Speaker

Abstract

This speaker is provided with an ultrasonic wave generating means (101), an amplitude modulation means (102) for modulating the amplitude of an ultrasonic wave generated from the ultrasonic wave generating means (101) to the amplitude of an envelope waveform corresponding to the sound wave of an audible frequency, and an acoustic plate (103) for vibrating due to acoustic radiation pressure of the ultrasonic wave having undergone the amplitude modulation. The amplitude modulation means (102) modulates the amplitude of the ultrasonic wave with a multiplier of the sound pressure level of the ultrasonic wave.

Description

Speaker

[0001] The present invention relates to a speaker having a vibrating portion with a MEMS structure.This application claims priority to Japanese Patent Application No. 2024-089098, filed on May 31, 2024, the contents of which are incorporated herein by reference.

[0002] MEMS speakers have been attracting attention as silicon drivers used in truly wireless (TWS) earphones and the like. MEMS speakers are sound-generating devices equipped with small mechanically movable parts and electronic circuit elements. For example, a large MEMS speaker equipped with multiple mechanically movable parts has been disclosed (see, for example, Patent Document 1). The MEMS speaker in Patent Document 1 makes it possible to digitally adjust the volume of the generated sound by adjusting the number of mechanically movable parts to be driven.

[0003] It is known that small MEMS speakers have difficulty generating low sounds. This is because the sound pressure of a sound generated by a speaker using a diaphragm is proportional to the product of three parameters: the area of ​​the vibrating part of the diaphragm, the amplitude, and the vibration frequency. For low sounds, all of these parameters become small. In other words, to generate low sounds with sufficient sound pressure, it is necessary to increase the area and amplitude of the vibrating part, but in a MEMS structure, both of these are difficult due to the constraints of the small size of the vibrating part. Furthermore, when generating low sounds, the sound pressure of the generated sound becomes weaker as the vibration frequency becomes smaller.

[0004] Another method for increasing the amplitude of the vibrating part is to make slits in the diaphragm, which is the vibrating part. However, in order to make slits in the diaphragm, or because air leaks from the slits in the diaphragm, it is necessary to cover the slits with a soft material, which poses problems such as increased manufacturing costs.

[0005] JP 2012-119842 A

[0006] The present invention has been made in view of the above circumstances, and has an object to provide a speaker that has a small vibrating part and is capable of generating sound of any frequency.

[0007] In order to solve the above problems, the present invention employs the following means.

[0008] (1) A speaker according to one aspect of the present invention comprises an ultrasonic generating means, an amplitude modulation means for modulating the amplitude of ultrasonic waves generated by the ultrasonic generating means to the amplitude of an envelope waveform corresponding to a sound wave of an audible frequency, and an acoustic plate that vibrates with the acoustic radiation pressure of the ultrasonic waves after amplitude modulation, and the amplitude modulation means modulates the amplitude of the ultrasonic waves by a multiplier of the sound pressure level of the ultrasonic waves.

[0009] (2) In the speaker described in (1) above, an auxiliary plate having a higher hardness than the acoustic plate may be attached to the center of the acoustic plate.

[0010] (3) In the speaker according to either (1) or (2) above, the acoustic plate may be a single or multiple cantilever plates, or a complete or slit membrane.

[0011] (4) In the speaker described in any one of (1) to (3) above, the amplitude modulation means may be configured to match the frequency of the applied vibration voltage to the natural frequency of the ultrasonic wave generating means, thereby causing resonance.

[0012] (5) In the speaker described in any one of (1) to (4) above, the ultrasonic generating means is a piezoelectric actuator consisting of a first vibrating body, a piezoelectric body supported on one surface of the first vibrating body, and a first voltage application means for applying a first vibration voltage to the piezoelectric body, and the amplitude modulation means is connected to the first voltage application means, and the acoustic plate may be arranged opposite to and spaced apart from the piezoelectric body.

