Patent Description:
Speakers employing a piezoelectric film (such speakers may hereinafter be referred to as piezoelectric speakers) have been known. Piezoelectric speakers have an advantage in that they are small in volume and light.

Patent Literature <NUM> describes using a piezoelectric speaker as a sound reducing speaker to configure a sound reducing system. Specifically, in this sound reducing system, a piezoelectric film is directly stuck using an adhesive to a wooden board serving as a wall material which is a support.

The present invention aims to provide a sound reducing system well radiating a sound wave for sound reduction from a piezoelectric film.

According to a study by the present inventors, sound in the audible range is easily emitted from a piezoelectric film when an appropriate layer is interposed between the piezoelectric film and a support. An adhesive for fixing the piezoelectric film is also interposed between the piezoelectric film and the support (Patent Literature <NUM>). However, the reproducibility of an implementation of such interposition is low because the adhesive is applied at a place where a sound reducing system should be built. Therefore, an adhesive applied to a piezoelectric film to fix the piezoelectric film to a support is, at least by itself, not suitable for improving a sound reducing system employing a piezoelectric speaker.

Patent Literature <NUM> describes an oscillation noise reducing system mounted with a piezoelectric speaker as an acousto-electric transducer or oscillation-electricity transducer on a suppression object, equipped with an energy consuming means which converts the electric energy obtained from the piezoelectric speaker to thermal energy and consumes the thermal energy. Patent Literature <NUM> describes an electro-acoustic converter that includes two electro-acoustic converting units each comprising: an electro-acoustic converting film including a polymer composite piezoelectric body formed by dispersing piezoelectric particles in a viscoelastic matrix comprising a polymer material that is viscoelastic at room temperature, and two thin-film electrodes laminated onto the two surfaces of the polymer composite piezoelectric body; and a resilient supporting body which is disposed in close contact with one of the main surfaces of the electro-acoustic converting film in such a way as to cause the electro-acoustic converting film to curve to form a curved portion. Patent Literature <NUM> describes a composite piezoelectric vibrator that includes a polymer film composed of fiber, lightweight foam and pressure sensitive adhesive that are combined together by an adhesive and a piezoelectric actuator adhered to surface of the polymer film or embedded in the polymer film. Patent Literature <NUM> describes a film-type audio-speaker that uses a piezoelectric film as a vibration element to provide a reinforcement of the low to medium frequency band sound waves generated. The film-type audio-speaker uses a piezoelectric film as a vibration element and comprises a piezoelectric film with an electrically conductive polymer or metal layer coated on its surface, a terminal portion electrically connected to the piezoelectric film; and an audio-speaker driving unit connected to the terminal portion. Patent Literature <NUM> describes an electroacoustic transducer includes: an electroacoustic transduction unit having an electroacoustic transduction film including a polymer composite piezoelectric body in which piezoelectric body particles are dispersed in a viscoelastic matrix formed of a polymer material having viscoelasticity at a normal temperature and thin film electrodes respectively laminated on both surfaces of the polymer composite piezoelectric body, and an elastic support disposed in close contact with one principal surface to bend the shape of the electroacoustic transduction film; and a first cushion layer that is in contact with the electroacoustic transduction film on an electroacoustic transduction film side of the electroacoustic transduction unit to cover the electroacoustic transduction unit and is formed of a three-dimensional solid knitted fabric.

The present invention provides a sound reducing system as defined in appended claim <NUM>.

The above sound reducing system is suitable for well radiating a sound wave for sound reduction from a piezoelectric film.

A piezoelectric speaker according to a first embodiment will be described using <FIG>. A piezoelectric speaker <NUM> includes a piezoelectric film <NUM>, a fixing face <NUM>, and a film holding portion <NUM>. The fixing face <NUM> is used for fixing the piezoelectric film <NUM> to a support.

The film holding portion <NUM> is disposed between the piezoelectric film <NUM> and the fixing face <NUM>. The film holding portion <NUM> includes an interposed layer <NUM>, a pressure-sensitive adhesive or adhesive layer <NUM> (which may hereinafter be simply referred to as a pressure-sensitive adhesive layer <NUM>), and a pressure-sensitive adhesive or adhesive layer <NUM> (which may hereinafter be simply referred to as a pressure-sensitive adhesive layer <NUM>). In the example in <FIG>, the fixing face <NUM> is formed of a surface (principal surface) of the pressure-sensitive adhesive layer <NUM>. That is, the fixing face <NUM> is a pressure-sensitive adhesive face or an adhesive face.

The piezoelectric film <NUM> includes a piezoelectric body <NUM>, an electrode <NUM>, and an electrode <NUM>. The pressure-sensitive adhesive layer <NUM>, the interposed layer <NUM>, the pressure-sensitive adhesive layer <NUM>, and the piezoelectric film <NUM> are laminated in this order.

Hereinafter, the pressure-sensitive adhesive layer <NUM> may be referred to as a first pressure-sensitive adhesive layer <NUM>, the pressure-sensitive adhesive layer <NUM> may be referred to as a second pressure-sensitive adhesive layer <NUM>, the electrode <NUM> may be referred to as a first electrode <NUM>, and the electrode <NUM> may be referred to as a second electrode <NUM>.

The piezoelectric body <NUM> has the shape of a film. The piezoelectric body <NUM> is vibrated by application of voltage. A ceramic film, a resin film, and the like can be used as the piezoelectric body <NUM>. Examples of the material of the piezoelectric body <NUM> that is a ceramic film include lead zirconate, lead zirconate titanate, lead lanthanum zirconate titanate, barium titanate, Bi-layered compounds, compounds having a tungsten bronze structure, and solid solutions of barium titanate and bismuth ferrite. Examples of the material of the piezoelectric body <NUM> that is a resin film include polyvinylidene fluoride and polylactic acid. The material of the piezoelectric body <NUM> that is a resin film may be a polyolefin such as polyethylene or polypropylene. The piezoelectric body <NUM> may be a non-porous body or may be a porous body.

The thickness of the piezoelectric body <NUM> is, for example, <NUM> to <NUM> and may be <NUM> to <NUM>.

