A golf ball 2 includes a core 4 , an envelope layer 6 positioned outside the core 4 , a mid layer 8 positioned outside the envelope layer 6 , and a cover 10 positioned outside the mid layer 8 . The core 4 includes a spherical center 12 and a core outer layer 14 positioned outside the center 12 . The center 12 has a diameter of 1 mm or greater and 15 mm or less. The difference (H4−H3) between the JIS-C hardness H4 of the core 4 at its surface and the JIS-C hardness H3 of the core outer layer 14 at its innermost portion is equal to or greater than 10. The cover 10 has a Shore D hardness H7 less than 40. The Shore D hardnesses H5, H6 and H7 of the envelope layer 6 , the mid layer 8 , and the cover 10 satisfy that H5>H6≧H7.
1. A golf ball comprising a core, an envelope layer positioned outside the core, a mid layer positioned outside the envelope layer, and a cover positioned outside the mid layer, wherein:
the core includes a center and a core outer layer positioned outside the center;
the center has a diameter of 1 mm or greater and 15 mm or less;
a difference (H4−H3) between a JIS-C hardness H4 of the core at its surface and a JIS-C hardness H3 of the core outer layer at its innermost portion is equal to or greater than 10;
the cover has a Shore D hardness H7 less than 40; and
a Shore D hardness H5 of the envelope layer, a Shore D hardness H6 of the mid layer, and a Shore D hardness H7of the cover satisfy the following mathematical formula (I)
H5>H6≧H7 (I).
2. The golf ball according to claim 1 , wherein a difference (H4−H1) between the hardness H4 and a JIS-C hardness H1 of the center at its central point is equal to or greater than 30.
3. The golf ball according to claim 1 , wherein a difference (H3−H2) between the hardness H3 and a JIS-C hardness H2 of the center at its surface is equal to or less than 35.
4. The golf ball according to claim 1 , wherein a hardness of the envelope layer is greater than a surface hardness of the core.
5. The golf ball according to claim 1 , wherein the hardness H6 is greater than the hardness H7.
6. The golf ball according to claim 5 , wherein:
a difference (H5−H6) between the hardness H5 and the hardness H6 is equal to or greater than 5 and equal to or less than 40; and
a difference (H6−H7) between the hardness H6 and the hardness H7 is equal to or greater than 3 and equal to or less than 28.
7. The golf ball according to claim 1 , wherein a JIS-C hardness H1 of the center at its central point is equal to or greater than 20 and equal to or less than 50.
8. The golf ball according to claim 1 , wherein:
the hardness H3 is equal to or greater than 45 and equal to or less than 75; and
the hardness H4 is equal to or greater than 65 and equal to or less than 95.
9. The golf ball according to claim 1 , wherein the hardness H5 is equal to or greater than 50.
10. The golf ball according to claim 9 , wherein the hardness H5 is equal to or greater than 50 and equal to or less than 80.
11. The golf ball according to claim 1 , wherein the hardness H6 is equal to or greater than 30 and equal to or less than 60.
12. The golf ball according to claim 1 , wherein the hardness H7 is equal to or greater than 10 and less than 40.
13. The golf ball according to claim 1 , wherein:
the center is formed by crosslinking a rubber composition;
a principal component of a base material of the rubber composition is a polybutadiene; and
the rubber composition includes sulfur as a crosslinking agent.
14. The golf ball according to claim 1 , wherein a principal component of a base material of the envelope layer is an ionomer resin.
15. The golf ball according to claim 1 , wherein a principal component of a base material of the mid layer is one or more selected from the group consisting of polyurethanes, polyesters, polyamides, polyolefins, polystyrenes and ionomer resins.
16. The golf ball according to claim 1 , wherein a principal component of a base material of the cover is a thermoplastic polyurethane elastomer.
17. The golf ball according to claim 1 , wherein the mid layer has a thickness Tm less than 1.2 mm.
18. The golf ball according to claim 1 , wherein the cover has a thickness Tc of 0.6 mm or less.
19. The golf ball according to claim 1 , wherein a sphere consisting of the core and the envelope layer has an amount of compressive deformation of 2.0 mm or greater and 3.6 mm or less.
20. The golf ball according to claim 1 , wherein a sphere consisting of the core, the envelope layer and the mid layer has an amount of compressive deformation of 2.3 mm or greater and 4.0 mm or less.