Patent Publication Number: US-2022223136-A1

Title: Active noise control system

Description:
TECHNICAL FIELD 
     The present invention relates to an active noise control system. 
     BACKGROUND ART 
     An active noise control system (hereinafter, referred to also as ANC system) is known. In the ANC system, noise is reduced by opposite-phase sound. Patent 
     Literature 1 describes an example of the ANC system. 
     Patent Literature 1 describes an ANC system according to which noise that is diffracted above a sound insulating wall to propagate is reduced. Specifically, in the ANC system of Patent Literature 1, a speaker having a characteristic of a line sound source is attached to the sound insulating wall. According to the description in Patent Literature 1, the characteristic of the line sound source is such that a radiated sound wave propagates cylindrically with a center axis that is identical with the line sound source. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2004-004583 A 
     Patent Literature 2: JP 2016-122187 A 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In the case where a structure is on a noise propagation path, diffraction may occur at a first end portion and a second end portion of the structure that face each other. Wave fronts generated by the diffraction at these end portions propagate so as to go around behind the structure. Specifically, the wave front generated by the diffraction at the first end portion and the wave front generated by the diffraction at the second end portion propagate so as to approach an axis passing between these end portions and extending in a direction away from the structure. The characteristic of the line sound source in Patent Literature 1 is not suitable for reducing the diffracted sounds generated in this manner at the first end portion and the second end portion. 
     Solution to Problem 
     The present invention provides an active noise control system including; 
     a structure; and 
     a speaker attached to the structure, wherein 
     the speaker includes a radiation surface, 
     the radiation surface has a first region, a second region, and a third region between the first region and the second region, and 
     when an axis passing through the third region and extending away from the radiation surface is defined as a reference axis, the speaker forms a first wavefront propagating from the first region so as to approach the reference axis and a second wavefront propagating from the second region so as to approach the reference axis. 
     Advantageous Effects of Invention 
     In the case where the above structure is on a noise propagation path, diffraction may occur at a first end portion and a second end portion of the structure that face each other. A wave front generated by diffraction at the first end portion and a wave front generated by diffraction at the second end portion propagate so as to approach the reference axis. Meanwhile, in the above ANC system, the first wave front propagates from the first region so as to approach the reference axis, and the second wave front propagates from the second region so as to approach the reference axis. Thus, the wave front derived from diffraction at the first end portion and the wave front derived from diffraction at the second end portion have common propagation directions with the first wave front and the second wave front derived from the ANC system. This is suitable for reducing diffracted sounds generated by diffraction of noise at the first end portion and the second end portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an ANC system. 
         FIG. 2  is a diagram illustrating diffracted waves. 
         FIG. 3  is a diagram illustrating a wave front formed by a speaker of the ANC system. 
         FIG. 4  is a diagram illustrating a wave front formed by a conventional dynamic speaker. 
         FIG. 5  is a diagram illustrating a wave front formed by a conventional plane speaker. 
         FIG. 6A  is a diagram illustrating vibration of a radiation surface of the speaker. 
         FIG. 6B  is a diagram illustrating a supporting structure for a piezoelectric film. 
         FIG. 7  is a perspective view for illustrating a first margin and a second margin. 
         FIG. 8  is a plan view for illustrating the first margin and the second margin. 
         FIG. 9  is a plan view for illustrating the first margin and the second margin. 
         FIG. 10  is a plan view for illustrating the first margin and the second margin. 
         FIG. 11  is a plan view for illustrating the first margin and the second margin. 
         FIG. 12  is a plan view for illustrating the first margin and the second margin. 
         FIG. 13A  is a configuration diagram of a feedforward ANC system. 
         FIG. 13B  is a configuration diagram of a single-channel ANC system. 
         FIG. 13C  is a configuration diagram of a multi-channel ANC system. 
         FIG. 13D  is a configuration diagram of a controller. 
         FIG. 14A  is a configuration diagram of a feedback ANC system. 
         FIG. 14B  is a configuration diagram of a single-channel ANC system. 
         FIG. 14C  is a configuration diagram of a multi-channel ANC system. 
         FIG. 14D  is a configuration diagram of a controller. 
         FIG. 15  is a cross-sectional view taken along a section parallel to a thickness direction of a piezoelectric speaker. 
         FIG. 16  is a top view of the piezoelectric speaker when viewed from the opposite side to a fixing surface. 
         FIG. 17  shows a piezoelectric speaker according to another structure example. 
         FIG. 18  is a view for illustrating structure of a produced sample. 
         FIG. 19  is a view for illustrating structure for sample measurement. 
         FIG. 20  is a view for illustrating structure for sample measurement. 
         FIG. 21  is a block diagram of an output system. 
         FIG. 22  is a block diagram of an evaluation system. 
         FIG. 23A  is a table showing evaluation results of samples. 
         FIG. 23B  is a table showing evaluation results of samples. 
         FIG. 24  is a graph showing a relationship between the holding degree of an interposed layer and a frequency at which emission of sound starts. 
         FIG. 25  is a graph showing the frequency characteristics of Sample E 1  in terms of sound pressure level. 
         FIG. 26  is a graph showing the frequency characteristics of Sample E 2  in terms of sound pressure level. 
         FIG. 27  is a graph showing the frequency characteristics of Sample E 3  in terms of sound pressure level. 
         FIG. 28  is a graph showing the frequency characteristics of Sample E 4  in terms of sound pressure level. 
         FIG. 29  is a graph showing the frequency characteristics of Sample E 5  in terms of sound pressure level. 
         FIG. 30  is a graph showing the frequency characteristics of Sample E 6  in terms of sound pressure level. 
         FIG. 31  is a graph showing the frequency characteristics of Sample E 7  in terms of sound pressure level. 
         FIG. 32  is a graph showing the frequency characteristics of Sample E 8  in terms of sound pressure level. 
         FIG. 33  is a graph showing the frequency characteristics of Sample E 9  in terms of sound pressure level. 
         FIG. 34  is a graph showing the frequency characteristics of Sample E 10  in terms of sound pressure level. 
         FIG. 35  is a graph showing the frequency characteristics of Sample E 11  in terms of sound pressure level. 
         FIG. 36  is a graph showing the frequency characteristics of Sample E 12  in terms of sound pressure level. 
         FIG. 37  is a graph showing the frequency characteristics of Sample E 13  in terms of sound pressure level. 
         FIG. 38  is a graph showing the frequency characteristics of Sample E 14  in terms of sound pressure level. 
         FIG. 39  is a graph showing the frequency characteristics of Sample E 15  in terms of sound pressure level. 
         FIG. 40  is a graph showing the frequency characteristics of Sample E 16  in terms of sound pressure level. 
         FIG. 41  is a graph showing the frequency characteristics of Sample E 17  in terms of sound pressure level. 
         FIG. 42  is a graph showing the frequency characteristics of Sample R 1  in terms of sound pressure level. 
         FIG. 43  is a graph showing the frequency characteristics of background noise in terms of sound pressure level. 
         FIG. 44  is a configuration diagram of an ANC evaluation system. 
         FIG. 45A  is a diagram showing a sound pressure distribution at a speaker OFF time. 
         FIG. 45B  is a diagram showing a sound pressure distribution at a speaker OFF time. 
         FIG. 45C  is a diagram showing a sound pressure distribution at a speaker OFF time. 
         FIG. 46  is a diagram showing propagation of a wave front at the speaker OFF times. 
         FIG. 47A  is a diagram showing a sound pressure distribution at a speaker OFF time. 
         FIG. 47B  is a diagram showing a sound pressure distribution at a speaker OFF time. 
         FIG. 47C  is a diagram showing a sound pressure distribution at a speaker OFF time. 
         FIG. 48  is a diagram showing propagation of a wave front at the speaker OFF times. 
         FIG. 49A  is a diagram showing a sound pressure distribution derived from the piezoelectric speaker. 
         FIG. 49B  is a diagram showing a sound pressure distribution derived from the piezoelectric speaker. 
         FIG. 49C  is a diagram showing a sound pressure distribution derived from the piezoelectric speaker. 
         FIG. 50  is a diagram showing propagation of a wave front derived from the piezoelectric speaker. 
         FIG. 51A  is a diagram showing a sound pressure distribution derived from the piezoelectric speaker. 
         FIG. 51B  is a diagram showing a sound pressure distribution derived from the piezoelectric speaker. 
         FIG. 51C  is a diagram showing a sound pressure distribution derived from the piezoelectric speaker. 
         FIG. 52  is a diagram showing propagation of a wave front derived from the piezoelectric speaker. 
         FIG. 53A  is a diagram showing a sound pressure distribution derived from a dynamic speaker. 
         FIG. 53B  is a diagram showing a sound pressure distribution derived from the dynamic speaker. 
         FIG. 53C  is a diagram showing a sound pressure distribution derived from the dynamic speaker. 
         FIG. 54  is a diagram showing propagation of a wave front derived from the dynamic speaker. 
         FIG. 55A  is a diagram showing a sound pressure distribution derived from the dynamic speaker. 
         FIG. 55B  is a diagram showing a sound pressure distribution derived from the dynamic speaker. 
         FIG. 55C  is a diagram showing a sound pressure distribution derived from the dynamic speaker. 
         FIG. 56  is a diagram showing propagation of a wave front derived from the dynamic speaker. 
         FIG. 57A  is a diagram showing a sound pressure distribution derived from a plane speaker. 
         FIG. 57B  is a diagram showing a sound pressure distribution derived from the plane speaker. 
         FIG. 57C  is a diagram showing a sound pressure distribution derived from the plane speaker. 
         FIG. 58  is a diagram showing propagation of a wave front derived from the plane speaker. 
         FIG. 59A  is a diagram showing a sound pressure distribution derived from the plane speaker. 
         FIG. 59B  is a diagram showing a sound pressure distribution derived from the plane speaker. 
         FIG. 59C  is a diagram showing a sound pressure distribution derived from the plane speaker. 
         FIG. 60  is a diagram showing propagation of a wave front derived from the plane speaker. 
         FIG. 61A  is diagram illustrating a sound reducing effect. 
         FIG. 61B  is diagram illustrating the sound reducing effect. 
         FIG. 61C  is diagram illustrating the sound reducing effect. 
         FIG. 62A  is diagram illustrating the sound reducing effect. 
         FIG. 62B  is diagram illustrating the sound reducing effect. 
         FIG. 62C  is diagram illustrating the sound reducing effect. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. The following description is only illustrative of the embodiment of the present invention and has no intention to limit the present invention. In the following description, the same or similar components are given the same reference numerals, and description thereof may be omitted. 
     [Active Noise Control System] 
       FIG. 1  shows an active noise control system (ANC system)  500  according to an embodiment. The ANC system  500  includes a structure  80  and a speaker  10 . The speaker  10  is attached to the structure  80 . 
     In the illustrated example, the structure  80  is a plate-like body. The structure  80 , which is a plate-like body, for example has a dimension of 20 cm to 600 cm (may have a dimension of 20 cm to 200 cm) in the longitudinal direction, a dimension of 20 cm to 600 cm (may have a dimension of 20 cm to 200 cm) in the lateral direction, and a dimension of 0.1 cm to 15 cm in the front-back direction. Here, the longitudinal direction, the lateral direction, and the front-back direction are perpendicular to each other. The dimension in the longitudinal direction and the dimension in the lateral direction may be the same as or different from each other. 
     A specific example of the structure  80  is a partition. 
     The speaker  10  has a radiation surface  15 . The radiation surface  15  radiates a sound wave by vibrating. This sound wave reduces noise. In the illustrated example, the radiation surface  15  is a continuous radiation surface. 
     Specifically, the structure  80  has end portions  81  and  82  facing each other. The ANC system  500  is suitable for reducing diffracted sounds generated at the end portions  81  and  82 . This point will be described below with reference to  FIG. 2  and  FIG. 3 . 