[0013] (6) In the speaker described in (5) above, the piezoelectric elements supported on one side of the first vibrating body may be arranged in an array or configured in multiple layers, with electrodes sandwiched between them.

[0014] (7) In the speaker described in (5) above, a groove may be formed on one surface of the first vibrating body around a region in contact with the piezoelectric body.

[0015] (8) In the speaker described in any one of (1) to (4) above, the ultrasonic generating means is a piezoelectric actuator consisting of a first vibrating body, a piezoelectric body supported on one surface of the first vibrating body, and a first voltage application means for applying a first vibration voltage to the piezoelectric body, the amplitude modulation means is connected to the first voltage application means, and the acoustic plate has one end fixed and the other end arranged so as to be in contact with the first vibrating body, and may further include a first substrate arranged opposite to and spaced apart from the piezoelectric body or the acoustic plate.

[0016] (9) In the speaker described in any one of (1) to (4) or (6) above, the ultrasonic generating means is an electrostatic actuator composed of a second substrate having a fixed shape, a deformable second vibrating body, a spacer, and a second voltage application means for applying a second vibration voltage between the second substrate and the second vibrating body, and one surface of the second substrate and one surface of the second vibrating body are arranged opposite each other with the spacer in between, the amplitude modulation means is connected to the second voltage application means, and the acoustic plate may be arranged to face the other surface of the second vibrating body.

[0017] According to the present invention, it is possible to provide a speaker that has a small vibration part and is capable of generating sound of any frequency.

[0018] FIG. 1 is a cross-sectional view of a speaker according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view of a speaker according to a first modification of the first embodiment. FIG. 3 is a cross-sectional view of a speaker according to a second embodiment of the present invention. FIG. 4 is a cross-sectional view of a speaker according to a third embodiment of the present invention. FIG. 5 is a frequency spectrum obtained from a speaker according to a first comparative example. FIG. 6 is a frequency spectrum obtained from a speaker according to a first example. FIG. 7 is a frequency spectrum obtained from a speaker according to a second example. FIG. 8 is a frequency spectrum obtained from a speaker according to a third example. FIG. 9 is a graph showing the voltage characteristics of the amplitude of ultrasonic waves generated by the ultrasonic wave generating means of Examples 2 and 3.

[0019] Hereinafter, a speaker according to an embodiment of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show characteristic portions enlarged for the sake of clarity, and the dimensional proportions of the components may not be the same as those in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications may be made within the scope of the present invention.

[0020] First Embodiment FIG. 1 is a cross-sectional view of a speaker 100 according to a first embodiment of the present invention. The speaker 100 primarily comprises an ultrasonic wave generating unit 101, an amplitude modulation unit 102, and an acoustic panel 103. The speaker 100 may further comprise a wall material 107 surrounding a space sandwiched between the ultrasonic wave generating unit 101 and the acoustic panel 103. The wall material 107 may be cylindrical, for example, but may have other shapes. However, in order to equalize the pressure in the space surrounded by the wall material 107 with the pressure in the external space, the wall material 107 is provided with at least one communication hole connecting the two spaces. Furthermore, the acoustic panel 103 is preferably a complete membrane (a uniform membrane). However, if necessary, it may not be a complete membrane but may have a slit formed therein, or may be a single cantilever plate (cantilever beam), an assembly of multiple cantilever plates, or a plate-like member including a cantilever plate.

[0021] The ultrasonic wave generating means 101 is a device that generates ultrasonic waves (with a frequency of approximately 40 kHz to 20 MHz). In this embodiment, a case where the ultrasonic wave generating means 101 is a piezoelectric actuator 101A is illustrated. The piezoelectric actuator 101A is mainly composed of a film-like first vibrating body 104, a piezoelectric body 105, a first voltage applying means 106, and a support body 109.