The first electrode <NUM> and the second electrode <NUM> are in contact with the piezoelectric body <NUM> so as to sandwich the piezoelectric body <NUM>. The first electrode <NUM> and the second electrode <NUM> each have the shape of a film. The first electrode <NUM> and the second electrode <NUM> are each connected to a lead wire which is not illustrated. The first electrode <NUM> and the second electrode <NUM> can be formed on the piezoelectric body <NUM> by vapor deposition, plating, sputtering, or the like. A metal foil can be used as each of the first electrode <NUM> and the second electrode <NUM>. A metal foil can be stuck to the piezoelectric body <NUM> using a double-faced tape, a pressure-sensitive adhesive, an adhesive, or the like. Examples of the materials of the first electrode <NUM> and the second electrode <NUM> include metals, and specific examples thereof include gold, platinum, silver, copper, palladium, chromium, molybdenum, iron, tin, aluminum, and nickel. Examples of the materials of the first electrode <NUM> and the second electrode <NUM> also include carbon and electrically conductive polymers. Examples of the materials of the first electrode <NUM> and the second electrode <NUM> also include alloys of the above metals. The first electrode <NUM> and the second electrode <NUM> may include, for example, a glass component.

The thickness of the first electrode <NUM> and that of the second electrode <NUM> are each, for example, <NUM> to <NUM>, and may be <NUM> to <NUM>.

In the example in <FIG>, the first electrode <NUM> covers one entire principal surface of the piezoelectric body <NUM>. The first electrode <NUM> may cover only a portion of the one principal surface of the piezoelectric body <NUM>. The second electrode <NUM> covers the other entire principal surface of the piezoelectric body <NUM>. The second electrode <NUM> may cover only a portion of the other principal surface of the piezoelectric body <NUM>.

The interposed layer <NUM> is disposed between the piezoelectric film <NUM> and the fixing face <NUM>. In the present embodiment, the interposed layer <NUM> is disposed between the piezoelectric film <NUM> and the first pressure-sensitive adhesive layer <NUM>. The interposed layer <NUM> may be a layer other than an adhesive layer and a pressure-sensitive adhesive layer, or may be an adhesive layer or a pressure-sensitive adhesive layer. The interposed layer <NUM> is a porous body layer and/or a resin layer. Here, the resin layer is a concept including a rubber layer and an elastomer layer. Therefore, the interposed layer <NUM> that is a resin layer may be a rubber layer or an elastomer layer. Examples of the interposed layer <NUM> that is a resin layer include an ethylene propylene rubber layer, a butyl rubber layer, a nitrile rubber layer, a natural rubber layer, a styrene-butadiene rubber layer, a silicone layer, a urethane layer, and an acrylic resin layer. Examples of the interposed layer <NUM> that is a porous body layer include foam layers. Specifically, examples of the interposed layer <NUM> that is a porous body layer and a resin layer include an ethylene propylene rubber foam layer, a butyl rubber foam layer, a nitrile rubber foam layer, a natural rubber foam layer, a styrene-butadiene rubber foam layer, a silicone foam layer, and a urethane foam layer. Examples of the interposed layer <NUM> that is not a porous body layer but a resin layer include acrylic resin layers. Examples of the interposed layer <NUM> that is not a resin layer but a porous body layer include porous metal body layers. The term "resin layer" as used herein refers to a resin-including layer. The term "resin layer" as used herein refers to a layer that may include a resin in an amount of <NUM>% or more, in an amount of <NUM>% or more, in an amount of <NUM>% or more, or in an amount of <NUM>% or more. The same applies to, for example, a rubber layer, an elastomer layer, an ethylene propylene rubber layer, a butyl rubber layer, a nitrile rubber layer, a natural rubber layer, a styrene-butadiene rubber layer, a silicone layer, an urethane layer, an acrylic resin layer, a metal layer, a resin film, and a ceramic film. The interposed layer <NUM> may be a blended layer including two or more materials.

The elastic modulus of the interposed layer <NUM> is, for example, <NUM> N/m<NUM> to <NUM> N/m<NUM>, and may be <NUM> N/m<NUM> to <NUM> N/m<NUM>.

In an example, the pore diameter of the interposed layer <NUM> that is a porous body layer is <NUM> to <NUM>, and may be <NUM> to <NUM>. In another example, the pore diameter of the interposed layer <NUM> that is a porous body layer is, for example, <NUM> to <NUM>, and may be <NUM> to <NUM> or <NUM> to <NUM>. The porosity of the interposed layer <NUM> that is a porous body layer is, for example, <NUM>% to <NUM>%, and may be <NUM>% to <NUM>% or <NUM>% to <NUM>%.

A known foam (for example, the foam used in Patent Literature <NUM>) can be used as the interposed layer <NUM> that is a foam layer. The interposed layer <NUM> that is a foam layer may have an open-cell structure, a closed-cell structure, or a semi-open-/semi-closed-cell structure. The term "open-cell structure" refers to a structure having an open cell rate of <NUM>%. The term "closed-cell structure" refers to a structure having an open cell rate of <NUM>%. The term "semi-open-/semi-closed-cell structure" refers to a structure having an open cell rate of greater than <NUM>% and less than <NUM>%. The open cell rate can be calculated, for example, using the following equation after a test in which a foam layer is sunk in water: open cell rate (%) = {(volume of absorbed water)/(volume of cell part)} × <NUM>. In a specific example, the "volume of absorbed water" can be obtained by sinking and leaving a foam layer in water under a reduced pressure of -<NUM> mmHg (<NUM> mmHg being equal to approximately <NUM> Pa) for <NUM> minutes, measuring the mass of water having replaced the air in cells of the foam layer, and converting the mass of water in the cells into volume on the assumption that the density of water is <NUM>/cm<NUM>. The term "volume of cell part" refers to a value calculated using the following equation: volume of cell part (cm<NUM>) = {(mass of foam layer)/(apparent density of foam layer)} - {(mass of foam layer)/(density of material)}. The term "density of material" refers to the density of a matrix (solid, or non-hollow, body) forming the foam layer.

The foaming factor (the ratio between the density before foaming and that after foaming) of the interposed layer <NUM> that is a foam layer is, for example, <NUM> to <NUM>, and may be <NUM> to <NUM>.

The interposed layer <NUM> in an uncompressed state has a thickness of, for example, <NUM> to <NUM>, and may have a thickness of <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. The interposed layer <NUM> in an uncompressed state is typically thicker than the piezoelectric film <NUM> in an uncompressed state. The thickness of the interposed layer <NUM> in an uncompressed state is, for example, <NUM> or more times the thickness of the piezoelectric film <NUM> in an uncompressed state, and may be <NUM> or more times or <NUM> or more times the thickness of the piezoelectric film <NUM> in an uncompressed state. The interposed layer <NUM> in an uncompressed state is typically thicker than the first pressure-sensitive adhesive layer <NUM> in an uncompressed state.