     As shown in  FIG. 2 , it is assumed that noise from a noise source  200  has propagated toward the structure  80 . In this case, diffraction may occur at the first end portion  81  and the second end portion  82 . Wave fronts generated by diffraction at the end portions  81  and  82  propagate so as to go around behind the structure  80 . Specifically, a wave front  81   w  generated by diffraction at the first end portion  81  and a wave front  82   w  generated by diffraction at the second end portion  82  propagate so as to approach an axis  80 X. Here, the axis  80 X is an axis passing between the first end portion  81  and the second end portion  82  and extending in a direction away from the structure  80 . Specifically, the axis  80 X is perpendicular to a mounting surface of the structure  80  on which the speaker  10  is mounted. The axis  80 X may pass through the center of the mounting surface. 
     The ANC system  500  is suitable for reducing diffracted sounds generated in this manner at the end portions  81  and  82 . Specifically, as shown in  FIG. 3 , the radiation surface  15  has a first region  15   a , a second region  15   b , and a third region  15   c . The third region  15   c  is a region between the first region  15   a  and the second region  15   b . The speaker  10  forms a first wave front  16   a  propagating from the first region  15   a  so as to approach a reference axis  10 X, and a second wave front  16   b  propagating from the second region  15   b  so as to approach the reference axis  10 X. Specifically, in the present embodiment, such first wave front  16   a  and second wave front  16   b  are formed by the radiation surface  15  vibrating. Here, the reference axis  10 X is an axis passing through the third region  15   c  and extending away from the radiation surface  15 . It should be noted that a wave front refers to a surface composed of linked points having the same wave phase. 
     It can be also said that the wave front  81   w  derived from diffraction at the first end portion  81  and the wave front  82   w  derived from diffraction at the second end portion  82  propagate so as to approach the reference axis  10 X shown in  FIG. 3 . 
     Accordingly, the wave front  81   w  derived from diffraction at the first end portion  81  and the wave front  82   w  derived from diffraction at the second end portion  82  have common propagation directions with the first wave front  16   a  and the second wave front  16   b  derived from the ANC system  500 . This is suitable for reducing diffracted sounds generated by diffraction of noise at the first end portion  81  and the second end portion  82 . 
     It is not impossible to mount two speakers separated from each other on the structure  80 , one speaker forming a wave front corresponding to the first wave front  16   a , and the other speaker forming a wave front corresponding to the second wave front  16   b . However, such a case needs for example adjustment on the difference in phase between sounds to be output from the two speakers. Meanwhile, in the present embodiment, the first wave front  16   a  and the second wave front  16   b  can be formed by the radiation surface  15  (continuous radiation surface in the illustrated example) of one speaker  10 . This is advantageous in view of simplifying the control on the speaker  10 . 
     In the present embodiment, the reference axis  10 X is perpendicular to the third region  15   c  in a state where the third region  15   c  does not vibrate. A deviation angle  01  of the first wave front  16   a  relative to the reference axis  10 X in the propagation direction falls within a range of for example 5° to 85°, and may fall within a range of 15° to 75° or a range of 25° to 65°. A deviation angle θ 2  of the second wave front  16   b  relative to the reference axis  10 X in the propagation direction falls within a range of for example 5° to 85°, and may fall within a range of 15° to 75° or a range of 25° to 65°. The third region  15   c  may be plane in a state where the third region  15   c  does not vibrate. Also, the entire radiation surface  15  may be plane in a state where the entire radiation surface  15  does not vibrate. The reference axis  10 X may be an axis passing through the center of the radiation surface  15 . 
     A conventional dynamic speaker  610  shown in  FIG. 4  radiates a substantially hemispherical wave from its radiation surface. The substantially hemispherical wave has a wave front  610 w that is also substantially hemispherical. In  FIG. 4 , an axis  610 X is an axis passing through the radiation surface of the dynamic speaker  610  and extending away from the radiation surface. 
     A conventional plane speaker  620  shown in  FIG. 5  radiates a substantially plane wave from its radiation surface. The substantially plane wave has a wave front  620 w that is also substantially plane. In  FIG. 5 , an axis  620 X is an axis passing through the radiation surface of the plane speaker  620  and extending away from the radiation surface. 
     As can be understood from  FIG. 3 ,  FIG. 4 , and  FIG. 5 , the conventional speakers  610  and  710  cannot achieve the combination according to the present embodiment composed of the first wave front  16   a  propagating from the first region  15   a  so as to approach the reference axis  10 X and the second wave front  16   b  propagating from the second region  15   b  so as to approach the reference axis  10 X. As shown in  FIG. 6A , the speaker  10  of the present embodiment is configured to vibrate well even at the end portions of the radiation surface  15 . The radiation surface  15  as a whole has a high degree of freedom of vibration. This may contribute to formation of the first wave front  16   a  and the second wave front  16   b , although the details need to be studied in the future. In addition, the radiation surface  15  may vibrate in a mode that is close to a free-end vibration mode to a certain extent. Specifically, the radiation surface  15  may vibrate in a mode close to a primary free-end vibration mode to a certain extent. 
     An advantage of a sound reducing effect by the speaker  10  compared to the conventional speakers  610  and  710  tends to be exhibited when noise from the noise source  200  has a high frequency. 
     In a typical example, a portion of an end portion of the radiation surface  15  is formed in the first region  15   a , and a portion of an end portion of the radiation surface  15  is formed in the second region  15   b.    
     Here, a situation is considered in which the speaker  10  is not vibrating and the ANC system  500  does not exhibit its sound reducing function. In this situation, although depending on the size of the structure  80  and the wavelength of noise from the noise source  200 , diffraction of the noise from the noise source  200  at the first end portion  81  and the second end portion  82  of the structure  80  can cause appearance of a period during which the phase of a sound wave in the first region  15   a  and the phase of a sound wave in the second region  15   b  are the same in terms of whether positive or negative, the phase of the sound wave in the first region  15   a  and the phase of a sound wave in the third region  15   c  are opposite to each other in terms of whether positive or negative, and the phase of the sound wave in the second region  15   b  and the phase of the sound wave in the third region  15   c  are opposite to each other in terms of whether positive or negative. 
     With respect to this point, in the present embodiment, a period appears during which the phase of the first wave and the phase of the second wave are the same in terms of whether positive or negative, the phase of the first wave and the phase of the third wave are opposite to each other in terms of whether positive or negative, and the phase of the second wave and the phase of the third wave are opposite to each other in terms of whether positive or negative. Here, the first sound wave is a sound wave in the first region  15   a  formed by the speaker  10 . The second sound wave is a sound wave in the second region  15   b  formed by the speaker  10 . The third sound wave is a sound wave in the third region  15   c  formed by the speaker  10 . According to the present embodiment, noise derived from the noise source  200  having such a phase distribution as described above in the first region  15   a , the second region  15   b , and the third region  15   c  can be reduced by sound derived from the ANC system  500 . 
     As described above, the first sound wave is a sound wave in the first region  15   a  formed by the speaker  10 . The first sound wave is a concept including a sound wave at a position infinitely close to the first region  15   a  in a space facing the first region  15   a . Accordingly, measurement of the first sound wave can be achieved by measuring the sound wave at this “infinitely close position”. The same applies to the second sound wave and the third sound wave. 
     The fact that the phase distribution such as above of the first sound wave, the second wave, and the third sound wave is obtained is consistent with the assumption that the radiation surface  15  is vibrating in the mode close to the primary free-end vibration mode to a certain extent. 
     In the present embodiment, the ANC system  500  includes a controller  110 . A certain frequency range is set in the controller  110 . The controller  110  controls a frequency of sound to be output from the speaker  10  to have a value within the frequency range. The frequency range is, for example, 20 Hz to 20000 Hz, and may be 20 Hz to 6000 Hz. 
     In the present embodiment, when the radiation surface  15  is viewed in plan, the radiation surface  15  has a first end portion  15   j  and a second end portion  15   k  facing each other. When the radiation surface  15  is viewed in plan, a first margin M 1  between the first end portion  15   j  and one of the end portions of the structure  80  is 0 or more and 1/10 or less of a reference wavelength. When the radiation surface  15  is viewed in plan, a second margin M 2  between the second end portion  15   k  and the other end portion of the structure  80  is 0 or more and 1/10 or less of the reference wavelength. Here, the reference wavelength is a wavelength of sound having the upper limit frequency of the above frequency range. This is suitable for reducing diffracted sounds generated by diffraction of noise at the first end portion  81  and the second end portion  82 . The ratio 1/10 is derived from the fact that a sound reducing region by a typical ANC is 1/10 of a wavelength of noise to be controlled. 
     In fact, there are cases where the first margin M 1  and the second margin M 2  should be increased to a certain extent for the sake of commercialization. Taking this into consideration, the upper limits of the first margin M 1  and the second margin M 2  may be increased to exceed 1/10 of the reference wavelength. In view of performing a reasonable commercialization while achieving an effect of reducing diffracted sounds, the first margin M 1  can be set to 0 or more and ⅓ or less of the reference wavelength, for example. Also, the second margin M 2  can be set to 0 or more and ⅓ or less of the reference wavelength when the radiation surface  15  is viewed in plan. 
     The first margin M 1  is, for example, 0 cm to 50 cm, and may be 0 cm to 10 cm. The second margin M 2  is, for example, 0 cm to 50 cm, and may be 0 cm to 10 cm. 
     The first margin M 1  is the distance (specifically, the shortest distance) between the first end portion  15   j  and the one end portion of the structure  80  when the radiation surface  15  is viewed in plan. The second margin M 2  is the distance ( spec ifically, the shortest distance) between the second end portion  15   k  and the other end portion of the structure  80  when the radiation surface  15  is viewed in plan. In the present embodiment, the first margin M 1  is the distance between the first end portion  15   j  and the first end portion  81  when the radiation surface  15  is viewed in plan. In the present embodiment, the second margin M 2  is the distance between the second end portion  15   k  and the second end portion  82  when the radiation surface  15  is viewed in plan. 
     The first margin M 1  and the second margin M 2  will be further described with reference to  FIG. 7  to  FIG. 12 .  FIG. 8  to  FIG. 12  show a long direction  80 L and a short direction  80 S of the structure  80  when the radiation surface  15  is viewed in plan.  FIG. 8  to  FIG. 12  omit the controller  110 . 
     In the examples shown in  FIG. 7  and  FIG. 8 , when the radiation surface  15  is viewed in plan, a peripheral portion of the radiation surface  15  and a peripheral portion of the structure  80  completely coincide with each other over the entire periphery. Accordingly, the first margin M 1  and the second margin M 2  are 0. 
     In the examples shown in  FIG. 9  to  FIG. 12 , the first margin M 1  and the second margin M 2  are larger than 0. 
     In the example in  FIG. 9 , when the radiation surface  15  is viewed in plan, the distance between every portion of an outer periphery of the radiation surface  15  and the end portion of the structure  80  is ⅓ or less of the reference wavelength. Specifically, when the radiation surface  15  is viewed in plan, the distance between every portion of the outer periphery of the radiation surface  15  and the end portion of the structure  80  is 1/10 or less of the reference wavelength. 
     In the example in  FIG. 10 , when the radiation surface  15  is viewed in plan, the long direction of the radiation surface  15  is the same as the short direction  80 S of the structure  80 . The first margin M 1  and the second margin M 2  are margins in the short direction  80 S. In the example in  FIG. 10 , meanwhile, when the radiation surface  15  is viewed in plan, the margin between the end portion of the structure  80  and the end portion of the radiation surface  15  in the long direction  80 L is larger than ⅓ of the reference wavelength. 
