In phonetics, aspiration is the strong burst of air that accompanies either the release or, in the case of preaspiration, the closure of some obstruents. To feel or see the difference between aspirated and unaspirated sounds, one can put a hand or a lit candle in front of one's mouth, and say pin ([pʰɪn]) and then bin ([bɪn]). One should either feel a puff of air or see a flicker of the candle flame with pin that one does not get with bin. In most dialects of English, the initial consonant is aspirated in pin and unaspirated in bin.
The diacritic for aspiration in the International Phonetic Alphabet (IPA) is a superscript "h", [ʰ] . Unaspirated consonants are not normally marked explicitly, but there is a diacritic for non-aspiration in the Extensions to the IPA, the superscript equal sign, [⁼].
Voiceless consonants are produced with the vocal cords open and voiced consonants are produced when the vocal folds are fractionally closed. Voiceless aspiration occurs when the vocal cords remain open after a consonant is released. An easy way to measure this is by noting the consonant's voice onset time, as the voicing of a following vowel cannot begin until the vocal cords close.
English voiceless stop consonants are aspirated for most native speakers when they are word-initial or begin a stressed syllable, as in pen, ten, Ken. They are unaspirated for almost all speakers when immediately following word-initial s, as in spun, stun, skunk. After s elsewhere in a word they are normally unaspirated as well, except when the cluster is heteromorphemic and the stop belongs to an unbound morpheme; compare dis[t]end vs. dis[tʰ]aste. Word-final voiceless stops optionally aspirate.
Aspirated consonants are not always followed by vowels or other voiced sounds; indeed, in Eastern Armenian, aspiration is contrastive even at the ends of words. For example compare: bard͡z pillow, with bart͡s⁼ difficult and bart͡sʰ high.
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