A surface of the first pressure-sensitive adhesive layer <NUM> forms the fixing face <NUM>. The first pressure-sensitive adhesive layer <NUM> is a layer to be joined to a support. In the example in <FIG>, the first pressure-sensitive adhesive layer <NUM> is joined to the interposed layer <NUM>. Examples of the first pressure-sensitive adhesive layer <NUM> include a double-faced tape including a substrate and a pressure-sensitive adhesive applied to the both sides of the substrate. Examples of the substrate of the double-faced tape used as the first pressure-sensitive adhesive layer <NUM> include non-woven fabric. Examples of the pressure-sensitive adhesive of the double-faced tape used as the first pressure-sensitive adhesive layer <NUM> include pressure-sensitive adhesives including an acrylic resin. The first pressure-sensitive adhesive layer <NUM> may be a layer including no substrate and formed of a pressure-sensitive adhesive.

The thickness of the first pressure-sensitive adhesive layer <NUM> is, for example, <NUM> to <NUM>, and may be <NUM> to <NUM>.

The second pressure-sensitive adhesive layer <NUM> is disposed between the interposed layer <NUM> and the piezoelectric film <NUM>. Specifically, the second pressure-sensitive adhesive layer <NUM> is joined to the interposed layer <NUM> and the piezoelectric film <NUM>. Examples of the second pressure-sensitive adhesive layer <NUM> include a double-faced tape including a substrate and a pressure-sensitive adhesive applied to the both sides of the substrate. Examples of the substrate of the double-faced tape used as the second pressure-sensitive adhesive layer <NUM> include non-woven fabric. Examples of the pressure-sensitive adhesive of the double-faced tape used as the second pressure-sensitive adhesive layer <NUM> include pressure-sensitive adhesives including an acrylic resin. The second pressure-sensitive adhesive layer <NUM> may be a layer including no substrate and formed of a pressure-sensitive adhesive.

The thickness of the second pressure-sensitive adhesive layer <NUM> is, for example, <NUM> to <NUM>, and may be <NUM> to <NUM>.

In the present embodiment, the piezoelectric film <NUM> is integrated with the layers on the fixing face <NUM> side by bringing an adhesive face or a pressure-sensitive adhesive face into contact with the piezoelectric film <NUM>. Specifically, in the present embodiment, the adhesive face or the pressure-sensitive adhesive face is a face formed of a surface of the second pressure-sensitive adhesive or adhesive layer <NUM>.

In the present invention, the piezoelectric speaker <NUM> is applied to a sound reducing system <NUM> shown in <FIG>. The sound reducing system <NUM> is a system including at least one sound reducing speaker for radiating a sound wave for sound reduction. Specifically, a sound wave for sound reduction is a sound wave having, in a given region (a region where sound should be reduced) <NUM>, an antiphase which is a phase opposite to that of a sound wave to be canceled out. The sound reducing system <NUM> further includes a reference microphone <NUM>, a difference microphone <NUM>, and a controller <NUM>. Compared to a dynamic speaker, the piezoelectric speaker <NUM> requires a short time from arrival of a signal at the piezoelectric speaker <NUM> to emission of sound (the time may hereinafter be referred to as a delay time). Therefore, the piezoelectric speaker <NUM> is suitable for configuring a compact sound reducing system in that the piezoelectric speaker <NUM> not only is small but also can shorten the distance between the reference microphone <NUM> and the piezoelectric speaker <NUM>. It is also possible, for example, to attach the reference microphone <NUM>, the controller <NUM>, and the piezoelectric speaker <NUM> to one partition.

In the sound reducing system <NUM>, the at least one sound reducing speaker includes at least one piezoelectric speaker <NUM>, and, in the present embodiment, includes a plurality of piezoelectric speakers <NUM>. The sound reducing system <NUM> includes the support <NUM> supporting the piezoelectric speaker <NUM>. The piezoelectric speaker <NUM> is fixed to the support <NUM>. The fixing face <NUM> is in contact with the support <NUM>. Including the plurality of piezoelectric speakers <NUM> is advantageous in view of sound reduction in a wide region.

In a state where the piezoelectric speaker <NUM> is fixed to the support <NUM>, voltage is applied to the piezoelectric film <NUM> through the lead wires. This vibrates the piezoelectric film <NUM>, and thus a sound wave is radiated from the piezoelectric film <NUM>. In the example in <FIG>, the support <NUM> has a flat surface, the piezoelectric speaker <NUM> is fixed to the flat surface, and the piezoelectric film <NUM> is extended flat thereon. This implementation is advantageous in that a sound wave radiated from the piezoelectric film <NUM> is close to a plane wave. When the support <NUM> has a curved surface, the piezoelectric speaker <NUM> may be fixed onto the curved surface.

As shown in <FIG>, it is assumed that a sound wave to be canceled out arrives at a region <NUM> from a noise source <NUM> and has a waveform <NUM> in the region <NUM>. The piezoelectric speaker <NUM> radiates a sound wave having, on the arrival at the region <NUM>, a waveform <NUM> whose phase is opposite to that of the waveform <NUM>. These sound waves cancel out each other in the region <NUM>. In other words, a synthetic sound wave having a waveform <NUM> whose amplitude is reduced to <NUM> or a low level is generated by synthesis from these sound waves in the region <NUM>. The sound reducing system <NUM> reduces sound in such a manner.

In a specific example, each of the plurality of piezoelectric speakers <NUM> forms a wave front. A synthetic wave front synthesized from the wave fronts of the piezoelectric speakers <NUM> propagates to the region <NUM>. The propagation direction of the synthetic wave front can be controlled by controlling a phase difference between voltages applied to the piezoelectric speakers <NUM>.

In the sound reducing system <NUM> shown in <FIG>, feedforward control is performed using the reference microphone <NUM>, the difference microphone <NUM>, and the controller <NUM>. Specifically, the reference microphone <NUM> detects a sound from the noise source <NUM>. The reference microphone <NUM> is typically disposed on the noise source <NUM> side with respect to the piezoelectric speaker <NUM>. Based on the sound detected by the reference microphone <NUM>, the controller <NUM> adjusts a phase of a sound wave to be radiated from the piezoelectric speaker <NUM>. The difference microphone <NUM> is disposed in the region <NUM> and detects sound in the region <NUM>. Based on the sound detected by the difference microphone <NUM>, the controller <NUM> adjusts the amplitude of a sound wave to be radiated from the piezoelectric speaker <NUM> to reduce the amplitude of the synthetic sound wave in the region <NUM>.

