Acoustic matching layer

1. An acoustic matching layer comprising:

a base material having a plate shape, and including a joining surface and a vibration surface which are opposite surfaces of the base material having a predetermined thickness, the joining surface being joined to an ultrasonic wave generation source, and the vibration surface being configured to emit a sound wave to a gas;

portions partially provided in the vibration surface, the portions being depressed or penetrating toward the joining surface; and

a film-shaped material completely covering the portions, the film-shaped material being installed in the vibration surface so as to be in contact with the gas.

2. The acoustic matching layer according to claim 1 , wherein:

the base material is an array of a plurality of sheet-shaped materials; and

each of the portions is a space between the sheet-shaped materials.

3. The acoustic matching layer according to claim 1 , wherein:

the base material is an array of a plurality of rod-shaped materials; and

each of the portions is a space between the rod-shaped materials.

4. The acoustic matching layer according to claim 1 , wherein a scale of at least one of the portions is smaller than a wavelength of the sound wave which has propagated.

5. The acoustic matching layer according to claim 4 , wherein the scale of the at least one of the portions is 1/10 or less of the wavelength of the sound wave which has propagated.

6. The acoustic matching layer according to claim 1 , wherein at least a part of the base material is a resin.

7. The acoustic matching layer according to claim 1 , wherein at least a part of the base material is a ceramic.

8. The acoustic matching layer according to claim 1 , wherein at least a part of the base material is a metal.

9. The acoustic matching layer according to claim 1 , wherein at least a part of the base material is glass.