     In the example in  FIG. 11 , when the radiation surface  15  is viewed in plan, the long direction of the radiation surface  15  is the same as the long direction  80 L of the structure  80 . The first margin M 1  and the second margin M 2  are margins in the long direction  80 L. In the example in  FIG. 11 , meanwhile, when the radiation surface  15  is viewed in plan, the margin between the end portion of the structure  80  and the end portion of the radiation surface  15  in the short direction  80 S is larger than ⅓ of the reference wavelength. 
     Although not shown, in another example, when the radiation surface  15  is viewed in plan, the long direction of the radiation surface  15  is different from the long direction  80 L and the short direction  80 S of the structure  80 . The first margin M 1  and the second margin M 2  are margins in the short direction  80 S. In the other example, meanwhile, when the radiation surface  15  is viewed in plan, the margin between the end portion of the structure  80  and the end portion of the radiation surface  15  in the long direction  80 L is larger than ⅓ of the reference wavelength. 
     In a specific example, an assembly of the structure  80  and the speaker  10  of the examples shown in  FIG. 7  to  FIG. 11  and the other example described above is disposed such that the short direction  80 S is parallel to the horizontal direction and the long direction  80 L is parallel to the vertical direction. In another specific example, the assembly is disposed such that the short direction  80 S is parallel to the vertical direction and the long direction  80 L is parallel to the horizontal direction. In still another specific example, the assembly is disposed such that the short direction  80 S is parallel to a direction that is inclined relative to the horizontal direction and the vertical direction, and the long direction  80 L is parallel to the direction that is inclined relative to the horizontal direction and the vertical direction, too. For reference,  FIG. 12  shows the assembly of  FIG. 10  to which this inclined disposition is applied. In  FIG. 12 , reference numeral HD indicates the horizontal direction, and reference numeral VD indicates the vertical direction. 
     The first margin M 1  and the second margin M 2  may be the same or different from each other. One of the first margin M 1  and the second margin M 2  may be 0, and the other may be larger than 0. 
     The dimension in the longitudinal direction and the dimension in the lateral direction of the radiation surface  15  when viewed in plan may be the same. In this case, the “long direction of the radiation surface  15 ” and the “short direction of the radiation surface  15 ” in the above description can be replaced with a “first direction of the radiation surface  15 ” and a “second direction of the radiation surface  15 ”. In the case where this replacement is performed, the first direction and the second direction may be directions perpendicular to each other. 
     When the radiation surface  15  is viewed in plan, the dimension in the longitudinal direction and the dimension in the lateral direction of the structure  80  may be the same. In this case, the “long direction of the structure  80 ” and the “short direction of the structure  80 ” in the above description can be replaced with a “third direction of the structure  80 ” and a “fourth direction of the structure  80 ”. In the case where this replacement is performed, the third direction and the fourth direction may be directions perpendicular to each other. 
     As can be understood from the description with reference to  FIG. 7  to  FIG. 12 , the direction in which the speaker  10  is mounted on the structure  80  is not particularly limited. Of course, this is also the case where the structure  80  is a partition. 
     [Feedforward ANC System] 
     In a specific example, the ANC system  500  performs feedforward control. Hereinafter, the ANC system  500  performing feedforward control is referred to also as feedforward ANC system  500 A or ANC system  500 A. Further, the controller  110  in the ANC system  500 A is referred to also as controller  110 A. An ANC system  500 A according to an example will be described with reference to  FIG. 13A  to  FIG. 13D . 
     As shown in  FIG. 13A , the feedforward ANC system  500 A includes a reference microphone  130 , an error microphone  140 , and a controller  110 A. 
     As shown in  FIG. 13A , it is assumed that a sound wave to be cancelled out reaches a region  300  from the noise source  200 , and has a waveform  290  in the region  300 . The speaker  10  radiates a sound wave that is to have, upon reaching the region  300 , a waveform  90  opposite in phase to the waveform  290 . These sound waves cancel out each other in the region  300 . In other words, these sound waves are synthesized in the region  300  to generate a synthetic sound wave having a waveform  390  whose amplitude is reduced to  0  or a low level. In the ANC system  500 A, sound reduction is achieved in this manner. 
     In the ANC system  500 A shown in  FIG. 13A , feedforward control is performed using the reference microphone  130 , the error microphone  140 , and the controller  110 A. Specifically, the reference microphone  130  is disposed on the noise source  200  side when viewed from the speaker  10 . The reference microphone  130  detects sound from the noise source  200 . The error microphone  140  is disposed in the region  300  and detects sound in the region  300 . Based on the sounds detected by the reference microphone  130  and the error microphone  140 , the controller  110 A adjusts a sound wave to be radiated from the speaker  10 . 
     In the example in  FIG. 13A , the ANC system  500 A has only one error microphone  140 . Such an ANC system  500 A may be referred to as single-channel ANC system  500 A. 
     The ANC system  500 A may have a plurality of error microphones  140 . Such an ANC system  500 A may be referred to as multi-channel ANC system  500 A. 
       FIG. 13B  schematically shows the single-channel ANC system  500 A.  FIG. 13C  schematically shows the multi-channel ANC system  500 A. The single-channel ANC system  500 A is advantageous in view of achieving simple control. Meanwhile, the multi-channel ANC system  500 A can reduce noise at a point of each of the error microphones  140 . Providing a plurality of points at which noise can be reduced by the plurality of error microphones  140  (control points) is advantageous in view of achieving sound reduction in a large space. 
       FIG. 13D  is a configuration diagram of a controller  110 A according to an example. The controller  110 A has a preamplifier (hereinafter, amplifier is referred to also as amp)  111 , a low-pass filter  112 , an analog-to-digital converter (hereinafter, referred to also as AD converter)  113 , a power amp  114 , a low-pass filter  115 , a digital-to-analog converter (hereinafter, referred to also as DA converter)  116 , a preamp  117 , a low-pass filter  118 , an AD converter  119 , and a calculation unit  120 A. 
     The preamp  111  amplifies an output signal of the reference microphone  130 . The low-pass filter  112  passes a low-pass component of an output signal of the preamp  111 . The AD converter  113  converts an output signal of the low-pass filter  112  into a digital signal. As a result, a reference signal x(n) at a time n is output from the AD converter  113 . 
     The preamp  117  amplifies an output signal of the error microphone  140 . The low-pass filter  118  passes a low-pass component of an output signal of the preamp  117 . The AD converter  119  converts an output signal of the low-pass filter  118  into a digital signal. As a result, an error signal e(n) at the time n is output from the AD converter  119 . 
     The calculation unit  120 A generates a control signal y(n) at the time n from the reference signal x(n) and the error signal e(n). The calculation unit  120 A includes, for example, a digital signal processor (DSP) or a field-programmable gate array (FPGA). The calculation unit  120 A operates based on, for example, a filtered-x algorithm. 
     The DA converter  116  converts the control signal y(n) into an analog signal. The low-pass filter  115  passes a low-pass component of an output signal of the DA converter  116 . The power amp  114  amplifies an output signal of the low-pass filter  115 . A signal output from the power amp  114  is transmitted as a control signal to the speaker  10 . Based on this signal, sound is output from the radiation surface  15 . 
     As can be understood from the above description, the ANC system  500 A includes the error microphone  140 , the reference microphone  130 , and the controller  110 A. The reference microphone  130 , the structure  80 , the speaker  10 , and the error microphone  140  are arranged in this order. The controller  110 A performs feedforward control of controlling sound to be output from the speaker  10  based on an output signal of the reference microphone  130  and an output signal of the error microphone  140 . Feedforward control enables reduction of not only a periodic signal but also a non-periodic signal. 
     [Feedback ANC System] 
     In a specific example, the ANC system  500  performs feedback control. 
     Hereinafter, the ANC system  500  performing feedback control is referred to also as feedback ANC system  500 B or ANC system  500 B. Further, the controller  110  in the ANC system  500 B is referred to also as controller  110 B. An ANC system  500 B according to an example will be described with reference to  FIG. 14A  to  FIG. 14D . 
     As shown in  FIG. 14A , the feedback ANC system  500 B includes an error microphone  140  and a controller  110 B. 
     As shown in  FIG. 14A , it is assumed that a sound wave to be cancelled out reaches the region  300  from the noise source  200 , and has a waveform  290  in the region  300 . The speaker  10  radiates a sound wave that is to have, upon reaching the region  300 , a waveform  90  opposite in phase to the waveform  290 . These sound waves cancel out each other in the region  300 . In other words, these sound waves are synthesized in the region  300  to generate a synthetic sound wave having a waveform  390  whose amplitude is reduced to 0 or a low level. In the ANC system  500 B, sound reduction is achieved in this manner. 
     In the ANC system  500 B shown in  FIG. 14A , feedback control is performed using the error microphone  140  and the controller  110 B. Specifically, the error microphone  140  is disposed in the region  300  and detects sound in the region  300 . Based on the sound detected by the error microphone  140 , the controller  110 B adjusts a sound wave to be radiated from the speaker  10 . 
     In the example in  FIG. 14A , the ANC system  500 B has only one error microphone  140 . Such an ANC system  500 B may be referred to as single-channel ANC system  500 B. 
     The ANC system  500 B may have a plurality of error microphones  140 . Such an ANC system  500 B may be referred to as multi-channel ANC system  500 B. 
       FIG. 14B  schematically shows the single-channel ANC system  500 B.  FIG. 14C  schematically shows the multi-channel ANC system  500 B. The single-channel ANC system  500 B is advantageous in view of achieving simple control. Meanwhile, the multi-channel ANC system  500 B can reduce noise at a point of each of the error microphones  140 . Providing a plurality of control points by the plurality of error microphones  140  is advantageous in view of achieving sound reduction in a large sp ace. 
       FIG. 14D  is a configuration diagram of a controller  110 B according to an example. The controller  110 B includes the power amp  114 , the low-pass filter  115 , the DA converter  116 , the preamp  117 , the low-pass filter  118 , the AD converter  119 , and a calculation unit  120 B. 
     The preamp  117  amplifies an output signal of the error microphone  140 . The low-pass filter  118  passes a low-pass component of an output signal of the preamp  117 . The AD converter  119  converts an output signal of the low-pass filter  118  into a digital signal. As a result, an error signal e(n) at the time n is output from the AD converter  119 . 
     The operation unit  120 B generates a control signal y(n) at the time n from the error signal e(n). The operation unit  120 B includes, for example, a DSP or an FPGA. The operation unit  120 B operates based on, for example, the filtered-x algorithm. 
     The DA converter  116  converts the control signal y(n) into an analog signal. 
     The low-pass filter  115  passes a low-pass component of an output signal of the DA converter  116 . The power amp  114  amplifies an output signal of the low-pass filter  115 . A signal output from the power amp  114  is transmitted as a control signal to the speaker  10 . Based on this signal, sound is output from the radiation surface  15 . 
     As can be understood from the above description, the ANC system  500 B includes the error microphone  140  and the controller  110 B. The structure  80 , the speaker  10 , and the error microphone  140  are arranged in this order. The controller  110 B performs feedback control of controlling sound to be output from the speaker  10  based on an output signal of the error microphone  140 . Feedback control enables reduction of a periodic signal with no need for the reference microphone  130  of  FIG. 13A . 
     As can be understood from the description on the ANC systems  500 A and  500 B, the controller  110  of the ANC system  500  can have at least one amp. The controller  110  can have at least one low-pass filter. The controller  110  can have at least one AD converter. The controller  110  can have at least one DA converter. These elements can contribute to control on sound to be output from the speaker  10 . 
     The ANC system  500  may be provided in an office and the like. In a specific example, the speaker  10  is attached to the structure  80  that is a partition. The noise source  200  is a person in a certain conference space. The region  300  is another conference space. 
     [First Structure Example of Speaker  10 ] 
     A speaker  10  according to a first structure example will be described with reference to  FIG. 15  and  FIG. 16 . In the first structure example, the speaker  10  is a piezoelectric speaker including a piezoelectric film. Hereinafter, the speaker  10  according to the first structure example is referred to also as piezoelectric speaker  10 . 