A sound reducing system of a variant does not include the reference microphone <NUM>. Feedback control is performed using the difference microphone <NUM> and the controller <NUM>. Specifically, the difference microphone <NUM> adjusts the phase and the amplitude of a sound wave radiated from the piezoelectric speaker <NUM> to reduce the amplitude of a sound wave in the region <NUM>. Consequently, in this case as well, a sound wave from the noise source <NUM> is canceled out in the region <NUM> by a sound wave having an antiphase and generated by the piezoelectric speaker <NUM>.

In the sound reducing system <NUM> of the present embodiment, the support <NUM> is an article produced for a purpose other than supporting the piezoelectric film <NUM> and is used to support the piezoelectric film <NUM>. Therefore, the sound reducing system <NUM> does not require a dedicated article for supporting the piezoelectric film <NUM>. Such a system is advantageous in view of not narrowing a space. Specifically, in the present embodiment, the support <NUM> is a) a partition wall separating a room from outdoors or a different room, the separated room including a space where the sound reducing system <NUM> is to reduce sound or a space where the sound reducing system <NUM> is to prevent sound from escaping to an outside, b) a product installed in the separated room in an immovable or movable manner and having a function other than as a sound reducing speaker, c) a device or a tool designed to be capable of being carried or worn by a person, or d) a noise barrier installed outdoors.

Examples of the support <NUM> that is the above a) include a wall of a building, a ceiling, window glass, a vehicle body, a door, and a barrier defining a space containing a person. Examples of the support <NUM> that is the above b) include office furniture such as partitions, chairs, and tables, household electrical appliances, and sashes. Examples of the support <NUM> that is the above c) include helmets.

The area of a surface of the support <NUM>, the surface facing the fixing face <NUM>, is typically equal to or greater than the area of the fixing face <NUM>. The former area is, for example, <NUM> or more times greater than the latter area, and may be <NUM> or more times or <NUM> or more times greater than the latter area. The support <NUM> typically has a high stiffness (a product of Young's modulus and the second moment of area), a high Young's modulus, and/or a great thickness, compared to the interposed layer <NUM>. The support <NUM> may have the same stiffness, Young's modulus, and/or thickness as that of the interposed layer <NUM>, or may have a lower stiffness, a lower Young's modulus, and/or a smaller thickness than that of the interposed layer <NUM>. The support <NUM> has a Young's modulus of, for example, <NUM> GPa or more, and may have a Young's modulus of <NUM> GPa or more or <NUM> GPa or more. The upper limit of the Young's modulus of the support <NUM> is, for example, but not particularly limited to, <NUM> GPa. Since various articles can be employed as the support <NUM>, it is difficult to define the range of the thickness thereof. The thickness of the support <NUM> is, for example, <NUM> or more, and may be <NUM> or more, <NUM> or more, or <NUM> or more. The upper limit of thickness of the support <NUM> is, for example, but not particularly limited to, <NUM>. The position and/or the shape of the support <NUM> typically does not vary depending on the piezoelectric speaker <NUM>. The support <NUM> is typically produced on the assumption that the support <NUM> is not bent.

In an example, the sound reducing system <NUM> is used to reduce sound in a region where a person is. Specifically, the region <NUM> is a region where a person is. In another example, the sound reducing system <NUM> is used to prevent sound from escaping from the region where a person is. Specifically, the region where a person is is the noise source <NUM>. The size of the region <NUM> is not particularly limited. In an example, the region <NUM> is a whole room, and in another example, the region <NUM> is a portion of a room.

The whole of the sound reducing system <NUM> according to the present embodiment and the components thereof will be further described.

In the sound reducing system <NUM>, the film holding portion <NUM> is disposed between the piezoelectric film <NUM> and the support <NUM>.

In the sound reducing system <NUM>, (i) the film holding portion <NUM> includes a pressure-sensitive adhesive layer and the fixing face <NUM> is formed of a surface of the pressure-sensitive adhesive layer and (ii) the film holding portion <NUM> may include a porous body layer.

The sound reducing system <NUM> as described above is suitable for well radiating a sound wave for sound reduction from the piezoelectric film <NUM>. The first pressure-sensitive adhesive layer <NUM> can fall under the pressure-sensitive adhesive layer in the above (i). The interposed layer <NUM> can fall under the porous body layer in the above (ii).

In the sound reducing system <NUM>, the interposed layer <NUM> is disposed between the piezoelectric film <NUM> and the support <NUM>.

It is likely that lower-frequency sound in the audible range is easily generated from the piezoelectric film <NUM> owing to the interposed layer <NUM> adequately holding one principal surface of the piezoelectric film <NUM>, although the detail of the effect needs to be studied in the future. Given this, the interposed layer <NUM> can be disposed on a region accounting for <NUM>% or more of the area of the piezoelectric film <NUM> when the piezoelectric film <NUM> is viewed in plan. The interposed layer <NUM> may be disposed on a region accounting for <NUM>% or more of the area of the piezoelectric film <NUM>, on a region accounting for <NUM>% or more of the area of the piezoelectric film <NUM>, or on the entire region of the piezoelectric film <NUM> when the piezoelectric film <NUM> is viewed in plan. <NUM>% or more of a principal surface <NUM> of the piezoelectric speaker <NUM>, the principal surface <NUM> being opposite to the fixing face <NUM>, can be composed of the piezoelectric film <NUM>. <NUM>% or more of the principal surface <NUM> may be composed of the piezoelectric film <NUM>. The entire principal surface <NUM> may be composed of the piezoelectric film <NUM>.

In the present embodiment, the second pressure-sensitive adhesive layer <NUM> prevents separation of the piezoelectric film <NUM> and the interposed layer <NUM>. In view of adequate holding, which is mentioned above, the second pressure-sensitive adhesive layer <NUM> and the interposed layer <NUM> can be disposed on a region accounting for <NUM>% or more of the area of the piezoelectric film <NUM> when the piezoelectric film <NUM> is viewed in plan. The second pressure-sensitive adhesive layer <NUM> and the interposed layer <NUM> may be disposed on a region accounting for <NUM>% or more of the area of the piezoelectric film <NUM>, on a region accounting for <NUM>% or more of the area of the piezoelectric film <NUM>, or on the entire region of the piezoelectric film <NUM> when the piezoelectric film <NUM> is viewed in plan.

When the interposed layer <NUM> is a porous body, the rate of the region where the interposed layer <NUM> is disposed is not defined from a microscopical perspective in consideration of pores in the porous structure of the interposed layer <NUM>, but rather from a relatively macroscopic perspective. For example, when the piezoelectric film <NUM>, the interposed layer <NUM> that is a porous body, and the second pressure-sensitive adhesive layer <NUM> are plate-like bodies having the same outline in plan, the second pressure-sensitive adhesive layer <NUM> and the interposed layer <NUM> are described as being disposed on a region accounting for <NUM>% of the area of the piezoelectric film <NUM>.