[0022] The first vibrating body 104 is a non-piezoelectric body, and is realized by a complete film (uniform film), a film with slits or the like formed therein, a single cantilever beam, or an assembly of multiple cantilevers, etc. The first vibrating body 104 is made of, for example, silicon, glass, metal, resin, etc.

[0023] The piezoelectric body 105 is connected to the first voltage application means 106 while being supported on one surface 104a of the first vibrating body. The piezoelectric body 105 is made of a piezoelectric material such as PZT, AlN, ScAlN, or KNN, and deforms when a voltage is applied. The piezoelectric body 105 can be formed by various methods, such as thin-film formation by sputtering, thick-film formation by the sol-gel method, or polishing from a bulk material. When the piezoelectric body deforms, the first vibrating body 104 also deforms, generating ultrasonic waves. Furthermore, the piezoelectric body 105 and the first vibrating body 104 may be given a curvature to focus the ultrasonic waves. The piezoelectric body 105 may have a multi-layer structure with electrodes sandwiched between the layers (between adjacent layers). The number of piezoelectric bodies 105 on the first vibrating body 104 may be one or more. Furthermore, the first vibrating body 104 and the piezoelectric body 105 may be considered as one unit, and this unit may be arranged in an array.

[0024] The first voltage application means (first voltage source) 106 is connected to both ends of the piezoelectric body 105 in the thickness direction (here, the vertical direction) and is configured to apply an oscillating voltage (first oscillating voltage) between the ends. The frequency of the oscillating voltage is preferably 40 kHz or higher. From the viewpoint of applying voltage efficiently, the first voltage application means 106 may be connected to the piezoelectric body 105 via an electrode (not shown) made of a conductive material. Applying the oscillating voltage causes periodic distortion in the piezoelectric body 105, resulting in vibration of the first vibrating body 104. This vibration generates ultrasonic waves S. The frequency of the generated ultrasonic waves S is the same as the frequency of the oscillating voltage.

[0025] When multiple ultrasonic generators each consisting of a piezoelectric body 105 or a first vibrating body 104 and a piezoelectric body 105 are arranged, each piezoelectric body 105 or a unit consisting of a first vibrating body 104 and a piezoelectric body 105 is controlled by a first voltage application means 106 connected to each piezoelectric body 105 so as to vibrate individually or simultaneously. The piezoelectric body 105 may be made of a single material or may have a configuration in which piezoelectric bodies and electrodes are alternately stacked. By alternately applying the voltage output from the first voltage application means 106 to each layer, it is possible to increase the displacement generated even with the same applied voltage.

[0026] The support 109 is an object that supports the other surface 104c of the first vibrating body. Note that the support 109 may support the entire piezoelectric body 105 as long as it can support the piezoelectric body 105, but it may also be shaped to support a portion that does not overlap with the piezoelectric body 105 (a portion surrounding the central portion that overlaps with the piezoelectric body 105). An adhesive layer 108 or the like may be sandwiched between the first vibrating body 104 and the support 109.

[0027] The amplitude modulation means 102 is a device that modulates the amplitude of the ultrasonic waves S generated from the ultrasonic wave generating means 101 with the amplitude of an envelope waveform corresponding to a sound wave of a desired frequency (audible frequency, etc.). The amplitude modulation means 102 modulates the amplitude of the ultrasonic waves S with a multiplier of the sound pressure level of the ultrasonic waves S. The amplitude modulation means 102 modulates the amplitude of the ultrasonic signal so that the desired audible sound signal intended to be generated from the acoustic panel 103 becomes an envelope. Furthermore, in order to reduce distortion such as harmonics of the generated audible sound, a signal obtained by performing an operation (for example, a square root or exponentiation operation) on the audible sound signal may be used for modulation. This operation may be realized by an analog circuit or by digital processing. The amplitude modulation means 102 is connected to the first voltage application means 106. By using the amplitude modulation means 102 to modulate the amplitude of the vibration of the voltage applied to the piezoelectric body 105, the amplitude of the ultrasonic wave S generated from the vibrating piezoelectric body 105 can be generated and modulated in such a way that it is enveloped in the amplitude waveform of the desired audible sound signal or a waveform obtained by performing an operation on it.