     The piezoelectric speaker  10  includes a piezoelectric film  35 , a first joining layer  51 , an interposed layer  40 , and a second joining layer  52 . The first joining layer  51 , the interposed layer  40 , the second joining layer  52 , and the piezoelectric film  35  are laminated in this order. 
     The piezoelectric film  35  includes a piezoelectric body  30 , a first electrode  61 , and a second electrode  62 . 
     The piezoelectric body  30  has the shape of a film. The piezoelectric body  30  is vibrated by application of voltage. A ceramic film, a resin film, and the like can be used as the piezoelectric body  30 . Examples of the material of the piezoelectric body  30  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  30  that is a resin film include polyvinylidene fluoride and polylactic acid. The material of the piezoelectric body  30  that is a resin film may be a polyolefin such as polyethylene or polypropylene. The piezoelectric body  30  may be a non-porous body or may be a porous body. 
     The thickness of the piezoelectric body  30  falls within a range of for example 10 μm to 300 μm, and may fall within a range of 30 μm to 110 μm. 
     The first electrode  61  and the second electrode  62  are in contact with the piezoelectric body  30  so as to sandwich the piezoelectric body  30  therebetween. The first electrode  61  and the second electrode  62  each have the shape of a film. The first electrode  61  and the second electrode  62  are each connected to a lead wire which is not illustrated. The first electrode  61  and the second electrode  62  can be formed on the piezoelectric body  30  by vapor deposition, plating, sputtering, or the like. A metal foil can be used as each of the first electrode  61  and the second electrode  62 . A metal foil can be stuck to the piezoelectric body  30  using a double-faced tape, a pressure-sensitive adhesive, an adhesive, or the like. Examples of the materials of the first electrode  61  and the second electrode  62  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  61  and the second electrode  62  also include carbon and electrically conductive polymers. Examples of the materials of the first electrode  61  and the second electrode  62  also include alloys of the above metals. The first electrode  61  and the second electrode  62  may include, for example, a glass component. 
     The thickness of the first electrode  61  and that of the second electrode  62  each may fall within a range of for example 10 nm to 150 μm, and may fall within a range of 20 nm to 100 μm. 
     In the examples in  FIG. 15  and  FIG. 16 , the first electrode  61  entirely covers one of principal surfaces of the piezoelectric body  30 . The first electrode  61  may only partially cover the one principal surface of the piezoelectric body  30 . The second electrode  62  entirely covers the other principal surface of the piezoelectric body  30 . 
     The second electrode  62  may only partially cover the other principal surface of the piezoelectric body  30 . 
     In the first structure example, the interposed layer  40  is disposed between the piezoelectric film  35  and the first joining layer  51 . The interposed layer  40  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. In the first structure example, the interposed layer  40  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. Accordingly, the interposed layer  40  that is a resin layer may be a rubber layer or an elastomer layer. Examples of the interposed layer  40  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  40  that is a porous body layer include foam layers. Specifically, examples of the interposed layer  40  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  40  that is not a porous body layer and is a resin layer include acrylic resin layers. Examples of the interposed layer  40  that is not a resin layer and is a porous body layer include porous metal body layers. Here, the resin layer refers to a layer containing a resin, and refers to a layer that may contain a resin in an amount of 30% or more, in an amount of 45% or more, in an amount of 60% or more, or in an amount of 80% 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, a urethane layer, an acrylic resin layer, and a metal layer. Further, the same applies to a resin film, a ceramic film, and the like that can be employed as the piezoelectric body  30 . The interposed layer  40  may be a blended layer including two or more materials. 
     The elastic modulus of the interposed layer  40  is, for example, 10000 N/m 2  to 20000000 N/m 2 , and may be 20000 N/m 2  to 100000 N/m 2 . 
     In an example, the pore diameter of the interposed layer  40  that is a porous body layer is 0.1 mm to 7.0 mm, and may be 0.3 mm to 5.0 mm. In another example, the pore diameter of the interposed layer  40  that is a porous body layer is, for example, 0.1 mm to 2.5 mm, and may be 0.2 mm to 1.5 mm or 0.3 mm to 0.7 mm. 
     The porosity of the interposed layer  40  that is a porous body layer is, for example, 70% to 99%, and may be 80% to 99% or 90% to 95%. 
     A known foam (for example, the foam used in Patent Literature 2) can be used as the interposed layer  40  that is a foam layer. The interposed layer  40  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 100%. The term “closed-cell structure” refers to a structure having an open cell rate of 0%. The term “semi-open-/semi-closed-cell structure” refers to a structure having an open cell rate of greater than 0% and less than 100%. 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)}×100. 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 −750 mmHg for 3 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 1.0 g/cm 3 . The term “volume of cell part” refers to a value calculated using the following equation: volume of cell part (cm 3 )={(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  40  that is a foam layer is, for example, 5 to 40, and may be 10 to 40. 
     The interposed layer  40  in an uncompressed state has a thickness of, for example, 0.1 mm to 30 mm, and may have a thickness of 1 mm to 30 mm, 1.5 mm to 30 mm, or 2 mm to 25 mm. The interposed layer  40  in an uncompressed state is typically thicker than the piezoelectric film  35  in an uncompressed state. The thickness of the interposed layer  40  in an uncompressed state is, for example, 3 or more times the thickness of the piezoelectric film  35  in an uncompressed state, and may be 10 or more times or 30 or more times the thickness of the piezoelectric film  35  in an uncompressed state. The interposed layer  40  in an uncompressed state is typically thicker than the first joining layer  51  in an uncompressed state. 
     A surface of the first joining layer  51  forms the fixing surface  17 . The first joining layer  51  is a layer to be joined to the structure  80 . In the example in  FIG. 15 , the first joining layer  51  is joined to the interposed layer  40 . 
     In the first structure example, the first joining layer  51  is a layer having pressure-sensitive adhesiveness or adhesiveness. In other words, the first joining layer  51  is an adhesive layer or a pressure-sensitive adhesive layer. The fixing surface  17  is an adhesive surface or a pressure-sensitive adhesive surface. The first joining layer  51  can be stuck to the structure  80 . In the example in  FIG. 1 , the first joining layer  51  is in contact with the interposed layer  40 . 
     Examples of the first joining layer  51  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 joining layer  51  include non-woven fabric. Examples of the pressure-sensitive adhesive of the double-faced tape used as the first joining layer  51  include pressure-sensitive adhesives including an acrylic resin. The first joining layer  51  may be a layer including no substrate and formed of a pressure-sensitive adhesive. 
     The thickness of the first joining layer  51  is, for example, 0.01 mm to 1.0 mm, and may be 0.05 mm to 0.5 mm. 
     The second joining layer  52  is disposed between the interposed layer  40  and the piezoelectric film  35 . In the first structure example, the second joining layer  52  is a layer having pressure-sensitive adhesiveness or adhesiveness. In other words, the second joining layer  52  is an adhesive layer or a pressure-sensitive adhesive layer. Specifically, the second joining layer  52  is joined to the interposed layer  40  and the piezoelectric film  35 . 
     Examples of the second joining layer  52  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 joining layer  52  include non-woven fabric. Examples of the pressure-sensitive adhesive of the double-faced tape used as the second joining layer  52  include pressure-sensitive adhesives including an acrylic resin. The second joining layer  52  may be a layer including no substrate and formed of a pressure-sensitive adhesive. 
     The thickness of the second joining layer  52  is, for example, 0.01 mm to 1.0 mm, and may be 0.05 mm to 0.5 mm. 
     In the first structure example, the piezoelectric film  35  is integrated with the layers on the fixing surface  17  side by bringing an adhesive surface or a pressure-sensitive adhesive surface into contact with the piezoelectric film  35 . Specifically, in the first structure example, the adhesive surface or the pressure-sensitive adhesive surface is a face formed of a surface of the second pressure-sensitive adhesive or adhesive layer  52 . 
     The piezoelectric speaker  10  is applicable to the ANC system  500 . Compared with dynamic speakers, the piezoelectric speaker  10  requires a short time from reach of an electric signal to the speaker to output of sound (hereinafter, this time is referred to also as delay time). Accordingly, the piezoelectric speaker  10  is suitable for configuring a compact ANC system because of not only being small in size but also being able to reduce the distance between the reference microphone  130  and the piezoelectric speaker  10 . It is also possible, for example, to attach the reference microphone  130 , the controller  110 , and the piezoelectric speaker  10  to a single partition. 
     While the piezoelectric speaker  10  is fixed to the structure  80 , a voltage is applied to the piezoelectric film  35  through a lead wire. This vibrates the piezoelectric film  35 , and thus a sound wave is radiated from the piezoelectric film  35 . 
     The piezoelectric speaker  10  and the ANC system  500  to which the piezoelectric speaker  10  is applied will be further described. 
     The piezoelectric speaker  10  can be fixed to the structure  80  by the fixing surface  17 . In such a manner, the ANC system  500  employing the piezoelectric speaker  10  can be configured. In the ANC system  500 , the interposed layer  40  is disposed between the piezoelectric film  35  and the structure  80 . 
     It is likely that lower-frequency sound in the audible range is easily generated from the piezoelectric film  35  owing to the interposed layer  40  adequately holding one of the principal surfaces of the piezoelectric film  35 , although the detail of the effect needs to be studied in the future. Given this, the interposed layer  40  can be disposed on a region accounting for 25% or more of the area of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. The interposed layer  40  may be disposed on a region accounting for 50% or more of the area of the piezoelectric film  35 , on a region accounting for 75% or more of the area of the piezoelectric film  35 , or on the entire region of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. Also, 50% or more of a principal surface  38  can be formed of the piezoelectric film  35 . The principal surface  38  is one of principal surfaces of the piezoelectric speaker  10  and is opposite to the fixing surface  17  that is the other principal surface. 75% or more of the principal surface  38  may be formed of the piezoelectric film  35 , or the entire principal surface 38 may be formed of the piezoelectric film  35 . 
     In the first structure example, the second joining layer  52  prevents the piezoelectric film  35  and the interposed layer  40  from separating from each other. In view of adequate holding, which is mentioned above, the second joining layer  52  and the interposed layer  40  can be disposed on a region accounting for 25% or more of the area of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. 
     The second joining layer  52  and the interposed layer  40  may be disposed on a region accounting for 50% or more of the area of the piezoelectric film  35 , on a region accounting for 75% or more of the area of the piezoelectric film  35 , or on the entire region of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. 
     In the case where the interposed layer  40  is a porous body, the rate of the region where the interposed layer  40  is disposed is defined not from a microscopical perspective in consideration of pores in the porous structure of the interposed layer  40 , but rather from a relatively macroscopic perspective. For example, in the case where the piezoelectric film  35 , the interposed layer  40  that is a porous body, and the second joining layer  52  are plate-like bodies having the same outline in plan, the second joining layer  52  and the interposed layer  40  are described as being disposed on a region accounting for 100% of the area of the piezoelectric film  35 . 
     In the first structure example, the interposed layer  40  has a holding degree of 5×10 9  N/m 3  or less. The interposed layer  40  has a holding degree of, for example, 1×10 4  N/m 3  or more. The interposed layer  40  has a holding degree of preferably 5×10 8  N/m 3  or less, more preferably 2×10 8  N/m 3  or less, and even more preferably 1×10 5  to 5×10 7  N/m 3 . The holding degree (N/m 3 ) of the interposed layer  40  is a value obtained by dividing a product of the elastic modulus (N/m 2 ) of the interposed layer  40  and the surface filling area ratio of the interposed layer  40  by the thickness (m) of the interposed layer  40 , as represented by the following equation. The surface filling area ratio of the interposed layer  40  is the filling area ratio (a value obtained by subtracting the porosity from 1) of the principal surface on the piezoelectric film  35  side of the interposed layer  40 . In the case where pores of the interposed layer  40  are evenly distributed, the surface filling area ratio can be regarded as equal to a three-dimensionally determined filling area ratio of the interposed layer  40 . 