In the present embodiment, the interposed layer <NUM> has a holding degree of <NUM> × <NUM><NUM> N/m<NUM> or less. The interposed layer <NUM> has a holding degree of, for example, <NUM> × <NUM><NUM> N/m<NUM> or more. The interposed layer <NUM> preferably has a holding degree of <NUM> × <NUM><NUM> N/m<NUM> or less, more preferably <NUM> × <NUM><NUM> N/m<NUM> or less, and even more preferably <NUM> × <NUM><NUM> to <NUM> × <NUM><NUM> N/m<NUM>. The holding degree (N/m<NUM>) of the interposed layer <NUM> is a value obtained by dividing a product of the elastic modulus (N/m<NUM>) of the interposed layer <NUM> and the surface filling factor of the interposed layer <NUM> by the thickness (m) of the interposed layer <NUM>, as represented by the following equation. The surface filling factor of the interposed layer <NUM> is the filling factor (a value obtained by subtracting the porosity from <NUM>) of the principal surface on the piezoelectric film <NUM> side of the interposed layer <NUM>. When pores of the interposed layer <NUM> are evenly distributed, the surface filling factor can be regarded as equal to a three-dimensionally determined filling factor of the interposed layer <NUM>.

The holding degree can be considered to be a parameter representing the degree of holding the piezoelectric film <NUM> by means of the interposed layer <NUM>. The above equation indicates that the greater the elastic modulus of the interposed layer <NUM> is, the greater the degree of holding becomes. The above equation indicates that the greater the surface filling factor of the interposed layer <NUM> is, the greater the degree of holding becomes. The above equation indicates that the smaller the thickness of the interposed layer <NUM> is, the greater the degree of holding becomes. Although the relationship between the holding degree of the interposed layer <NUM> and sound generated from the piezoelectric film <NUM> needs to be studied in the future, it is likely that an excessively great holding degree prevents the piezoelectric film <NUM> from deforming, which is necessary to emit lower-frequency sound. On the other hand, when the holding degree is excessively small, it is likely that the piezoelectric film <NUM> does not sufficiently deform in its thickness direction and extends and contracts only in its in-plane direction (the direction perpendicular to the thickness direction) and thus generation of lower-frequency sound is prevented. It is thought that since the holding degree of the interposed layer <NUM> is set within an adequate range, extension and contraction of the piezoelectric film <NUM> in the in-plane direction is adequately converted into deformation thereof in the thickness direction and that results in appropriate bending of the piezoelectric film <NUM> as a whole and makes it easy to generate lower-frequency sound.

The support <NUM> may have a greater holding degree than that of the interposed layer <NUM>. In this case as well, lower-frequency sound can be generated from the piezoelectric film <NUM> because of the contribution of the interposed layer <NUM>. The support <NUM> may have the same holding degree as that of the interposed layer <NUM>, or may have a smaller holding degree than that of the interposed layer <NUM>. The holding degree (N/m<NUM>) of the support <NUM> is a value obtained by dividing a product of the elastic modulus (N/m<NUM>) of the support <NUM> and the surface filling factor of the support <NUM> by the thickness (m) of the support <NUM>. The surface filling factor of the support <NUM> is the filling factor (a value obtained by subtracting the porosity from <NUM>) of the principal surface on the piezoelectric film <NUM> side of the support <NUM>.

In the present embodiment, the fixing face <NUM> is disposed so that at least a portion of the piezoelectric film <NUM> overlaps the fixing face <NUM> (in the example in <FIG>, overlaps the first pressure-sensitive adhesive layer <NUM>) when the piezoelectric film <NUM> is viewed in plan. In view of stably fixing the piezoelectric speaker <NUM> to the support <NUM>, the fixing face <NUM> can be disposed on a region accounting for <NUM>% or more of the area of the piezoelectric film <NUM> when the piezoelectric film <NUM> is viewed in plan. The fixing face <NUM> may be disposed on a region accounting for <NUM>% or more of the area of the piezoelectric film <NUM> or on the entire region of the piezoelectric film <NUM> when the piezoelectric film <NUM> is viewed in plan.

In the present embodiment, layers located between the piezoelectric film <NUM> and the fixing face <NUM> and adjacent to each other are joined together. The location between the piezoelectric film <NUM> and the fixing face <NUM> includes the piezoelectric film <NUM> and the fixing face <NUM>. Specifically, the first pressure-sensitive adhesive layer <NUM> and the interposed layer <NUM> are joined together, the interposed layer <NUM> and the second pressure-sensitive adhesive layer <NUM> are joined together, and the second pressure-sensitive adhesive layer <NUM> and the piezoelectric film <NUM> are joined together. This allows the piezoelectric film <NUM> to be stably disposed regardless of the orientation in which the piezoelectric film <NUM> is attached to the support <NUM>. This also makes it easy to attach the piezoelectric film <NUM> to the support <NUM>. Moreover, because of the contribution of the interposed layer <NUM>, sound is emitted from the piezoelectric film <NUM> regardless of the orientation in which the piezoelectric film <NUM> is attached. Thus, in the present embodiment, the combination of these allows achievement of a piezoelectric speaker of high usability. The expression "layers adjacent to each other are joined" means that layers adjacent to each other are entirely or partly joined together. In the illustrated example, the layers adjacent to each other are joined together in a given portion extending in the thickness direction of the piezoelectric film <NUM> in the order from the piezoelectric film <NUM>, to the interposed layer <NUM>, and to the fixing face <NUM>.

In the present embodiment, the piezoelectric film <NUM> and the interposed layer <NUM> each have a substantially uniform thickness. This is often advantageous from various points of view, for example, in view of storage of the piezoelectric speaker <NUM>, the usability thereof, and control of sound emitted from the piezoelectric film <NUM>. Having a "substantially uniform thickness" refers to, for example, having the smallest thickness which is <NUM>% or more and <NUM>% or less of the largest thickness. The smallest thickness of each of the piezoelectric film <NUM> and the interposed layer <NUM> may be <NUM>% or more and <NUM>% or less of the largest thickness.

In the present embodiment, the piezoelectric film <NUM> and the film holding portion <NUM> each have a substantially uniform thickness. The piezoelectric film <NUM> and the film holding portion <NUM> may have the smallest thickness which is <NUM>% or more and <NUM>% or less of the largest thickness.