[0028] In this embodiment, focusing on the fact that the acoustic radiation pressure generated by the ultrasonic waves S is proportional to the multiplier of the sound pressure, and that the sound pressure is proportional to the multiplier of the frequency, a force is applied to the acoustic plate 103 using this acoustic radiation pressure. The magnitude of the acoustic radiation pressure is modulated with a desired signal (audible sound signal), causing the acoustic plate 103 to vibrate. This method allows even low-frequency acoustic signals to be driven with a large force in the ultrasonic range, thereby enabling loud bass output. In this embodiment, the sound pressure of the ultrasonic waves S generated from the piezoelectric body 105 can be increased by increasing the acoustic amplitude.

[0029] Note that, as a preferred modulation multiplier, for example, modulation by the power of 0.5 makes the radiation pressure linear. Furthermore, while speakers using a diaphragm such as a conventional acoustic plate 103 typically select one with a low Q value to avoid resonance, in this embodiment, resonance is used to increase the amplitude by a factor of the Q value. The amplitude modulation means 102 adjusts the frequency of the applied vibration voltage to the natural frequency of the ultrasonic generator 101, thereby causing resonance and enabling the amplitude of the generated ultrasonic waves S to be modulated to a greater extent, thereby generating ultrasonic waves S with a higher sound pressure. When the ultrasonic generator 101 is a piezoelectric actuator, adjusting the frequency to the combined natural frequency of the piezoelectric body 105 and the first vibrator 104 causes resonance, thereby enabling the amplitude of the generated ultrasonic waves S to be modulated to a greater extent, thereby generating ultrasonic waves S with a higher sound pressure. When the ultrasonic generator 101 is an electrostatic actuator described later in FIG. 5, adjusting the frequency to the natural frequency of the second vibrator (ultrasonic acoustic plate) 115 causes resonance and enables the amplitude of the generated ultrasonic waves S to be modulated to a greater extent, thereby generating ultrasonic waves S with a higher sound pressure.

[0030] The acoustic plate 103 is, for example, fixed at its outer periphery 103a and positioned opposite but spaced apart from the piezoelectric body 105. It is preferable that the membrane surface 103b of the acoustic plate 103 be positioned approximately parallel to the surface of the opposing piezoelectric body 105. From the viewpoint of applying sufficient sound pressure from the ultrasonic waves S to the acoustic plate 103, the distance D between the acoustic plate 103 and the piezoelectric body 105 is preferably 10 mm or less. By setting the distance D to 10 mm or less, high-frequency ultrasonic waves can be propagated to the acoustic plate 103 with reduced attenuation. The acoustic plate 103 is not limited to a membrane, and may be a collection of cantilevers, an acoustic plate with slits formed therein, or a membrane attached to one of these to obstruct air flow.

[0031] As described above, the speaker 100 of this embodiment has a small vibration unit (here, the piezoelectric body 105) with a MEMS structure, and is configured to amplitude-modulate the ultrasonic waves S generated by this vibration unit to the audible frequency of the desired audible sound, and vibrate the acoustic plate 103 with acoustic radiation pressure using the sound pressure of the amplitude-modulated ultrasonic waves S to emit the desired audible sound. Amplitude modulation allows the amplitude of the ultrasonic waves S to be changed to adjust the sound pressure, so that sound of any frequency can be generated by vibrating the acoustic plate 103 with the adjusted sound pressure. For example, amplitude modulation can be used to increase the amplitude of the ultrasonic waves S to increase the sound pressure, and the acoustic plate 103 can be vibrated with that sound pressure to generate a low-frequency sound.

