Soundex

Soundex is phonetic algorithm, an algorithm for indexing names by their sound, when pronounced in English. The basic aim is for names with the same pronunciation to be encoded to the same string. Soundex is the most widely known of all phonetic algorithms and is often used (incorrectly) to describe "phonetic algorithm".

Soundex was developed by Robert Russell and Margaret Odell and patented in 1918 and 1922 (US patents 1,261,167 and 1,435,663). A variation called American Soundex was used in the 1930s for a retrospective analysis of the US censuses from 1890 through 1920. The Soundex code came to prominence in the 1960s when it was the subject of several articles in the Communications and Journal of the Association for Computing Machinery (CACM and JACM), and especially when described in Donald Knuth's magnum opus, The Art of Computer Programming.

The Soundex code for a name consists of a letter followed by three numbers: the letter is the first letter of the name, and the numbers encode the remaining consonants. Similar sounding consonants share the same number so, for example, the labial B, F, P and V are all encoded as 1. Vowels can affect the coding, but are never coded directly unless they appear at the start of the name.

As a response to deficiencies in the Soundex algorithm, Lawrence Philips developed Metaphone algorithm for the same purpose.