Resin is a material less likely to be cracked than, for example, ceramics. In a specific example, the piezoelectric body <NUM> of the piezoelectric film <NUM> is a resin film and the interposed layer <NUM> is a resin layer not functioning as a piezoelectric film. This specific example is advantageous in that the piezoelectric speaker <NUM> is cut, for example, with scissors or by hand without cracking the piezoelectric body <NUM> or the interposed layer <NUM> (the piezoelectric speaker <NUM> that is cuttable, for example, with scissors or by hand contributes to greater design flexibility of the sound reducing system <NUM> and makes it easy to configure the sound reducing system <NUM>). Additionally, in this specific example, the piezoelectric body <NUM> or the interposed layer <NUM> is unlikely to be cracked by bending the piezoelectric speaker <NUM>. Moreover, that the piezoelectric body <NUM> is a resin film and the interposed layer <NUM> is a resin layer is advantageous in that the piezoelectric speaker <NUM> is fixed onto a curved surface without cracking the piezoelectric body <NUM> or the interposed layer <NUM>.

In the example in <FIG>, the piezoelectric film <NUM>, the interposed layer <NUM>, the first pressure-sensitive adhesive layer <NUM>, and the second pressure-sensitive adhesive layer <NUM> each have the shape of a plate which is neither divided nor frame-shaped, and share the same outline when viewed in plan. Some or all of the piezoelectric film <NUM>, the interposed layer <NUM>, the first pressure-sensitive adhesive layer <NUM>, and the second pressure-sensitive adhesive layer <NUM> may have the shape of a frame. Some or all thereof may be divided into two or more. Their outlines may be misaligned.

In the example in <FIG>, the piezoelectric film <NUM>, the interposed layer <NUM>, the first pressure-sensitive adhesive layer <NUM>, and the second pressure-sensitive adhesive layer <NUM> are each a rectangle having a short side and a long side when viewed in plan. The piezoelectric film <NUM>, the interposed layer <NUM>, the first pressure-sensitive adhesive layer <NUM>, and the second pressure-sensitive adhesive layer <NUM> each may be, for example, a square, a circle, or an oval.

The piezoelectric speaker may also include a layer other than the layers shown in <FIG>.

It goes without saying that the film holding portion <NUM> can be described as likely to include a layer which can be adopted as the interposed layer <NUM>. This applies to a second embodiment described later. For example, the film holding portion <NUM> can be described as likely to include a resin layer not functioning as the piezoelectric film <NUM>. The film holding portion <NUM> can be described as likely to include a porous body layer. The film holding portion <NUM> can be described as likely to include an ethylene propylene rubber foam layer.

Likewise, the film holding portion <NUM> can be described as likely to include a layer that can be adopted as the first pressure-sensitive adhesive layer <NUM>. The film holding portion <NUM> can be described as likely to include a layer that can be adopted as the second pressure-sensitive adhesive layer <NUM>. For example, the film holding portion <NUM> can be described as likely to include a pressure-sensitive adhesive layer or an adhesive layer.

A piezoelectric speaker <NUM> according to the second embodiment will be described using <FIG>. The features identical to those of the first embodiment may not be described hereinafter.

A piezoelectric speaker <NUM> includes the piezoelectric film <NUM>, a fixing face <NUM>, and a film holding portion <NUM>. The fixing face <NUM> can be used to fix the piezoelectric film <NUM> to a support.

The film holding portion <NUM> is located between the piezoelectric film <NUM> and the fixing face <NUM>. (The location between the piezoelectric film <NUM> and the fixing face <NUM> includes the fixing face <NUM>. The same applies to the first embodiment. ) In the example in <FIG>, the film holding portion <NUM> is composed of an interposed layer <NUM>. The fixing face <NUM> is formed of a surface (principal surface) of the interposed layer <NUM>.

The interposed layer <NUM> is a porous body layer and/or a resin layer. The interposed layer <NUM> is a pressure-sensitive adhesive layer or an adhesive layer. A pressure-sensitive adhesive including an acrylic resin can be used as the interposed layer <NUM>. Another pressure-sensitive adhesive, for example, a pressure-sensitive adhesive including rubber, silicone, or urethane may be used as the interposed layer <NUM>. The interposed layer <NUM> may be a blended layer including two or more materials.

The interposed layer <NUM> in an uncompressed state has a thickness of, for example, <NUM> to <NUM>, and may have a thickness of <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. The interposed layer <NUM> in an uncompressed state is typically thicker than the piezoelectric film <NUM> in an uncompressed state. The thickness of the interposed layer <NUM> in an uncompressed state is, for example, <NUM> or more times the thickness of the piezoelectric film <NUM> in an uncompressed state, and may be <NUM> or more times or <NUM> or more times the thickness of the piezoelectric film <NUM> in an uncompressed state.

In the present embodiment, the interposed layer <NUM> has a holding degree of <NUM> × <NUM><NUM> N/m<NUM> or less. The interposed layer <NUM> has a holding degree of, for example, <NUM> × <NUM><NUM> N/m<NUM> or more. The interposed layer <NUM> preferably has a holding degree of <NUM> × <NUM><NUM> N/m<NUM> or less, more preferably <NUM> × <NUM><NUM> N/m<NUM> or less, and even more preferably <NUM> × <NUM><NUM> to <NUM> × <NUM><NUM> N/m<NUM>. The definition of the holding degree is as described previously.

In the present embodiment, the piezoelectric film <NUM> is integrated with the layer on the fixing face <NUM> side by bringing an adhesive face or a pressure-sensitive adhesive face into contact with the piezoelectric film <NUM>. Specifically, in the present embodiment, the adhesive face or the pressure-sensitive adhesive face is a face formed of the interposed layer <NUM>.

The piezoelectric speaker <NUM> can also be fixed to the support <NUM> of <FIG> with the aid of the fixing face <NUM>. The sound reducing system <NUM> employing the piezoelectric speaker <NUM> can be configured in such a manner.

The sound reducing system <NUM> as described above is suitable for well radiating a sound wave for sound reduction from the piezoelectric film <NUM>.

The present invention will be described in detail using Examples. It should be noted that Examples given below are only illustrative of the present invention and do not limit the present invention.

