic transducer (1), A support portion (10); four or more piezoelectric vibration sections (24, 241, 242) each including a piezoelectric film (21) made of a piezoelectric material, formed in a plate shape having a plate thickness direction along a single axis (Lc), arranged side by side in a circumferential direction (Dc) around the single axis, and configured in a cantilever shape having a fixed end (24a) provided on the outside in a radial direction (Dr) of the single axis and connected to the support section so as not to be displaced, and a free end (24b) provided on the inside in the radial direction and displaceable back and forth in an axial direction (Da) of the single axis relative to the support section; a stretchable film (30) that is laminated on the four or more piezoelectric vibration parts in the axial direction and bonded to the free end parts of each of the four or more piezoelectric vibration parts, and has a higher stretchability than the piezoelectric vibration parts; The four or more piezoelectric vibration parts are surrounded by the four or more piezoelectric vibration parts, and a hole periphery (25a) is formed by the free end parts of each of the four or more piezoelectric vibration parts, forming a central hole (25) penetrating in the axial direction, The central hole is covered with the stretchable film and includes a narrow portion (251) and a one-side widening portion (252) provided on one side of the narrow portion in a first direction (D1) perpendicular to the axial direction, The central hole has a shape expanding from a center position (Pc) of the central hole in both the first direction and the second direction toward the first direction more than a second direction (D2) perpendicular to the axial direction and the first direction, An electro-acoustic transducer, wherein the one-side widening portion is formed so that the hole width (Wh) in the second direction is larger than that of the narrow width portion, and extends to a position farther to the one side in the first direction than the narrow width portion with respect to the center position of the central hole.

2. The four or more piezoelectric vibration parts include the piezoelectric vibration parts (241) arranged opposite each other in the first direction across the central hole, and the piezoelectric vibration parts (242) arranged opposite each other in the second direction across the central hole, The electro-acoustic transducer of claim 1, wherein the mutual distance (G1) between the free ends of the piezoelectric vibration parts arranged opposite each other in the first direction across the central hole is greater than the mutual distance (G2) between the free ends of the piezoelectric vibration parts arranged opposite each other in the second direction across the central hole.

3. The central hole includes an other-side widening portion (253) provided on the other side of the narrow portion in the first direction, The electro-acoustic transducer of claim 1 , wherein the other-side widening portion is formed so that the hole width in the second direction is larger than that of the narrow width portion, and extends to a position farther toward the other side in the first direction than the narrow width portion relative to the center position of the central hole.

4. The stretchable membrane has a thick portion (321) that covers the narrow portion of the central hole, and a thin portion (322) that is disposed on the one side of the narrow portion and the thick portion in the first direction and covers the one side wide portion of the central hole, 4. The electroacoustic transducer according to claim 1, wherein the thin portion is formed to have a thickness (Ts) in the axial direction smaller than that of the thick portion.

5. The narrow width portion is formed by the free end portions of a pair of the piezoelectric vibration portions (242) that are arranged opposite each other in the second direction across the central hole, among the four or more piezoelectric vibration portions; 4. The electro-acoustic transducer according to claim 3, wherein a distance (G2) between the free ends of the pair of piezoelectric vibration portions is equal to the hole width of the narrow portion.

6. The number of the piezoelectric vibration parts is an even number, Among the four or more piezoelectric vibration parts, adjacent piezoelectric vibration parts in the circumferential direction are separated in the circumferential direction by slits (26) formed between the piezoelectric vibration parts, 6. The electro-acoustic transducer according to claim 1, wherein the plurality of slits are each connected to the central hole and extend radially from the central hole along the radial direction.

7. The number of the piezoelectric vibration parts is a multiple of 4, Among the four or more piezoelectric vibration parts, adjacent piezoelectric vibration parts in the circumferential direction are separated in the circumferential direction by slits (26) formed between the piezoelectric vibration parts, 6. The electro-acoustic transducer according to claim 1, wherein the plurality of slits are each connected to the central hole and extend radially from the central hole along the radial direction.

8. the four or more piezoelectric vibration parts as a whole are symmetrical in the first direction and also in the second direction; 6. The electro-acoustic transducer according to claim 1, wherein the central hole is also symmetrical in the first direction and in the second direction.

9. An electroacoustic transducer as described in any one of claims 1, 2, 3, and 5, wherein, when viewed in the axial direction, the free end of at least one of the four or more piezoelectric vibration parts has a different shape than the other piezoelectric vibration parts.

Images