Headphone

1. A headphone having a hanger and two speaker units respectively disposed at two ends of the hanger, each speaker unit comprising:

a first shell;

a second shell engaged with the first shell, a first air passage being formed between the first shell and the second shell, a second air passage being formed on the second shell, the second air passage having an opening direction different from an opening direction of the first air passage;

a cover engaged with the second shell and having a plurality of holes formed thereon; and

a speaker disposed between the first shell and the second shell;

wherein when the speaker outputs sound, airflows generated by the speaker are exhausted through the first air passage, the second air passage and the holes.

2. The headphone of claim 1 , wherein the first air passage is formed on a side of the second air passage that is further to the center of the speaker unit.

3. The headphone of claim 1 , wherein the second air passage is formed in a ring shape on the bottom of the second shell.

4. The headphone of claim 1 , further comprising a plurality of third air passages formed in a ring shape on an inner side of the second air passages on the second shell that is closer to the center of the speaker unit.

5. The headphone of claim 1 , wherein the first air passage has a ring shape.

6. The headphone of claim 1 , wherein the opening direction of the second air passage is perpendicular to the opening direction of the first air passage.

7. The headphone of claim 4 , wherein the third air passages are formed between the second shell and the cover.

8. The headphone of claim 4 , wherein the opening direction of the second air passage is parallel with the opening direction of the third air passages.

9. The headphone of claim 4 , further comprising a plurality of fourth air passages formed in a ring shape on the cover and in connection with the third air passages so that the airflows produced by the speaker are exhausted through the fourth air passages.

10. The headphone of claim 1 , wherein the cover further comprises an accommodation formed in a center of the cover for accommodating the speaker.

11. The headphone of claim 10 , wherein the accommodation has a first retaining part and a second retaining part, the first retaining part is configured for accommodating the speaker and the second retaining part is configured for installing a non-woven fabric.

12. The headphone of claim 11 , wherein the first retaining part has a larger radius than the radius of the second retaining part.

13. The headphone of claim 1 , further comprising a sponge disposed between the speaker and the first shell.

14. The headphone of claim 1 , further comprising an earlap disposed outside of the cover.

15. The headphone of claim 14 , wherein a portion of the earlap protrudes from an outer side of the second shell away from the center of the speaker unit.

16. The headphone of claim 15 , wherein the airflows produced by the speaker are exhausted through the second air passage, the holes and the earlap sequentially.

17. The headphone of claim 1 , wherein the amount of airflows exhausted through the first air passage is greater than the airflows exhausted through the second air passage.

18. The headphone of claim 1 , wherein the holes of the cover have an opening direction perpendicular to the opening direction of the first air passage.