A structure shown in <FIG> was produced by sticking a fixing face <NUM> of a piezoelectric speaker <NUM> to a supporting member <NUM> fixed. Specifically, a <NUM>-mm-thick stainless steel plate (SUS plate) was used as the supporting member <NUM>. A <NUM>-mm-thick pressure-sensitive adhesive sheet (double-faced tape) including non-woven fabric both sides of which were impregnated with an acrylic adhesive was used as the first pressure-sensitive adhesive layer <NUM>. A <NUM>-mm-thick closed-cell foam obtained by foaming a mixture including ethylene propylene rubber and butyl rubber by a foaming factor of about <NUM> was used as the interposed layer <NUM>. A <NUM>-mm-thick pressure-sensitive adhesive sheet (double-faced tape) including non-woven fabric as a substrate to the both sides of which a pressure-sensitive adhesive including a solventless acrylic resin was applied was used as the second pressure-sensitive adhesive layer <NUM>. A polyvinylidene fluoride film on each side of which a copper electrode (including nickel) was vapor-deposited (total thickness: <NUM>) was used as the piezoelectric film <NUM>. The first pressure-sensitive adhesive layer <NUM>, the interposed layer <NUM>, the second pressure-sensitive adhesive layer <NUM>, and the piezoelectric film <NUM> of Example <NUM> each have dimensions of <NUM> long by <NUM> wide when viewed in plan, each have the shape of a plate which is neither divided nor frame-shaped, and have outlines overlapping when viewed in plan. (The same applies to Examples and Reference Example described later. ) The supporting member <NUM> has dimensions of <NUM> long by <NUM> wide when viewed in plan and covers the entire first pressure-sensitive adhesive layer <NUM>. A sample of Example <NUM> having the structure as shown in <FIG> was produced in this manner.

A <NUM>-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about <NUM> was used as an interposed layer <NUM>. This foam includes sulfur. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick foam formed of the same material and having the same configuration as those of the interposed layer <NUM> of Example <NUM> was used as an interposed layer <NUM> in Example <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about <NUM> was used as an interposed layer <NUM>. This foam does not include sulfur and is more flexible than the foams used as the interposed layers <NUM> of Examples <NUM> to <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about <NUM> was used as an interposed layer <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A porous metal body was used as an interposed layer <NUM>. This porous metal body is made of nickel and has a pore diameter of <NUM> and a thickness of <NUM>. A pressure-sensitive adhesive layer same as a first pressure-sensitive adhesive layer <NUM> as used in Example <NUM> was used as a second pressure-sensitive adhesive layer <NUM>. Except for those, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A first pressure-sensitive adhesive layer <NUM> and a second pressure-sensitive adhesive layer <NUM> as used in Example <NUM> were omitted, and only an interposed layer <NUM> was interposed between a piezoelectric film <NUM> as used in Example <NUM> and a support <NUM>. A <NUM>-mm-thick substrate-less pressure-sensitive adhesive sheet formed of an acrylic pressure-sensitive adhesive was used as the interposed layer <NUM>. Except for those, a sample of Example <NUM> having a structure in which a laminate as in <FIG> is attached to a supporting member <NUM> as in <FIG> was produced in the same manner as in Example <NUM>.

An interposed layer same as an interposed layer <NUM> as used in Example <NUM> was used as an interposed layer <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick urethane foam was used as an interposed layer <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick urethane foam was used as an interposed layer <NUM>. This urethane foam has a smaller pore diameter than that of the urethane foam used as the interposed layer <NUM> of Example <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick closed-cell acrylonitrile butadiene rubber foam was used as an interposed layer <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick closed-cell ethylene propylene rubber foam was used as an interposed layer <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick closed-cell foam in which natural rubber and styrene-butadiene rubber are blended was used as an interposed layer <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick closed-cell silicone foam was used as an interposed layer <NUM>. Except for that, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A <NUM>-mm-thick foam formed of the same materials and having the same configuration as those of the interposed layer <NUM> of Example <NUM> was used as an interposed layer <NUM>. A pressure-sensitive adhesive sheet same as the one used as the second pressure-sensitive adhesive layer <NUM> in Example <NUM> was used as the second pressure-sensitive adhesive layer <NUM>. A <NUM>-µm-thick resin sheet including a corn-derived polylactic acid as a main raw material was used as a piezoelectric body <NUM> of the piezoelectric film <NUM>. A first electrode <NUM> and a second electrode <NUM> of the piezoelectric film <NUM> are each formed of a <NUM>-µm-thick aluminum film and were formed by vapor deposition. A piezoelectric film <NUM> having a total thickness of <NUM> was thus obtained. Except for those, a sample of Example <NUM> was produced in the same manner as in Example <NUM>.

A piezoelectric film <NUM> as used in Example <NUM> was employed as a sample of Reference Example <NUM>. In Reference Example <NUM>, the sample was placed on a board parallel to the ground without being adhered to the board.

The methods for evaluation of the samples according to Examples and Reference Example are as follows.

The thickness of each of the interposed layers was measured using a thickness gauge.

A small piece was cut out from each of the interposed layers. The small piece was subjected to a compression test at ordinary temperature using a tensile tester ("RSA-G2" manufactured by TA Instruments). A stress-strain curve was thus obtained. The elastic modulus was calculated from the initial slope of the stress-strain curve.

An enlarged image of each of the interposed layers was obtained using a microscope. The average of the pore diameters of the interposed layer was determined by image analysis of the enlarged image. The average thus determined was employed as the pore diameter of the interposed layer.

A small rectangular cuboid piece was cut out from each of the interposed layers. The apparent density was determined from the volume and the mass of the small rectangular cuboid piece. The apparent density was divided by the density of a matrix (solid, or non-hollow, body) forming the interposed layer. The filling factor was thus calculated. Then, the filling factor was subtracted from <NUM>. The porosity was thus obtained.

For Examples <NUM> to <NUM>, the filling factor calculated as above is employed as the surface filling factor. For Examples <NUM> and <NUM>, the surface filling factor is <NUM>% because the interposed layers have a surface skin layer.

A structure for measurement of the samples of Examples <NUM> to <NUM> and <NUM> to <NUM> is shown in <FIG>. An electrically conductive copper foil tape <NUM> (CU-35C manufactured by <NUM>) having dimensions of <NUM> thick by <NUM> long by <NUM> wide was attached to a corner of each side of the piezoelectric film <NUM>. An alligator clip <NUM> with a cover was attached to each of the electrically conductive copper foil tapes <NUM>. The electrically conductive copper foil tapes <NUM> and the alligator clips <NUM> with covers compose a portion of an electrical pathway used for application of AC voltage to the piezoelectric film <NUM>.

A structure for measurement of the sample of Example <NUM> is shown in <FIG>. The structure in <FIG> lacks the first pressure-sensitive adhesive layer <NUM> and the second pressure-sensitive adhesive layer <NUM> of <FIG>. The structure in <FIG> includes the interposed layer <NUM>.

