Speaker

1. A speaker, comprising:

a pair of front-side and back-side vibration portions each having a dielectric layer having insulating properties and made of an elastomer or resin, a front-side electrode layer placed on a front side of the dielectric layer and having conductive properties, and a back-side electrode layer placed on a back side of the dielectric layer and having conductive properties;

an interposed member placed between the pair of front-side and back-side vibration portions so that the front-side vibration portion protrudes toward a front side and the back-side vibration portion protrudes toward a back side; and

a circuit that transmits signal waves based on sound to be reproduced to the pair of front-side and back-side vibration portions so that the signal waves transmitted to the pair of front-side and back-side vibration portions have opposite phases to each other, and drives the pair of front-side and back-side vibration portions, wherein

the interposed member has a larger spring constant in a front-back direction than in a surface direction.

2. The speaker according to claim 1 , wherein

the circuit superimposes bias voltages of a same polarity on the signal waves with the opposite phases to each other, and applies respective resultant voltages to the front-side electrode layer of the front-side vibration portion and the back-side electrode layer of the back-side vibration portion, and

the back-side electrode layer of the front-side vibration portion and the front-side electrode layer of the back-side vibration portion are grounded.

3. The speaker according to claim 1 , wherein

at least of the pair of front-side and back-side vibration portions has a front-side shield layer placed on a front side of the front-side electrode layer, having insulating properties, and made of an elastomer, and a back-side shield layer placed on a back side of the back-side electrode layer, having insulating properties, and made of an elastomer.

4. The speaker according to claim 3 , wherein

the at least one of the pair of front-side and back-side vibration portions is a film assembly in which the front-side shield layer, the front-side electrode layer, the dielectric layer, the back-side electrode layer, and the back-side shield layer are stacked from the front side toward the back side.

5. The speaker according to claim 4 , further comprising:

a front-side frame member placed on a front side of the front-side film assembly, and a back-side frame member placed on a back side of the back-side film assembly, wherein

the front-side film assembly and the back-side film assembly are held and fixed in the front-back direction by the front-side frame member and the back-side frame member.

6. The speaker according to claim 5 , wherein

each of the front-side frame member and the back-side frame member has a pair of opposite sides facing each other in the surface direction.

7. The speaker according to claim 1 , wherein

the interposed member has a plurality of interposed bodies stacked in the front-back direction, and

at least two of the plurality of interposed bodies have different spring constants from each other in the front-back direction.

8. A speaker, comprising:

a pair of front-side and back-side vibration portions each having a dielectric layer having insulating properties and made of an elastomer or resin, a front-side electrode layer placed on a front side of the dielectric layer and having conductive properties, and a back-side electrode layer placed on a back side of the dielectric layer and having conductive properties;

an interposed member placed between the pair of front-side and back-side vibration portions so that the front-side vibration portion protrudes toward a front side and the back-side vibration portion protrudes toward a back side; and

a circuit that transmits signal waves based on sound to be reproduced to the pair of front-side and back-side vibration portions so that the signal waves transmitted to the pair of front-side and back-side vibration portions have opposite phases to each other, and drives the pair of front-side and back-side vibration portions, wherein

the interposed member has a plurality of interposed bodies stacked in a front-back direction, and

at least two of the plurality of interposed bodies have different spring constants from each other in the front-back direction.

9. The speaker according to claim 8 , wherein

the plurality of interposed bodies are an inner interposed body, and a pair of outer interposed bodies stacked on both sides of the inner interposed body in the front-back direction and contacting the pair of front-side and back-side vibration portions, and

the inner interposed body has a smaller spring constant in the front-back direction than the pair of outer interposed bodies.

10. A speaker, comprising:

a pair of front-side and back-side vibration portions each having a dielectric layer having insulating properties and made of an elastomer or resin, a front-side electrode layer placed on a front side of the dielectric layer and having conductive properties, and a back-side electrode layer placed on a back side of the dielectric layer and having conductive properties;

an interposed member placed between the pair of front-side and back-side vibration portions so that the front-side vibration portion protrudes toward a front side and the back-side vibration portion protrudes toward a back side; and

a circuit that transmits signal waves based on sound to be reproduced to the pair of front-side and back-side vibration portions so that the signal waves transmitted to the pair of front-side and back-side vibration portions have opposite phases to each other, and drives the pair of front-side and back-side vibration portions, wherein

the circuit superimposes bias voltages of a same polarity on the signal waves with the opposite phases to each other, and applies respective resultant voltages to the front-side electrode layer of the front-side vibration portion and the back-side electrode layer of the back-side vibration portion, and

the back-side electrode layer of the front-side vibration portion and the front-side electrode layer of the back-side vibration portion are grounded.

11. The speaker according to claim 10 , wherein

at least one of the pair of front-side and back-side vibration portions has a front-side shield layer placed on a front side of the front-side electrode layer, having insulating properties, and made of an elastomer, and a back-side shield layer placed on a back side of the back-side electrode layer, having insulating properties, and made of an elastomer.

12. The speaker according to claim 11 , wherein

the at least one of the pair of front-side and back-side vibration portions is a film assembly in which the front-side shield layer, the front-side electrode layer, the dielectric layer, the back-side electrode layer, and the back-side shield layer are stacked from the front side toward the back side.

13. The speaker according to claim 12 , further comprising:

a front-side frame member placed on a front side of the front-side film assembly, and a back-side frame member placed on a back side of the back-side film assembly, wherein

the front-side film assembly and the back-side film assembly are held and fixed in a front-back direction by the front-side frame member and the back-side frame member.

14. The speaker according to claim 13 , wherein

each of the front-side frame member and the back-side frame member has a pair of opposite sides facing each other in a surface direction.

15. The speaker according to claim 10 , wherein

the interposed member has a plurality of interposed bodies stacked in a front-back direction, and

at least two of the plurality of interposed bodies have different spring constants from each other in the front-back direction.

16. The speaker according to claim 8 , wherein

the plurality of interposed bodies are an inner interposed body, and a pair of outer interposed bodies stacked on both sides of the inner interposed body in the front-back direction and contacting the pair of front-side and back-side vibration portions, and

the inner interposed body has a larger spring constant in the front-back direction than the pair of outer interposed bodies.

17. The speaker according to claim 8 , wherein

at least one of the pair of front-side and back-side vibration portions has a front-side shield layer placed on a front side of the front-side electrode layer, having insulating properties, and made of an elastomer, and a back-side shield layer placed on a back side of the back-side electrode layer, having insulating properties, and made of an elastomer.

18. The speaker according to claim 17 , wherein

the at least one of the pair of front-side and back-side vibration portions is a film assembly in which the front-side shield layer, the front-side electrode layer, the dielectric layer, the back-side electrode layer, and the back-side shield layer are stacked from the front side toward the back side.

19. The speaker according to claim 18 , further comprising:

a front-side frame member placed on a front side of the front-side film assembly, and a back-side frame member placed on a back side of the back-side film assembly, wherein

the front-side film assembly and the back-side film assembly are held and fixed in the front-back direction by the front-side frame member and the back-side frame member.

20. The speaker according to claim 19 , wherein

each of the front-side frame member and the back-side frame member has a pair of opposite sides facing each other in a surface direction.