       Holding degree (N/m 3 )=Elastic modulus (N/m 2 )×Surface filling area ratio÷Thickness (m)
 
     The holding degree can be considered to be a parameter representing the degree of holding the piezoelectric film  35  by means of the interposed layer  40 . The above equation indicates that the greater the elastic modulus of the interposed layer  40  is, the greater the degree of holding becomes. The above equation indicates that the greater the surface filling area ratio of the interposed layer  40  is, the greater the degree of holding becomes. The above equation indicates that the smaller the thickness of the interposed layer  40  is, the greater the degree of holding becomes. Although the relationship between the holding degree of the interposed layer  40  and sound generated from the piezoelectric film  35  needs to be studied in the future, it is likely that an excessively great holding degree prevents the piezoelectric film  35  from deforming, which is necessary to emit lower-frequency sound. On the other hand, in the case where the holding degree is excessively small, it is likely that the piezoelectric film  35  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  40  is set within an adequate range, extension and contraction of the piezoelectric film  35  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  35  as a whole and makes it easy to generate lower-frequency sound. 
     As can be understood from the above description, there may be a layer other than the interposed layer  40  between the piezoelectric film  35  and the fixing surface  17 . The other layer is, for example, the second pressure-adhesive layer  52 . 
     The structure  80  may have a greater holding degree than that of the interposed layer  40 . In this case as well, lower-frequency sound can be generated from the piezoelectric film  35  because of the contribution by the interposed layer  40 . The structure  80  may have the same holding degree as that of the interposed layer  40 , or may have a smaller holding degree than that of the interposed layer  40 . The holding degree (N/m 3 ) of the structure  80  is a value obtained by dividing a product of the elastic modulus (N/m 2 ) of the structure  80  and the surface filling area ratio of the structure  80  by the thickness (m) of the structure  80 . The surface filling area ratio of the structure  80  is the filling area ratio (a value obtained by subtracting the porosity from  1 ) of the principal surface on the piezoelectric film  35  side of the structure  80 . 
     The structure  80  typically has a high stiffness (the product of Young&#39;s modulus and the second moment of area), a high Young&#39;s modulus, and/or a great thickness, compared to the interposed layer  40 . The structure  80  may have the same stiffness, Young&#39;s modulus, and/or thickness as that of the interposed layer  40 , or may have a lower stiffness, a lower Young&#39;s modulus, and/or a smaller thickness than that of the interposed layer  40 . The Young&#39;s modulus of the structure  80  is, for example, 1 GPa or more, and may be 10 GPa or more, or 50 GPa or more. The upper limit of the Young&#39;s modulus of the structure  80  is not particularly limited, and is for example 1000 GPa. 
     In the illustrated example, the piezoelectric film  35  is not completely surrounded by the interposed layer  40 . In the illustrated example, a virtual straight line passes through the interposed layer  40  and the piezoelectric film  35  in this order, and then reaches the outside of the speaker  10  without passing through the interposed layer  40 . Here, the phrase “virtual straight line passes” means that such a straight line can be drawn. In the illustrated example, the interposed layer  40  extends only toward the fixing surface  17  when viewed from the piezoelectric film  35 . 
     In the illustrated example, the principal surface  38 , which is opposite to the fixing surface  17 , of the piezoelectric film  35 , forms the radiation surface  15 . That is, the principal surface  38  is one of principal surfaces of the piezoelectric film  35  which is more distant from the interposed layer  40  than the other is, and forms the radiation surface  15 . In this structure, since the principal surface of the piezoelectric film  35  on the interposed layer  40  side is held by the interposed layer  40 , extension and contraction of the piezoelectric film  35  in the in-plane direction can be adequately converted into deformation thereof in the thickness direction. Other embodiment may be employed. 
     Specifically, a first layer may be provided on the opposite side of the piezoelectric film  35  from the interposed layer  40 . For example, the first layer is used for protecting the piezoelectric film  35 . In this case, a principal surface of the first layer can form the radiation surface  15 . Alternatively, a second layer other than the first layer can form the radiation surface  15 . 
     The thickness of the first layer is, for example, 0.05 mm to 5 mm. The material of the first layer is, for example, a polyester-based material. Here, the polyester-based material refers to a material containing polyester, and refers to a material that may contain 30% or more polyester, 45% or more polyester, 60% or more polyester, and 80% or more polyester. In an example, the material of the interposed layer  40  is different from the material of the first layer. In the case where the material of the interposed layer  40  is different from the material of the first layer, it is possible to make a difference between the degree to which the principal surface on the interposed layer  40  side of the piezoelectric film  35  is held and the degree to which the principal surface on the first layer side of the piezoelectric film  35 . This can allow to adequately convert extension and contraction of the piezoelectric film  35  in the in-plane direction into deformation thereof in the thickness direction. The holding degree of the interposed layer  40  may be different from the holding degree of the first layer. Here, the holding degree (N/m 3 ) of the first layer is a value obtained by dividing the product of the elastic modulus (N/m 2 ) of the first layer and the surface filling area ratio of the first layer by the thickness (m) of the first layer. The surface filling area ratio of the first layer is the filling area ratio (a value obtained by subtracting the porosity from  1 ) of the principal surface on the piezoelectric film  35  side of the first layer. The interposed layer  40  and the first layer differing from each other in holding degree can allow to adequately convert extension and contraction of the piezoelectric film  35  in the in-plane direction into deformation thereof in the thickness direction. In a specific example, the interposed layer  40  has a higher holding degree than the first layer has. The first layer may have the shape of a film. The first layer may be non-woven fabric. 
     In the first structure example, the fixing surface  17  is disposed so that at least a portion of the piezoelectric film  35  overlaps the fixing surface  17  (the first joining layer  51  in the example in  FIG. 15 ) when the piezoelectric film  35  is viewed in plan. In view of stably fixing the piezoelectric speaker  10  to the structure  80 , the fixing surface  17  can be disposed on a region accounting for 50% or more of the area of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. The fixing surface  17  may be disposed on a region accounting for 75% or more of the area of the piezoelectric film  35  or on the entire region of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. 
     In the first structure example, adjacent layers between the piezoelectric film  35  and the fixing surface  17  are joined to each other. Here, the phrase “between the piezoelectric film  35  and the fixing surface  17 ” includes the piezoelectric film  35  and the fixing surface  17 . Specifically, the first joining layer  51  and the interposed layer  40  are joined to each other, the interposed layer  40  and the second joining layer  52  are joined to each other, and the second joining layer  52  and the piezoelectric film  35  are joined to each other. This allows the piezoelectric film  35  to be stably disposed regardless of the orientation in which the piezoelectric film  35  is attached to the structure  80 . This also makes it easy to attach the piezoelectric film  35  to the structure  80 . Moreover, because of the contribution of the interposed layer  40 , sound is emitted from the piezoelectric film  35  regardless of the orientation in which the piezoelectric film  35  is attached. Thus, in the first structure example, the combination of these allows achievement of a piezoelectric speaker of high usability. The phrase “adjacent layers are joined to each other” means that the adjacent layers are entirely or partially joined to each other. In the illustrated examples, the adjacent layers are joined to each other in a predetermined region extending along the thickness direction of the piezoelectric film  35  and passing through the piezoelectric film  35 , the interposed layer  40 , and the fixing surface  17  in this order. 
     In the first structure example, the piezoelectric film  35  and the interposed layer  40  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  10 , the usability thereof, and control of sound emitted from the piezoelectric film  35 . Having a “substantially uniform thickness” refers to, for example, having the smallest thickness which is 70% or more and 100% or less of the largest thickness. The smallest thickness of each of the piezoelectric film  35  and the interposed layer  40  may be 85% or more and 100% 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  30  of the piezoelectric film  35  is a resin film and the interposed layer  40  is a resin layer not functioning as a piezoelectric film. 
     This specific example is advantageous in view of cutting the piezoelectric speaker  10  with for example with scissors or by hand without cracking the piezoelectric body  30  or the interposed layer  40  (the fact that the piezoelectric speaker  10  is cuttable with for example scissors or by hand contributes to greater design flexibility of the ANC system  500  and facilitates to configure the ANC system  500 ). Additionally, in this specific example, the piezoelectric body  30  or the interposed layer  40  is less likely to crack even when the piezoelectric speaker  10  is bent. Moreover, it is advantageous that the piezoelectric body  30  is a resin film and the interposed layer  40  is a resin layer, in view of fixing the piezoelectric speaker  10  onto a curved surface without cracking the piezoelectric body  30  or the interposed layer  40 . 
     In the example in  FIG. 15 , the piezoelectric film  35 , the interposed layer  40 , the first joining layer  51 , and the second joining layer  52  share the same outline when viewed in plan. Their outlines may be misaligned. 
     In the example in  FIG. 15 , the piezoelectric film  35 , the interposed layer  40 , the first joining layer  51 , and the second joining layer  52  are each a rectangle having a short side and a long side when viewed in plan. The piezoelectric film  35 , the interposed layer  40 , the joining layer  51 , and the second joining layer  52  each may be, for example, a square, a circle, or an oval. 
     The piezoelectric speaker  10  may also include a layer other than the layers shown in  FIG. 15 . The layer other than the layer layers shown in  FIG. 15  is for example the first layer and the second layer described above. 
     [Second structure example of speaker  10 ] 
     A piezoelectric speaker  110  according to a second structure example will be described using  FIG. 17 . The features identical to those of the first structure example may not be described hereinafter. 
     The piezoelectric speaker  110  includes the piezoelectric film  35 , a fixing surface  117 , and an interposed layer  140 . The fixing surface  117  can be used to fix the piezoelectric film  35  to the structure  80 . 
     The interposed layer  140  is disposed between the piezoelectric film  35  and the fixing surface  117  (the phrase “between the piezoelectric film  35  and the fixing surface  117 ” includes the fixing surface  117 . The same applies to the first structure example.). The fixing surface  117  is formed of a surface (principal surface) of the interposed layer  140 . 
     The interposed layer  140  is a porous body layer and/or a resin layer. The interposed layer  140  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  140 . Another pressure-sensitive adhesive, for example, a pressure-sensitive adhesive including rubber, silicone, or urethane may be used as the interposed layer  140 . The interposed layer  140  may be a blended layer including two or more materials. 
     The elastic modulus of the interposed layer  140  is, for example, 10000 N/m 2  to 20000000 N/m 2 , and may be 20000 N/m 2  to 100000 N/m 2 . 
     The interposed layer  140  in an uncompressed state has a thickness of, for example, 0.1 mm to 30 mm, and may have a thickness of 1 mm to 30 mm, 1.5 mm to 30 mm, or 2 mm to 25 mm. The interposed layer  140  in an uncompressed state is typically thicker than the piezoelectric film  35  in an uncompressed state. The thickness of the interposed layer  140  in an uncompressed state is, for example,  3  or more times the thickness of the piezoelectric film  35  in an uncompressed state, and may be  10  or more times or  30  or more times the thickness of the piezoelectric film  35  in an uncompressed state. 
     In the second structure example, the interposed layer  140  has a holding degree of 5×10 9  N/m 3  or less. The interposed layer  140  has a holding degree of, for example, 1×10 4  N/m 3  or more. The interposed layer  140  has a holding degree of preferably 5×10 8  N/m 3  or less, more preferably 2×10 8  N/m 3  or less, and even more preferably 1×10 5  to 5×10 7  N/m 3 . The definition of the holding degree is as described previously. 
     In the second structure example, the piezoelectric film  35  is integrated with the layer on the fixing surface  117  side by bringing an adhesive surface or a pressure-sensitive adhesive surface into contact with the piezoelectric film  35 . Specifically, in the second structure example, the adhesive surface or the pressure-sensitive adhesive surface is a face formed of the interposed layer  140 . 