A structure for measurement of the sample of Reference Example <NUM> is based on the structures of <FIG> and <FIG>. Specifically, as in <FIG> and <FIG>, an electrically conductive copper foil tape <NUM> was attached to a corner of each side of the piezoelectric film <NUM>, and an alliglator clip <NUM> with a cover was attached to each of the tapes <NUM>. The resulting assembly was placed on a board parallel to the ground without being adhered to the board.

Block diagrams for measurement of the acoustic characteristics of the samples are shown in <FIG>. Specifically, an output system is shown in <FIG>, and an evaluation system is shown in <FIG>.

In the output system shown in <FIG>, a personal computer (a personal computer may hereinafter be simply described as a PC) <NUM> for audio output, an audio interface <NUM>, a speaker amplifier <NUM>, a sample <NUM> (any of the piezoelectric speakers of Examples and Reference Example) were connected in this order. The speaker amplifier <NUM> was also connected to an oscilloscope <NUM> so that output from the speaker amplifier <NUM> to the sample <NUM> could be monitored.

WaveGene was installed in the PC <NUM> for audio output. WaveGene is free software for generation of a test audio signal. QUAD-CAPTURE manufactured by Roland Corporation was used as the audio interface <NUM>. The sampling frequency of the audio interface <NUM> was set to <NUM>. A-<NUM> manufactured by Onkyo Corporation was used as the speaker amplifier <NUM>. DPO2024 manufactured by Tektronix, Inc. was used as the oscilloscope <NUM>.

In the evaluation system shown in <FIG>, a microphone <NUM>, an acoustic evaluation apparatus (PULSE) <NUM>, and a PC <NUM> for acoustic evaluation were connected in this order.

Type <NUM>-C-<NUM> manufactured by Bruel & Kjaer Sound & Vibration Measurement A/S was used as the microphone <NUM>. The microphone <NUM> was disposed <NUM> away from the sample <NUM>. Type <NUM>-A-<NUM> manufactured by Bruel & Kjaer Sound & Vibration Measurement A/S was used as the acoustic evaluation apparatus <NUM>.

The output system and the evaluation system were configured in the above manners. AC voltage was applied from the PC <NUM> for audio output to the sample <NUM> via the audio interface <NUM> and the speaker amplifier <NUM>. Specifically, a test audio signal whose frequency sweeps from <NUM> to <NUM> in <NUM> seconds was generated using the PC <NUM> for audio output. During this, voltage output from the speaker amplifier <NUM> was monitored using the oscilloscope <NUM>. Additionally, sound generated from the sample <NUM> was evaluated using the evaluation system. A test for measurement of the sound pressure frequency characteristics was performed in this manner.

The details of the output system and evaluation system settings are as follows.

The lower end of a frequency domain (exclusive of a sharp peak portion in which a frequency range where the sound pressure level is maintained higher than that of background noise by +<NUM> dB or more falls within ±<NUM>% of a peak frequency (a frequency at which the sound pressure level reaches a peak)) where the sound pressure level is higher than that of background noise by <NUM> dB or more was determined as a frequency at which emission of sound starts.

The evaluation results for Examples <NUM> to <NUM> and Reference Example <NUM> are shown in <FIG>. The frequency characteristics of background noise in terms of sound pressure level are shown in <FIG>. E1 to E17 in <FIG> correspond to Examples <NUM> to <NUM>.

<FIG> is referred back to for an exemplary piezoelectric speaker-supporting structure of the present invention. In the piezoelectric speaker <NUM>, the entire surface of the piezoelectric film <NUM> is fixed to the support (supporting structure) <NUM> with the pressure-sensitive adhesive layers <NUM> and <NUM> and the interposed layer <NUM> therebetween.

It is also conceivable that a portion of the piezoelectric film <NUM> is supported to be spaced away from the support <NUM> in order to prevent the support <NUM> from hindering vibration of the piezoelectric film <NUM>. An exemplary supporting structure based on this design concept is shown in <FIG>. In a hypothetical piezoelectric speaker <NUM> shown in <FIG>, a frame <NUM> supports a peripheral portion of the piezoelectric film <NUM> at a position distant from the support <NUM>.

It is easy to ensure a sufficient volume of sound emitted from a piezoelectric film already curved and fixed in one direction. Therefore, it is conceivable that, for example, in the piezoelectric speaker <NUM>, a nonuniformly thick interposition having a convex upper surface is disposed in a space <NUM> surrounded by the piezoelectric film <NUM>, the frame <NUM>, and the support <NUM> and a central portion of the piezoelectric film <NUM> is pushed upward. However, such an interposition is not joined to the piezoelectric film <NUM> so as not to hinder vibration of the piezoelectric film <NUM>. Therefore, even with the interposition disposed in the space <NUM>, it is only the frame <NUM> that supports the piezoelectric film <NUM> so as to determine vibration of the piezoelectric film <NUM>.

Claim 1:
A sound reducing system (<NUM>) comprising at least one sound reducing speaker for radiating a sound wave for sound reduction, a support (<NUM>), a difference microphone (<NUM>), and a controller (<NUM>), wherein
the at least one sound reducing speaker comprises a piezoelectric speaker (<NUM>) which is supported by the support (<NUM>),
the piezoelectric speaker (<NUM>) comprises a piezoelectric film (<NUM>), a fixing face (<NUM>, <NUM>) in contact with the support (<NUM>), and a film holding portion (<NUM>, <NUM>) disposed between the piezoelectric film (<NUM>) and the fixing face (<NUM>, <NUM>),
the difference microphone (<NUM>) is configured to be disposed in a region (<NUM>) where sound should be reduced and to detect sound in the region (<NUM>), such that when a sound wave to be canceled out arrives at the region (<NUM>) from a noise source (<NUM>) and has a second waveform (<NUM>) in the region (<NUM>), a control is performed using the difference microphone (<NUM>) and the controller (<NUM>) so that the piezoelectric speaker (<NUM>) radiates a sound wave having, on the arrival at the region (<NUM>), a first waveform (<NUM>) whose phase is opposite to that of the waveform (<NUM>), characterized in that
the film holding portion (<NUM>, <NUM>) comprises a first pressure-sensitive adhesive layer (<NUM>) and the fixing face (<NUM>, <NUM>) is formed of a surface of the first pressure-sensitive adhesive layer (<NUM>),
the film holding portion (<NUM>, <NUM>) comprises an interposed layer (<NUM>),
the film holding portion (<NUM>, <NUM>) further comprises a second pressure-sensitive adhesive or second adhesive layer (<NUM>), and
the second pressure-sensitive adhesive or second adhesive layer (<NUM>) is disposed between the interposed layer (<NUM>) and the piezoelectric film (<NUM>).