An ELT Glossary : Phoneme

A phoneme is a speech sound which, in a specific language, makes a difference in meaning. For example, in English the choice of the initial sound in words such as lip, rip, tip, pip, nip etc changes the meaning of the word. Thus the consonants involved - /l r t p n/ - are considered to be phonemes of English.

Phonemes are, however, language specific. Any language will only use a selection of the full range of sounds used in all the world's languages. For example, English does not use the sound [ʝ] which is a voiced palatal fricative, but it is used in Danish, Dutch and Greek.  Language learners tend to have difficulty pronouncing those phonemes that do not occur in their first language, but which occur in the language they are learning - hence the difficulty for many learners of English of pronouncing  the unvoiced and voiced dental fricatives represented by "th" in the written language - eg  the unvoiced /θ/ as in "bath", and the voiced /ð/ as in "bathe".  

To see the full list of the consonant sounds tabulated by  the International Phonetics Association and to hear how they are pronounced click here.

Then there are sounds which act as allophones of a phoneme (ie they can be interchanged without creating a difference in meaning) and may not be perceived by native speakers as being in any way different. For example, in some South East Asian languages (eg Korean) /l/ and /r/ are not phonemes but  allophones - they both occur in the language but as variants of the same phoneme, and are not necessarily perceived as "different" by native speakers. Conversely the phoneme /p/ in English also has two allophones - aspirated  as in "pin" [pʰas in "pin" [pʰɪnand unaspirated as in "spin". Try saying them with your hand in front of your mouth. You'll feel a little puff of air as you say "pin" which is not there when you say "spin". This means that the sounds are phonetically different, but native speakers of English generally won't perceive a difference, although they use the sounds automatically. However, in Korean, the aspiration distinction does make a difference in meaning - [p͈ul] means "horn",  while [pʰul]  means "grass". In Korean, these two sounds are therefore separate phonemes.

Related Reading

A classic work on the phonology of English - updated to reflect modern pronunciation.

Cruttenden, A. (2014) Gimson's Pronunciation of English, Routledge