     The piezoelectric speaker  110  can also be fixed to the structure  80  by the fixing surface  117 . In such a manner, the ANC system  500  employing the piezoelectric speaker  110  can be configured. 
     EXAMPLES 
     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. 
     (Sample E 1 ) 
     The fixing surface  17  of the piezoelectric speaker  10  was stuck to a supporting member  680  fixed. Structure as shown in  FIG. 18  was thus produced. Specifically, a 5-mm-thick stainless steel plate (SUS plate) was used as the supporting member  680 . A 0.16-mm-thick pressure-sensitive adhesive sheet (double-faced tape) including non-woven fabric both sides of which were impregnated with an acrylic pressure-sensitive adhesive was used as the first joining layer  51 . A 3-mm-thick closed-cell foam obtained by foaming a mixture including ethylene propylene rubber and butyl rubber by a foaming factor of about  10  was used as the interposed layer  40 . A 0.15-mm-thick pressure-sensitive adhesive sheet (double-faced tape) including non-woven fabric as a substrate having both sides to which a pressure-sensitive adhesive including a solventless acrylic resin was applied was used as the second joining layer  52 . A polyvinylidene fluoride film (total thickness of 33 μm) having both sides on which copper electrodes (including nickel) were vapor-deposited was used as the piezoelectric film  35 . The first joining layer  51 , the interposed layer  40 , the second joining layer  52 , and the piezoelectric film  35  of Sample E 1  each have a dimension of 37.5 mm in the longitudinal direction and a dimension of 37.5 mm in the lateral direction 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 Samples E 2  to E 17  and R 1  described later.) The supporting member  680  has a dimension of 50 mm in the longitudinal direction and a dimension of 50 mm in the lateral direction when viewed in plan and covers the entire first joining layer  51 . Sample E 1  having the structure as shown in  FIG. 18  was produced in this manner. 
     (Sample E 2 ) 
     A 3-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about  10  was used as an interposed layer  40 . This foam includes sulfur. Sample E 2  that is the same as Sample E 1  except the above was produced. 
     (Sample E 3 ) 
     A 5-mm-thick foam formed of the same material and having the same structure as those of the interposed layer  40  of Sample E 2  was used as an interposed layer  40  in Sample E 3 . Sample E 3  that is the same as Sample E 2  except the above was produced. 
     (Sample E 4 ) 
     A 10-mm-thick foam formed of the same material and having the same structure as those of the interposed layer  40  of Sample E 2  was used as an interposed layer  40  in Sample E 4 . Sample E 4  that is the same as Sample E 2  except the above was produced. 
     (Sample E 5 ) 
     A 20-mm-thick foam formed of the same material and having the same structure as those of the interposed layer  40  of Sample E 2  was used as an interposed layer  40  in Sample E 5 . Sample E 5  that is the same as Sample E 2  except the above was produced. 
     (Sample E 6 ) 
     A 20-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about  10  was used as an interposed layer  40 . This foam does not include sulfur and is more flexible than the foams used as the interposed layers  40  of Samples E 2  to E 5 . Sample E 6  that is the same as Sample E 1  except the above was produced. 
     (Sample E 7 ) 
     A 20-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about 20 was used as an interposed layer  40 . Sample E 7  that is the same as Sample E 1  except the above was produced. 
     (Sample E 8 ) 
     A porous metal body was used as an interposed layer  40 . This porous metal body is made of nickel and has a pore diameter of 0.9 mm and a thickness of 2.0 mm. 
     A pressure-sensitive adhesive layer the same as a first joining layer  51  as used in Sample E 1  was used as a second joining layer  52 . Sample E 8  that is the same as Sample E 1  except the above was produced. 
     (Sample E 9 ) 
     A first joining layer  51  and a second joining layer  52  as used in Sample E 1  were omitted, and only an interposed layer  140  was interposed between a piezoelectric film  35  and a structure  80  as used in Sample E 1 . A 3-mm-thick substrate-less pressure-sensitive adhesive sheet formed of an acrylic pressure-sensitive adhesive was used as the interposed layer  140 . Sample E 9  was produced that is the same as Sample E 1  except the above, which has the structure in which the laminate of  FIG. 17  is attached to the supporting member  680  of  FIG. 18 . 
     (Sample E 10 ) 
     An interposed layer the same as an interposed layer  140  as used in Sample E 9  was used as an interposed layer  40 . Sample E 10  that is the same as Sample E 8  except the above was produced. 
     (Sample E  11 ) 
     A 5-mm-thick urethane foam was used as an interposed layer  40 . Sample E 11  that is the same as Sample E 8  except the above was produced. 
     (Sample E 12 ) 
     A 10-mm-thick urethane foam was used as an interposed layer  40 . This urethane foam has a smaller pore diameter than that of the urethane foam used as the interposed layer  40  of Sample E 11 . Sample E 12  that is the same as Sample E 8  except the above was produced. 
     (Sample E 13 ) 
     A 5-mm-thick closed-cell acrylonitrile butadiene rubber foam was used as an interposed layer  40 . Sample E 13  that is the same as Sample E 8  except the above was produced. 
     (Sample E 14 ) 
     A 5-mm-thick closed-cell ethylene propylene rubber foam was used as an interposed layer  40 . Sample E 14  that is the same as Sample E 8  except the above was produced. 
     (Sample E 15 ) 
     A 5-mm-thick closed-cell foam in which natural rubber and styrene-butadiene rubber are blended was used as an interposed layer  40 . Sample E 15  that is the same as Sample E 8  except the above was produced. 
     (Sample E 16 ) 
     A 5-mm-thick closed-cell silicone foam was used as an interposed layer  40 . Sample E 16  that is the same as Sample E 8  except the above was produced. 
     (Sample E 17 ) 
     A 10-mm-thick foam formed of the same materials and having the same structure as those of the interposed layer  40  of Sample E 1  was used as an interposed layer  40 . A pressure-sensitive adhesive sheet the same as that in Sample E 1  was used as a second joining layer  52 . A 35-μm-thick resin sheet including a corn-derived polylactic acid as a main raw material was used as a piezoelectric body  30  of a piezoelectric film  35 . A first electrode  61  and a second electrode  62  of the piezoelectric film  35  are each formed of a 0.1-μm-thick aluminum film and were formed by vapor deposition. The piezoelectric film  35  having a total thickness of 35.2 μm was thus obtained. Sample E 17  that is the same as Sample E 1  except the above was produced. 
     (Sample R 1 ) 
     A piezoelectric film  35  as used in Sample E 1  was employed as Sample R 1 . In Sample R 1 , the sample was placed on a board parallel to the ground without being adhered to the board. 
     The methods for evaluation of Samples E 1  to E 17  and R 1  are as follows. 
     &lt;Thickness of Interposed Layer (Uncompressed State)&gt; 
     The thickness of each of the interposed layers was measured using a thickness gauge. 
     &lt;Elastic Modulus of Interposed Layer&gt; 
     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-G 2 ” 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. 
     &lt;Pore Diameter of Interposed Layer&gt; 
     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. 
     &lt;Porosity of Interposed Layer&gt; 
     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 area ratio was thus calculated. Then, the filling area ratio was subtracted from 1. The porosity was thus obtained. 
     &lt;Surface Filling Area Ratio of Interposed Layer&gt; 
     For Samples E 2  to E 16 , the filling area ratio calculated as above is employed as the surface filling area ratio. For Samples E 1  and E 17 , the surface filling area ratio is 100% because the interposed layers have a surface skin layer. 
     &lt;Frequency Characteristics of Sample in Terms of Sound Pressure Level&gt; 
     Structure for measurement of Samples E 1  to E 8  and E 10  to E 17  is shown in 
       FIG. 19 . An electrically conductive copper foil tape  70  (CU- 35 C manufactured by 3M) having a dimension of 70 μm in the thickness direction, a dimension of 5 mm in the longitudinal direction and. a dimension. of 70 mm in the lateral. direction was attached to a corner of each side of the piezoelectric film  35 . An alligator clip  75  with a cover was attached to each of the electrically conductive copper foil tapes  70 . 
     The electrically conductive copper foil tapes  70  and the alligator clips  75  with covers compose a portion of an electrical pathway used for application of AC voltage to the piezoelectric film  35 . 
     Structure for measurement of Sample E 9  is shown in  FIG. 20 . The structure in  FIG. 20  lacks the first joining layer  51  and the second joining layer  52  of  FIG. 19 . The structure in  FIG. 20  includes the interposed layer  140 . 
     Structure for measurement of Sample R 1  is based on the structures of  FIG. 19  and  FIG. 20 . Specifically, as in  FIG. 19  and  FIG. 20 , an electrically conductive copper foil tape  70  was attached to a corner of each side of the piezoelectric film  35 , and an alligator clip  75  with a cover was attached to each of the tapes  70 . 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. 21  and  FIG. 22 . Specifically, an output system is shown in  FIG. 21 , and an evaluation system is shown in  FIG. 22 . 
     In the output system shown in  FIG. 21 , an audio output personal computer (h e reinafter, personal computer is also simplified as PC)  401 , an audio interface  402 , a speaker amp  403 , a sample  404  (any of the piezoelectric speakers of Samples E 1  to E 17  and R 1 ) were connected in this order. The speaker amp  403  was also connected to an oscilloscope  405  so that output from the speaker amp  403  to the sample  404  could be monitored. 
     WaveGene was installed in the audio output PC  401 . WaveGene is free software for generation of a test audio signal. QUAD-CAPTURE manufactured by Roland Corporation was used as the audio interface  402 . The sampling frequency of the audio interface  402  was set to 192 kHz. A-924 manufactured by Onkyo Corporation was used as the speaker amp  403 . DP02024 manufactured by Tektronix, Inc. was used as the oscilloscope  405 . 
     In the evaluation system shown in  FIG. 22 , a microphone  501 , an acoustic evaluation apparatus (PULSE)  502 , and an acoustic evaluation PC  503  were connected in this order. 
     Type 4939-C-002 manufactured by Bruel &amp; Kjaer Sound &amp; Vibration 
     Measurement A/S was used as the microphone  501 . The microphone  501  was disposed 1 m away from the sample  404 . Type 3052-A-030 manufactured by Bruel &amp; Kjaer Sound &amp; Vibration Measurement A/S was used as the acoustic evaluation apparatus  502 . 
     The output system and the evaluation system were configured in the above manners. AC voltage was applied from the audio output PC  401  to the sample  404  via the audio interface  402  and the speaker amp  403 . Specifically, a test audio signal whose frequency sweeps from 100 Hz to 100 kHz in 20 seconds was generated, using the audio output PC  401 . During this, voltage output from the speaker amp  403  was monitored using the oscilloscope  405 . Additionally, sound generated from the sample  404  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. 
     [Output System Settings]
     Frequency range: 100 Hz to 100 kHz   Sweep time: 20 seconds   Effective voltage: 10 V   Output waveform: sine curve   

     [Evaluation System Settings]
     Measurement time: 22 seconds   Peak hold   Measurement range: 4 Hz to 102.4 kHz   Number of lines: 6400   

     &lt;Determination of Frequency at which Emission of Sound Starts&gt; 
     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 +3 dB or more falls within ±10% 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 3 dB or more was determined as a frequency at which emission of sound starts. 
     The evaluation results for Samples E 1  to E 17  and R 1  are shown in  FIG. 23A  to  FIG. 42 . The frequency characteristics of background noise in terms of sound pressure level are shown in  FIG. 43 . Reference numerals E 1  to E 17  in  FIG. 24  correspond to Samples E 1  to E 17 . 
     (Evaluation of ANC system) 
     An ANC evaluation system  800  shown in  FIG. 44  was configured by using the same piezoelectric speaker  10  as the piezoelectric speaker  10  of Sample E 1  except that the dimensions of the piezoelectric speaker  10  in plan view were set to 35 cm in the longitudinal direction and 50 cm in the lateral direction. 
