Metaphone

Key takeaways

Metaphone belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Metaphone to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Metaphone from memory before moving on to harder problems.

Reference excerpt

Metaphone is a phonetic algorithm, published by Lawrence Philips in 1990, for indexing words by their English pronunciation. It fundamentally improves on the Soundex algorithm by using information about variations and inconsistencies in English spelling and pronunciation to produce a more accurate encoding, which does a better job of matching words and names which sound similar. As with Soundex, similar-sounding words should share the same keys. Metaphone is available as a built-in operator in a number of systems. Philips later produced a new version of the algorithm, which he named Double Metaphone. Contrary to the original algorithm whose application is limited to English only, this version takes into account spelling peculiarities of a number of other languages. In 2009 Philips released a third version, called Metaphone 3, which achieves an accuracy of approximately 99% for English words, non-English words familiar to Americans, and first names and family names commonly found in the United States, having been developed according to modern engineering standards against a test harness of prepared correct encodings.

Procedure Original Metaphone codes use the 16 consonant symbols 0BFHJKLMNPRSTWXY. The '0' represents "th" (as an ASCII approximation of Θ), 'X' represents "sh" or "ch", and the others represent their usual English pronunciations. The vowels AEIOU are also used, but only at the beginning of the code. This table summarizes most of the rules in the original implementation:

Drop duplicate adjacent letters, except for C. If the word begins with 'KN', 'GN', 'PN', 'AE', 'WR', drop the first letter. Drop 'B' if after 'M' at the end of the word. 'C' transforms to 'X' if followed by 'IA' or 'H' (unless in latter case, it is part of '-SCH-', in which case it transforms to 'K'). 'C' transforms to 'S' if followed by 'I', 'E', or 'Y'. Otherwise, 'C' transforms to 'K'. 'D' transforms to 'J' if followed by 'GE', 'GY', or 'GI'. Otherwise, 'D' transforms to 'T'. Drop 'G' if followed by 'H' and 'H' is not at the end or before a vowel. Drop 'G' if followed by 'N' or 'NED' and is at the end. 'G' transforms to 'J' if before 'I', 'E', or 'Y', and it is not in 'GG'. Otherwise, 'G' transforms to 'K'. Drop 'H' if after vowel and not before a vowel. 'CK' transforms to 'K'. 'PH' transforms to 'F'. 'Q' transforms to 'K'. 'S' transforms to 'X' if followed by 'H', 'IO', or 'IA'. 'T' transforms to 'X' if followed by 'IA' or 'IO'. 'TH' transforms to '0'. Drop 'T' if followed by 'CH'. 'V' transforms to 'F'. 'WH' transforms to 'W' if at the beginning. Drop 'W' if not followed by a vowel. 'X' transforms to 'S' if at the beginning. Otherwise, 'X' transforms to 'KS'. Drop 'Y' if not followed by a vowel. 'Z' transforms to 'S'. Drop all vowels unless it is the beginning. This table does not constitute a complete description of the original Metaphone algorithm, and the algorithm cannot be coded correctly from it. Original Metaphone contained many errors and was superseded by Double Metaphone, and in turn Double Metaphone and original Metaphone were superseded by Metaphone 3, which corrects thousands of miscodings that will be produced by the first two versions. To implement Metaphone without purchasing a (source code) copy of Metaphone 3, the reference implementation of Double Metaphone can be used. Alternatively, version 2.1.3 of Metaphone 3, an earlier 2009 version without a number of encoding corrections made in the current version, version 2.5.4, has been made available under the terms of the BSD License via the OpenRefine project.

Double Metaphone The Double Metaphone phonetic encoding algorithm is the second generation of this algorithm. Its implementation was described in the June 2000 issue of C/C++ Users Journal. It makes a number of fundamental design improvements over the original Metaphone algorithm. It is called "Double" because it can return both a primary and a secondary code for a string; this accounts for some ambiguous cases as well as for multiple variants of surnames with common ancestry. For example, encoding the name "Smith" yields a primary code of SM0 and a secondary code of XMT, while the name "Schmidt" yields a primary code of XMT and a secondary code of SMT—both have XMT in common. Double Metaphone tries to account for myriad irregularities in English of Slavic, Germanic, Celtic, Greek, French, Italian, Spanish, Chinese, and other origins. Thus it uses a much more complex ruleset for coding than its predecessor; for example, it tests for approximately 100 different contexts of the use of the letter C alone.

Metaphone 3 A professional version was released in October 2009, developed by the same author, Lawrence Philips. It is a commercial product sold as source code. Metaphone 3 further improves phonetic encoding of words in the English language, non-English words familiar to Americans, and first names and family names commonly found in the United States. It improves encoding for proper names in particular to a considerable extent. The author claims that in general it improves accuracy for all words from the approximately 89% of Double Metaphone to 98%. Developers can also now set switches in code to cause the algorithm to encode Metaphone keys 1) taking non-initial vowels into account, as well as 2) encoding voiced and unvoiced consonants differently. This allows the result set to be more closely focused if the developer finds that the search results include too many words that don't resemble the search term closely enough. Metaphone 3 is sold as C++, Java, C#, PHP, Perl, and PL/SQL source, Ruby and Python wrappers accessing a Java jar, and also Metaphone 3 for Spanish and German pronunciation available as Java and C# source. The latest revision of the Metaphone 3 algorithm is v2.5.4, released March 2015. The Metaphone3 Java source code for an earlier version, 2.1.3, lacking a large number of encoding corrections made in the current version, version 2.5.4, was included as part of the OpenRefine project and is publicly viewable.

Common misconceptions There are some misconceptions about the Metaphone algorithms that should be addressed. The following statements are true:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Metaphone

Start with the simplest possible case. Write down what Metaphone claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Metaphone before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Metaphone ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Metaphone

In research: Metaphone appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Metaphone in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Metaphone is common in secondary-school and first-year university syllabi. It links to neighbouring topics Phonetic algorithms, so understanding it makes those chapters shorter.
In everyday life: Look for Metaphone outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Metaphone in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Metaphone means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Metaphone out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Metaphone in simple terms?

Metaphone is a phonetic algorithm, published by Lawrence Philips in 1990, for indexing words by their English pronunciation. It fundamentally improves on the Soundex algorithm by using information about variations and inconsistencies in English spelling and pronunciation to produce a more accurate…

Why does Metaphone matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Metaphone?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Metaphone.