[0032] Fig. 2 is a cross-sectional view of a speaker 110 according to a first modification of the first embodiment. In the speaker 110, an auxiliary plate 111 is attached to the center of the acoustic plate 103. Fig. 2 illustrates a case in which the auxiliary plate 111 is attached to the ultrasonic wave generating means 101 side of the acoustic plate 103 (here, the lower side), but it may also be attached to the opposite side of the ultrasonic wave generating means 101 (here, the upper side). The other configuration is the same as that of the speaker 100, and at least the same effects as those of the speaker 100 can be achieved. Configurations corresponding to those of the speaker 100 are indicated by the same reference numerals.

[0033] The auxiliary plate 111 has a higher hardness than the acoustic plate 103, but is heavy enough not to significantly distort the acoustic plate 103 in the initial state. Examples of materials for the auxiliary plate 111 include silicon, glass, metal, and resin.

[0034] By attaching the auxiliary plate 111, the vibration amplitude can be made uniform between the center and outer periphery of the acoustic plate 103, allowing the acoustic plate 103 to vibrate without bending the membrane surface. Compared to when the auxiliary plate 111 is not attached, the vibration at the outer periphery is increased, which adds to the displacement of the acoustic plate 103, thereby increasing the sound pressure of the sound output by the speaker 110. In order to increase the amplitude of the acoustic plate 103, a corrugated rigidity-reducing portion may be provided on the membrane surface 103b outside the auxiliary plate 111 and inside the outer periphery 103a.

[0035] 3 is a cross-sectional view of a speaker 130 according to a second modification of the first embodiment. In the speaker 130, a groove 104b is formed on one surface 104a of the first vibrating body around the area where the piezoelectric body 105 comes into contact. The other configurations are the same as those of the speaker 100, and the speaker 130 can achieve at least the same effects as those of the speaker 100. Configurations corresponding to those of the speaker 100 are denoted by the same reference numerals.

[0036] The voltage characteristics of the ultrasonic wave intensity generated by the piezoelectric body 105 differ depending on whether the applied voltage (drive voltage) is low or high. On the low-voltage side, the intensity of the generated ultrasonic waves increases as the voltage is increased, but on the high-voltage side, the intensity of the generated ultrasonic waves remains almost unchanged even when the voltage is increased, reaching a plateau. The voltage characteristics of the ultrasonic wave intensity on the high-voltage side are affected by the nonlinearity of the rigidity of the first vibrating body 104, i.e., the hard spring effect. The nonlinearity occurs when tensile stress is applied to the first vibrating body 104. Therefore, the nonlinearity can be reduced by alleviating this tensile stress.

[0037] In speaker 130, groove 104b is formed on one surface 104a of the first vibrating body around the area where piezoelectric body 105 comes into contact, thereby alleviating the tensile stress applied to first vibrating body 104. The shape of groove 104b may be any shape that matches the shape of the outer periphery of the piezoelectric body, and may be, for example, ring-shaped.

[0038] The depth of the groove 104b is not particularly limited. As shown in FIG. 3 , the groove 104b may penetrate the first vibrating body 104 and extend into the support 109. In this case, the piezoelectric body 105 is supported by a thin portion of the other surface 104c of the first vibrating body, sandwiched between the groove 104b and a space facing the central portion overlapping the piezoelectric body 105, thereby maintaining a high resonant frequency. Therefore, the speaker 130 can suppress the nonlinearity of the spring constant of the piezoelectric body 105 while maintaining the resonant frequency, thereby applying a high voltage to generate high-intensity ultrasonic waves. The above structure is particularly effective for a small ultrasonic generating means 101 using MEMS technology, such as that of this embodiment. The effects of this structure will be described later.

[0039] Second Embodiment Fig. 4 is a cross-sectional view of a speaker 200 according to a second embodiment of the present invention. In the speaker 200, the ultrasonic wave generating means 101 is a piezoelectric actuator 101A, which is built into an acoustic plate 103. The speaker 200 further includes a first substrate 112 that faces the piezoelectric body 105 or the other surface 104c of the first vibrating body and is spaced apart from it. The speaker 200 may also include a wall material 113 that surrounds a space (cavity) between the piezoelectric actuator 101A and the first substrate 112. The other configuration is the same as that of the speaker 100, and the same effects as those of the speaker 100 can be achieved. Components corresponding to those of the speaker 100 are designated by the same reference numerals.