     The piezoelectric speaker  10  was attached to a partition  780 . A noise source  700 , a reference microphone  730 , the partition  780 , the piezoelectric speaker  10 , and an error microphone  735  were disposed, such that the noise source  700 , the reference microphone  730 , the center of the partition  780 , the center of the piezoelectric speaker  10 , and the error microphone  735  were arranged in this order on a straight line. A control region  790  was set on the piezoelectric speaker  10  side when viewed from the partition  780 . A measurement microphone  740  was disposed in the control region  790 . 
     In  FIG. 44 , the x direction is the longitudinal direction of the control region  790 , the y direction is the lateral direction of the control region  790 , and the z direction is the depth direction of the control region  790 . The x direction, the y direction, and the z direction are perpendicular to each other. 
     The z direction is also a direction in which the noise source  700 , the reference microphone  730 , the center of the partition  780 , the center of the piezoelectric speaker  10 , and the error microphone  735  are arranged. The z direction is further a direction in which the radiation surface  15  of the piezoelectric speaker  10  faces. 
     The noise source  700  used was Eclipse TD508MK3 manufactured by Fujitsu Ten Limited. The partition  780  used was Desk side screen R manufactured by 
     Mihashi kougei, Inc. The reference microphone  730  used was ECM-PC60 manufactured by Sony Corporation. The error microphone  735  used was ECM-PC60 manufactured by Sony Corporation. The measurement microphone  740  used was ECM-PC60 manufactured by Sony Corporation. 
     The distance between the noise source  700  and the reference microphone  730  is 5 cm. The distance between the reference microphone  730  and the partition  780  is 60 cm. The distance between the radiation surface  15  of the piezoelectric speaker  10  and the error microphone  735  is 17.5 cm. These distances are the dimensions in the z direction. 
     The partition  780  has a rectangular plate-like shape in plan view. The partition  780  has a dimension. of 60 cm in the longitudinal. direction, a dimension of 45 cm in the lateral direction, and a dimension of 0.5 cm in the front-back direction. The control region  790  has a dimension of 60 cm in the longitudinal direction, a dimension. of 4.5 cm in the lateral. direction, and a dimension. of 60 cm in the depth direction. These longitudinal directions indicate the x direction. These lateral directions indicate the y direction. These front-back and depth directions indicate the z direction. 
     In the ANC evaluation system  800 , the first margin M 1  is 5 cm and the second margin M 2  is 5 cm. These margins are dimensions in the x direction. 
     In the ANC evaluation system  800 , an output signal personal computer (PC)  750 , a measurement PC  760 , and a controller  710  were used. The output signal PC  750  was connected to the noise source  700  and the measurement PC  760 . 
     The output signal PC  750  transmits a noise signal to the noise source  700 . The output signal PC  750  thus causes the noise source  700  to radiate a sine wave. 
     Also, the output signal PC  750  transmits a trigger signal to the measurement PC  760 . The trigger signal enables to give a common reference time to each measurement data piece. Specifically, sound pressure data pieces with the uniform time axis can be obtained for  176  measurement points described later. This enables mapping of sound pressure distributions shown in  FIG. 45A  to  FIG. 60  described later. 
     The reference microphone  730  detects sound from the noise source  700 . An output signal of the reference microphone  730  is transmitted to the controller  710 . 
     The error microphone  735  detects sound in the control region  790 . An output signal of the error microphone  735  is transmitted to the controller  710 . 
     Based on the output signals of the reference microphone  730  and the error microphone  735 , the controller  710  transmits a control signal to the piezoelectric speaker  10 . The controller  710  thus controls a sound wave to be radiated from the piezoelectric speaker  10 . 
     The measurement microphone  740  detects sound at a position where the measurement microphone  740  is disposed. An output signal of the measurement microphone  740  is transmitted to the measurement PC  760 . 
     The measurement PC  760  receives the trigger signal from the output signal PC  750  and the output signal of the measurement microphone  740 . 
     The control region  790  has a measurement cross-section  790 CS extending in the x direction and the z direction. In the ANC evaluation system  800 ,  176  measurement points are provided on the measurement cross-section  790 CS. Specifically, the measurement cross-section  790 CS is divided equally into 11 pieces in the x direction and is divided equally into 16 pieces in the z direction. The number of measurement points,  176 , is the product of 11, which is the number of divisions in the x direction, and  16 , which is the number of divisions in the z direction. The position of the measurement cross-section  790 CS in the y direction is the same as the center position of the radiation surface  15  in the y direction. The error microphone  735  is provided on the measurement cross-section  790 CS. 
     In the ANC evaluation system  800 , the measurement microphone  740  is successively moved to the  176  measurement points. Thus, in cooperation with the measurement PC  760 , the microphone  740  measures the sound pressures at the  176  measurement points. Specifically, the measurement PC  760  maps the distribution of the sound pressures at the  176  measurement points. This mapping visualizes the sound field of the measurement cross-section  790 CS. 
     Hereinafter, description will be given based on actual measurement data with reference to  FIG. 45A  to  FIG. 62C .  FIG. 45A  to  FIG. 62C  omit a portion of the control region  790  shown in  FIG. 44  that is distant from the partition  780 . In  FIG. 45A  to  FIG. 45C ,  FIG. 47A  to  FIG. 47C ,  FIG. 49A  to  FIG. 49C ,  FIG. 51A  to  FIG. 51C ,  FIG. 53A  to  FIG. 53C ,  FIG. 55A  to  FIG. 55C ,  FIG. 57A  to  FIG. 57C , and  FIG. 59A  to  FIG. 59C , the numerical value on the color bar indicates the sound pressure level in units of pascal (Pa). While the numerical value being positive means that the sound pressure is positive, the numerical value being negative means that the sound pressure is negative. 
     (Reference Example 1: Measurement of diffracted sound) 
     In a state where the piezoelectric speaker  10  radiated no sound and the noise source  700  radiated a sine wave, sound pressures at the 176 measurement points of the measurement cross-section  790 CS were measured for mapping.  FIG. 45A  to  FIG. 48  show the sound pressure distributions obtained by the mapping. In  FIG. 45A  to  FIG. 48 , the piezoelectric speaker  10  is not shown so as to facilitate an intuitive understanding that diffracted sound is measured. However, the measurement of Reference Example 1 was performed while the piezoelectric speaker  10  was attached to the partition  780 , in the same manner as in Example 1 described later. 
     Specifically,  FIG. 45A  to  FIG. 45C  show the sound pressure distributions derived from the noise source  700  relating to different times in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz.  FIG. 45A  to  FIG. 45C  are arranged in chronological order. A series of lines in  FIG. 46  represent propagation over time of a certain wave front generated by the noise source  700  radiating the sine wave of 500 Hz.  FIG. 47A  to  FIG. 47C  show the sound pressure distribution from the noise source  700  relating to different times in the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz.  FIG. 47A  to  FIG. 47C  are arranged in chronological order. A series of lines in  FIG. 48  represent propagation over time of a certain wave front generated by the noise source  700  radiating the sine wave of 800 Hz. 
     In  FIG. 46 , the lines in the series of lines represent respective positions of the “certain wave front” at different times. In general, in  FIG. 46 , one of two adjacent lines that is further away from the partition  780  than the other is indicates the “certain wave front” at a more advanced time. Block arrows in  FIG. 46  represent the propagation direction of the wave fronts. The same descriptions of the series of lines and the block arrows apply to  FIG. 48 ,  FIG. 50 ,  FIG. 52 ,  FIG. 54 ,  FIG. 56 ,  FIG. 58 , and  FIG. 60 . 
       FIG. 46  was prepared by the following procedure. First, a plurality of sound pressure distribution maps based on actual measurements relating to different times, similar to those in  FIG. 45A  to  FIG. 45C , were obtained. Next, in each of the plurality of sound pressure distribution maps, a line corresponding to the certain wave front was manually drawn. Then, the plurality of sound pressure distribution maps on which the lines have been drawn were overlapped each other. Thus, the diagram shown in  FIG. 46  was obtained in which the series of lines representing propagation of the wave fronts were drawn. The same description of the drawing procedure applies to  FIG. 48 ,  FIG. 50 ,  FIG. 52 ,  FIG. 54 ,  FIG. 56 ,  FIG. 58 , and  FIG. 60 . 
       FIG. 45A  to  FIG. 48  show that diffraction occurs at end portions of the partition  780  that face each other.  FIG. 45A  to  FIG. 48  also show that wave fronts generated by diffraction at these end portions propagate so as to go around behind the partition  780 . Specifically,  FIG. 45A  to  FIG. 48  show that the wave fronts generated by diffraction at these end portions propagate so as to approach an axis passing through the center of the partition  780  and extending in the z direction. 
     Wave front propagation shown in  FIG. 45A  to  FIG. 48  occurs in the same manner as in  FIG. 2 . 
     Example 1 
     Measurement of Sound Output from Piezoelectric Speaker  10   
     In a state where the noise source  700  radiated a sine wave in the same manner as in Reference Example 1, the controller  710  was used to vibrate the piezoelectric speaker  10  thereby to cause the piezoelectric speaker  10  to generate a sound wave for sound reduction. At this time, a control signal to be transmitted to piezoelectric speaker  10  was stored in the controller  710 . Then, in a state where the noise source  700  radiated no sound, the controller  710  was caused to transmit the stored control signal to the piezoelectric speaker  10 . In this manner, vibration of the piezoelectric speaker  10  was reproduced in the state where the noise source  700  radiated no sound, and sound pressures at the  176  measurement points of the measurement cross-section  790 CS were measured for mapping.  FIG. 49A  to  FIG. 52  show the sound pressure distributions obtained by the mapping. 
     Specifically,  FIG. 49A  to  FIG. 49C  show the sound pressure distributions derived from the piezoelectric speaker  10  relating to different times in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz. FIG. 
       49 A to  FIG. 49C  are arranged in chronological order. A series of lines in  FIG. 50  represent propagation over time of a certain wave front generated by the piezoelectric speaker  10  in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz.  FIG. 51A  to  FIG. 51C  show the sound pressure distributions derived from the piezoelectric speaker  10  relating to different times in the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz.  FIG. 51A  to  FIG. 51C  are arranged in chronological order. A series of lines in  FIG. 52  represent propagation over time of a certain wave front generated by the piezoelectric speaker  10  in the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz. 
       FIG. 49A  to  FIG. 52  show that the wave front propagates so as to approach, from two outer regions of the radiation surface  15  of the piezoelectric speaker  10  with a center region sandwiched therebetween, an axis passing through the center region and extending in the z direction. Wave front propagation shown in  FIG. 49A  to  FIG. 52  occurs in the same manner as in  FIG. 3 . Specifically, a wave front of a diffracted wave generated by diffraction of noise from the noise source  700  at the partition  780  and the wave front derived from the piezoelectric speaker  10  have a common point that the both wave fronts propagate while approaching the above axis. 
     Further, from  FIG. 45A  to  FIG. 48 , it is understood that diffraction at the partition  780  causes appearance of a period during which the phase of the sound wave in the first region  15   a  and the phase of the sound wave in the second region  15   b  are the same in terms of whether positive or negative, the phase of the sound wave in the first region  15   a  and the phase of the sound wave in the third region  15   c  are opposite to each other in terms of whether positive or negative, and the phase of the sound wave in the second region  15   b  and the phase of the sound wave in the third region  15   c  are opposite to each other in terms of whether positive or negative of the phase (see  FIG. 1  to  FIG. 3  and related descriptions for the regions  15   a ,  15   b  and  15   c ). From  FIG. 49A  to  FIG. 52 , it is understood that the piezoelectric speaker  10  causes appearance of a period during which the phase of the first sound wave and the phase of the second sound wave are the same in terms of whether positive or negative, the phase of the first sound wave and the phase of the third sound wave are opposite to each other in terms of whether positive or negative, and the phase of the second sound wave and the phase of the third sound wave are opposite to each other in terms of whether positive or negative (see the description given using  FIG. 1  to  FIG. 3  for the first sound wave, the second sound wave and the third sound wave). The phase distribution in the first region  15   a , the second region  15   b , and the third region  15   c  is also common to noise derived from the noise source  700  and sound derived from the piezoelectric speaker  10 . 
