Why is the [CMRR][1] of an op-amp defined as \$20\log(A_d/A_c)\$?

I know that the value of \$A_d\$ is far greater than \$A_c\$, so we take the log, but why are we multiplying 20 with it?

  [1]: https://en.wikipedia.org/wiki/Common-mode_rejection_ratio