[0040] The first vibrating body 104 is built into the acoustic plate 103 and is disposed, for example, in a cutout portion formed in the acoustic plate 103. The first substrate 112 is disposed opposite to but spaced apart from the piezoelectric body 105 or the other surface 104c of the first vibrating body. The acoustic plate 103 is disposed so that one end is fixed and the other end is in contact with the first vibrating body 104. FIG. 4 illustrates an example in which the first substrate 112 is disposed opposite to but spaced apart from the other surface 104c of the first vibrating body. The acoustic plate 103 is fixed to one end 113a of the wall material (here, the upper end), and one surface 112a of the first substrate is fixed to the other end 113b of the wall material (here, the lower end).

[0041] In the speaker 200, the piezoelectric actuator 101A, to which an oscillation voltage is applied, generates ultrasonic waves S, a portion of which is irradiated onto the first substrate 112. The ultrasonic waves S' reflected by the first substrate 112 are irradiated onto the piezoelectric actuator 101A, and the resulting acoustic radiation pressure drives the piezoelectric actuator 101A and the acoustic plate 103. As described above, sound is output from the speaker 200 by modulating the ultrasonic waves with an acoustic signal. In this embodiment, the first vibrating body 104 and the acoustic plate 103 function together as an acoustic plate for audible sound.

[0042] 5 is a cross-sectional view of a speaker 300 according to a third embodiment of the present invention. In the speaker 300, the ultrasonic wave generating means 101 is an electrostatic actuator 101B. The other configuration is the same as that of the speaker 100, and the same effects as those of the speaker 100 can be achieved. The same reference numerals are used to designate components corresponding to those of the speaker 100.

[0043] The electrostatic actuator 101B is mainly composed of a second substrate 114, a second vibrating body 115, a spacer 116, and a second voltage application means 117. The second substrate 114 is a rigid substrate with a fixed shape. The material of the second substrate 114 is not particularly limited, but may be, for example, a semiconductor or conductor such as silicon or metal. Alternatively, it may be an insulating material such as glass, with only the surface made conductive. The second vibrating body 115 is a substrate that is softer and more deformable than the second substrate 114. The material of the second vibrating body 115 is not particularly limited, but may be, for example, a semiconductor such as silicon or a conductor such as metal. Alternatively, it may be an insulating material such as glass or resin with a thin film conductor formed on its surface. One surface 114a of the second substrate and one surface 115a of the second vibrating body are arranged to face each other with one or more spacers 116 sandwiched therebetween. The material of the spacer 116 is not particularly limited, but may be, for example, a high-resistance semiconductor or insulator such as silicon, glass, or resin. The space between the surface 114a of the second substrate and the surface 115a of the second vibrating body may be filled with air or may be a vacuum.

[0044] The second voltage application means (second voltage source) 117 is connected to the second substrate 114 and the second vibrating body 115 and is configured to apply an oscillating voltage (second oscillating voltage) between them. The frequency of the oscillating voltage is preferably 40 kHz or higher. By applying the oscillating voltage, an electrostatic force generated between the second substrate 114 and the second vibrating body 115 deforms the second vibrating body 115, and this vibration generates ultrasonic waves S. The frequency of the generated ultrasonic waves S is approximately the same as the frequency of the oscillating voltage.

[0045] In addition, as in the first embodiment, the speaker of this embodiment using this electrostatic actuator 101B may be configured in such a way that an auxiliary plate 111 is provided in the center of the acoustic plate 103, that multiple electrostatic actuators 101B are arranged in an array, or that an ultrasonic generating means 101 is arranged on (inside) the acoustic plate 103.