     Comparative Example 1 
     Measurement of Sound Output from Dynamic Speaker  610   
     The piezoelectric speaker  10  of Example 1 was replaced with the dynamic speaker  610 . This dynamic speaker  610  is Fostex P650K manufactured by Foster Electric Company, Limited. In the same manner as in Example 1 except this replacement, sound pressures derived from the dynamic speaker  610  at the 176 measurement points of the measurement cross-section  790 CS were measured for mapping.  FIG. 53A  to  FIG. 56  show the sound pressure distributions obtained by the mapping. The dynamic speaker  610  is embedded in the partition  780 . 
     Specifically,  FIG. 53A  to  FIG. 53C  show the sound pressure distributions derived from the dynamic speaker  610  relating to different times in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz.  FIG. 53A  to  FIG. 53C  are arranged in chronological order. A series of lines in  FIG. 54  represent propagation over time of a certain wave front generated by the dynamic speaker  610  in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz.  FIG. 55A  to  FIG. 55C  show the sound pressure distributions derived from the dynamic speaker  610  relating to different times in the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz. FIG. 
       55 A to  FIG. 55C  are arranged in chronological order. A series of lines in  FIG. 56  represent propagation over time of a certain wave front generated by the dynamic speaker  610  in the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz. 
       FIG. 53A  to  FIG. 56  show that a substantially hemispherical wave is radiated from the radiation surface of the dynamic speaker  610 , and the substantially hemispherical wave has also a substantially hemispherical wave front. Wave front propagation shown in  FIG. 53A  to  FIG. 56  occurs in the same manner as in  FIG. 4 . 
     Comparative Example 2 
     Measurement of Sound Output from Plane Speaker  620   
     The piezoelectric speaker  10  of Example 1 was replaced with the plane speaker  620 . This plane speaker  620  is FPS2030M3P1R manufactured by FPS Inc. In the same manner as in Example 1 except this replacement, sound pressures derived from the plane speaker  620  at the  176  measurement points of the measurement cross-section  790 CS were measured for mapping.  FIG. 57A  to  FIG. 60  show the sound pressure distributions obtained by the mapping. 
     Specifically,  FIG. 57A  to  FIG. 57C  show the sound pressure distributions derived from the plane speaker  620  relating to different times in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz.  FIG. 57A  to  FIG. 57C  are arranged in chronological order. A series of lines in  FIG. 58  represent propagation over time of a certain wave front generated by the plane speaker  620  in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz.  FIG. 59A  to  FIG. 59C  show the sound pressure distributions derived from the plane speaker  620  relating to different times in the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz.  FIG. 59A  to  FIG. 59C  are arranged in chronological order. A series of lines in  FIG. 60  represent propagation over time of a certain wave front generated by the plane speaker  620  in the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz. 
       FIG. 57A  to  FIG. 60  show that a substantially plane wave is radiated from the radiation surface of the plane speaker  620 , and the substantially plane wave also has a substantially plane wave front. Wave front propagation shown in  FIG. 57A  to 
       FIG. 60  occurs in the same manner as in  FIG. 5 . 
     (Sound Reducing Effect) 
     The difference in sound reducing effect between Example 1 and Comparative Example 2 will be described with reference to  FIG. 61A  to  FIG. 62C . In the following description, terms “speaker ON time” and “speaker OFF time” may be used. 
     A speaker ON time indicates a time when sound for sound reduction is radiated from the speaker. A speaker OFF time indicates a time when sound for sound reduction is not radiated from the speaker. 
     Color maps of  FIG. 61A  and  FIG. 62A  show sound reducing states at a certain time when a sine wave is radiated from the noise source  700 . In  FIG. 61A  and FIG. 
       62 A, the color maps on the left show the sound reducing states by the piezoelectric speaker  10  of Example 1, and the color maps on the right show the sound reducing states by the plane speaker  620  of Comparative Example 2.  FIG. 61A  shows a sound pressure distribution at the certain time in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz.  FIG. 62A  shows a sound pressure distribution at the certain time in the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz. 
     In  FIG. 61A  and  FIG. 62A , numerical values on the right side of color bars indicate the amplification factor in units of dB. The amplification factor being X represents that a sound pressure is amplified by XdB at a speaker ON time with reference to a speaker OFF time. The amplification factor being negative indicates that a sound reducing effect is exhibited. In contrast, the amplification factor being positive indicates that noise is amplified. Reduction area (R.A) indicates the ratio of an area where the amplification factor is −6 dB or less (i.e., area where the sound reducing effect is exhibited well) on the measurement cross-section  790 CS. 
     Amplification area (A.A) indicates the ratio of an area where the amplification factor is more than 0 dB (i.e., area where the noise is amplified) on the measurement cross-section  790 CS. 
       FIG. 61B  shows a finely hatched region where the amplification factor in  FIG. 61A  is less than 0 dB and a coarsely hatched region where the amplification factor is more than 0.  FIG. 62B  shows a finely hatched region where the amplification factor in  FIG. 62A  is less than 0 dB and a coarsely hatched region where the amplification factor is more than 0. That is, in  FIG. 61B  and  FIG. 62B , the regions where noise is reduced are finely hatched and the amplification areas are coarsely hatched. The hatching in  FIG. 61B  and  FIG. 62B  is roughly done manually based on the visual observation of  FIG. 61A  and  FIG. 62A . The same applies to  FIG. 61C  and  FIG. 62C  described later. 
       FIG. 61C  shows a finely hatched region where the amplification factor in  FIG. 61A  is −6 dB or less and a coarsely hatched region where the amplification factor is more than 0.  FIG. 62C  shows a finely hatched region where the amplification factor in  FIG. 62A  is −6 dB or less and a coarsely hatched region where the amplification factor is more than 0. That is, in  FIG. 61C  and  FIG. 62C , the reduction regions are finely hatched and the amplification areas are coarsely hatched. 
     As shown in  FIG. 61A  to  FIG. 62C , in the case where the piezoelectric speaker  10  of Example 1 is used, the area where the noise is reduced and the reduction area are large and the amplification area is small compared with the case where the plane speaker  620  of Comparative Example 2 is used. 
     Specifically, in the use case of the piezoelectric speaker  10  of Example 1, in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz, the reduction area is about 58% and the amplification area is about 18%. In the case where the frequency of the sine wave radiated from the noise source  700  is 800 Hz, the reduction area is about 27% and the amplification area is about 18%. 
     Meanwhile, in the use case of the plane speaker  620  of Comparative Example 2, in the case where the frequency of the sine wave radiated from the noise source  700  is 500 Hz, the reduction area is about 38% and the amplification area is about 21%. In the case where the frequency of the sine wave radiated from the noise source 700 is 800 Hz, the reduction area is about 13% and the amplification area is about 61%. 
       FIG. 61A  to  FIG. 62C  demonstrate that the advantage of the sound reducing effect of the piezoelectric speaker  10  with respect to the plane speaker  620  is exhibited more prominently when the frequency of the sine wave radiated from the noise source  700  is 800 Hz than when the frequency is 500 Hz. 
     It is expected that in the case where the dynamic speaker  610  of Comparative Example 1 is used, the area where the noise is reduced and the reduction area are small and the amplification is large compared with the case where the plane speaker  620  of Comparative Example 2 is used. 
     [Supporting Structure for Piezoelectric Film and Degree of Freedom of Vibration] 
     The following refers to an example of a supporting structure for the piezoelectric speaker according to the present invention. As can be understood from  FIG. 6A ,  FIG. 15 ,  FIG. 17 , and  FIG. 18  and the descriptions relating to these figures, in the piezoelectric speaker  10 , the entire surface of the piezoelectric film  35  is fixed to the structure  80  with the joining layers  51  and  52  and the interposed layer  40  therebetween. 
     It is also conceivable that a portion of the piezoelectric film  35  is supported to be spaced away from the structure  80  in order to prevent the structure  80  from hindering vibration of the piezoelectric film  35 . An exemplary supporting structure based on this design concept is shown in  FIG. 6B . In a hypothetical piezoelectric speaker  108  shown in  FIG. 6B , a frame  88  supports a peripheral portion of the piezoelectric film  35  at a position distant from the structure  80 . 
     It is easy to ensure a sufficient volume of sound emitted from a piezoelectric film already curved and fixed in one direction. Accordingly, it is conceivable that, for example, in the piezoelectric speaker  108 , a nonuniformly thick interposed object having a convex upper surface is disposed in a space  48  surrounded by the piezoelectric film  35 , the frame  88 , and the structure  80  and a central portion of the piezoelectric film  35  is pushed upward. However, such an interposed object is not joined to the piezoelectric film  35  so as not to hinder vibration of the piezoelectric film  35 . Accordingly, even with the interposed object disposed in the space  48 , it is only the frame  88  that supports the piezoelectric film  35  so as to determine vibration of the piezoelectric film  35 . 
     As described above, the piezoelectric speaker  108  shown in  FIG. 6B  employs the supporting structure locally supporting the piezoelectric film  35 . On the other hand, the piezoelectric film  35  of the piezoelectric speaker  10  as in  FIG. 6A  and the like is not supported at a particular portion. Unexpectedly, the piezoelectric speaker  10  exhibits practical acoustic characteristics in spite of the fact that the entire surface of the piezoelectric film  35  is fixed to the structure  80 . Specifically, in the piezoelectric speaker  10 , even a peripheral portion of the piezoelectric film  35  possibly vibrates up and down. The piezoelectric film  35  can vibrate up and down as a whole. Accordingly, compared to the piezoelectric speaker  108 , the piezoelectric speaker  10  has a higher degree of freedom of vibration and is relatively advantageous in achieving good sound emission characteristics. 
     As described with reference to  FIG. 6A , the high degree of freedom of vibration may contribute to formation of the first wave front  16   a  and the second wave front  16   b . In  FIG. 6A , the case where the speaker  10  is the piezoelectric speaker  10  shown in  FIG. 15  is illustrated. In  FIG. 6A , the first joining layer  51  and the second joining layer  52  are not shown. A high degree of freedom of vibration can be obtained also in the case where the speaker  10  is the piezoelectric speaker  110  shown in  FIG. 17 . 
     According to the studies by the present inventors, the interposed layer being a porous body layer and/or a resin layer is suitable for achieving the degree of freedom of vibration. In fact, as shown in  FIG. 25  to  FIG. 41 , in Samples E 1  to E 17  in which the interposed layer is a porous body layer and/or a resin layer, practical acoustic characteristics are exhibited in spite of the fact that the entire surface of the piezoelectric film  35  is fixed to the supporting member  680 . Accordingly, it is considered that even in the case where the piezoelectric speaker  10  in the ANC evaluation system  800  is changed from a different size product of Sample E 1  to different size products of Samples E 2  to E 17 , a sound pressure distribution with the same tendency as in  FIG. 49A  to  FIG. 52  appears. 
     The ANC system  500  according to the present invention can be interpreted as follows: 
     an ANC system  500  including: 
     a structure  80 ; and 
     a speaker  10  attached to the structure  80 , wherein 
     the speaker  10  includes a radiation surface  15 , a piezoelectric film  35 , and an interposed layer  40  (or  140 ), 
     the interposed layer  40  is disposed between the structure  80  and the piezoelectric film  35 , and 
     the interposed layer  40  is a porous body layer and/or a resin layer.