Soundex is an encoding used to relate similar names, but can also be used as a general purpose scheme to find word with similar phonemes.
The American Soundex System
The soundex code consist of the first letter of the name followed by three digits. These three digits are determined by dropping the letters a, e, i, o, u, h, w and y and adding three digits from the remaining letters of the name according to the table below. There are only two additional rules. (1) If two or more consecutive letters have the same code, they are coded as one letter. (2) If there are an insufficient numbers of letters to make the three digits, the remaining digits are set to zero.
3 d, t
5 m, n