[0046] The effects of the present invention will be more clearly understood from the following examples. The present invention is not limited to the following examples, and can be practiced with appropriate modifications within the scope of the present invention.

[0047] Comparative Example 1: A small piezoelectric speaker using a bulk material was prepared to simulate the ultrasonic wave generating means (piezoelectric actuator) of the first embodiment. An oscillating voltage was applied to this piezoelectric element, causing the vibrating piezoelectric element to generate sound. The frequency spectrum of the generated sound was measured.

[0048] Figure 6 is a graph showing the measurement results. The horizontal axis of the graph represents frequency, and the vertical axis of the graph represents power spectral density function. When driven at a frequency of 1 kHz or higher, as can be seen from Figure 6, a sharp peak is observed at the drive frequency, whereas at drive frequencies lower than 1 kHz, no peaks other than noise are observed. This experimentally demonstrates that it is difficult for small piezoelectric speakers to output low-frequency sound.

[0049] Example 1: The speaker of the first embodiment (Fig. 1) was fabricated and operated. Specifically, the piezoelectric speaker was driven by an amplitude-modulated 40 kHz ultrasonic signal, and the ultrasonic waves were irradiated onto the acoustic plate, causing it to vibrate. The acoustic plate was made of a vinyl sheet with an estimated thickness of approximately 50 μm. The frequency spectrum of the sound generated from the acoustic plate was measured.

[0050] Figure 7 is a graph showing the measurement results. The horizontal and vertical axes of the graph are the same as those of the graph in Figure 6. Unlike the case in Figure 6, a sharp peak was observed even when driven at a low frequency of 1 kHz or less. Therefore, it can be seen that with the speaker of the present invention, by modulating the amplitude of the ultrasonic waves, it is possible to vibrate the membrane with an adjusted sound pressure, and as a result, it is possible to generate sound of any frequency from the membrane with sufficient sound pressure.

[0051] [Study of ultrasonic wave generating means] (Example 2) An ultrasonic wave generating means was studied. An ultrasonic wave generating means 101 was fabricated by forming a groove 104b around the area in contact with the piezoelectric body 105 on one surface 104a of the first vibrating body of the speaker of the first embodiment (Fig. 1). In this ultrasonic wave generating means 101, the displacement of the acoustic plate generated when the amplitude of the voltage applied to the piezoelectric body was set to 0.25V, 0.5V, 1V, 1.5V, and 3V was measured.

[0052] Figure 8 is a graph showing the measurement results. The peak amplitude increases in proportion to the applied voltage. As the applied voltage and amplitude increase, the tension acting on the membrane increases, and nonlinearity becomes more pronounced. As a result, it can be seen that a hard spring effect occurs, in which the resonant frequency shifts to the higher frequency side as the amplitude increases.

[0053] Example 3 The structure of the piezoelectric actuator 101A was examined. A speaker according to Modification 2 of the first embodiment was fabricated. The depth of the grooves (slits) was set to 15 μm. In this speaker, the amplitude of the acoustic plate generated when the amplitude of the voltage applied to the piezoelectric body was set to 0.25 V, 0.5 V, 1 V, 1.5 V, 2 V, and 3 V was measured.

[0054] Figure 9 is a graph showing the measurement results. The horizontal and vertical axes of the graph are the same as those of the graph in Figure 8. As in Figure 8, the spectrum peak increases in proportion to the applied voltage. However, due to the stress reduction caused by groove 104b, the shift in the resonant frequency of the piezoelectric body toward higher frequencies (hard spring effect) caused by the increased amplitude is reduced, resulting in linear operation.

[0055] 10 is a graph showing the voltage characteristics of the amplitude of ultrasonic waves generated by the ultrasonic generating means of Examples 2 and 3. The horizontal axis of the graph represents the applied voltage, and the vertical axis of the graph represents the amplitude of the acoustic plate. In the ultrasonic generating means of Example 2, which does not have grooves, the amplitude rise curve becomes gentle (plateaus) when the voltage is increased on the high voltage side. In contrast, in the ultrasonic generating means of Example 3, which has grooves, the amplitude rise curve approaches linearity when the voltage is increased on the high voltage side. A comparison of Examples 2 and 3 reveals that when grooves are formed in the first vibrating body, as in the speaker of Modification 2 of the first embodiment, the influence of nonlinearity is suppressed, and a large amplitude proportional to the applied voltage is obtained.

[0056] 100, 110, 130, 200, 300 Speaker 101 Ultrasonic wave generating means 101A Piezoelectric actuator 101B Electrostatic actuator 102 Amplitude modulation means 103 Acoustic plate 103a Outer periphery of acoustic plate 103b Membrane surface of acoustic plate 104 First vibrating body 104a One surface of first vibrating body 104b Groove 104c Other surface of first vibrating body 105 Piezoelectric body 106 First voltage application means 107, 113 Wall material 108 Adhesive layer 109 Support 111 Auxiliary plate 112 First substrate 112a One surface of first substrate 114 Second substrate 115 Second vibrating body 116 Spacer 117 Second voltage application means

Claims

1. A speaker comprising: ultrasonic wave generating means; amplitude modulation means for modulating the amplitude of ultrasonic waves generated by said ultrasonic wave generating means to the amplitude of an envelope waveform corresponding to a sound wave of an audible frequency; and an acoustic plate that vibrates due to the acoustic radiation pressure of said ultrasonic waves after amplitude modulation, wherein said amplitude modulation means modulates the amplitude of said ultrasonic waves by a multiplier of the sound pressure level of said ultrasonic waves.

2. A speaker according to claim 1, wherein an auxiliary plate having a higher hardness than said sound plate is attached to the center of said sound plate.

3. A loudspeaker according to claim 1, wherein the acoustic plate is a single or multiple cantilevered plates, or a membrane that is complete or has slits.

4. A speaker according to claim 1, wherein said amplitude modulation means is configured to match the frequency of the applied vibration voltage to the natural frequency of said ultrasonic wave generating means, thereby causing resonance.

5. The speaker according to claim 1, wherein the ultrasonic wave generating means is a piezoelectric actuator comprising a first vibrating body, a piezoelectric body supported on one surface of the first vibrating body, and a first voltage applying means for applying a first vibration voltage to the piezoelectric body, the amplitude modulation means is connected to the first voltage applying means, and the acoustic plate is positioned opposite to and spaced apart from the piezoelectric body.

6. A speaker according to claim 5, wherein the piezoelectric elements supported on one surface of the first vibrating body are arranged in an array or are configured in multiple layers, with electrodes sandwiched between them.

7. The speaker according to claim 5, wherein a groove is formed on one surface of the first vibrating body around the area in contact with the piezoelectric body.

8. The speaker described in claim 1, further comprising: a first substrate arranged opposite to and spaced apart from the piezoelectric body or the acoustic plate; wherein the ultrasonic wave generating means is a piezoelectric actuator consisting of a first vibrating body, a piezoelectric body supported on one surface of the first vibrating body, and a first voltage applying means for applying a first vibration voltage to the piezoelectric body; the amplitude modulating means is connected to the first voltage applying means; the acoustic plate is arranged so that one end is fixed and the other end is in contact with the first vibrating body; and 9. A speaker as described in any one of claims 1, 2 or 6, characterized in that the ultrasonic wave generating means is an electrostatic actuator composed of a second substrate having a fixed shape, a deformable second vibrating body, a spacer, and a second voltage applying means for applying a second vibration voltage between the second substrate and the second vibrating body, one surface of the second substrate and one surface of the second vibrating body are arranged opposite each other with the spacer in between, the amplitude modulation means is connected to the second voltage applying means, and the acoustic plate is arranged so as to face the other surface of the second vibrating body.

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