|No audible release|
A stop with no audible release, also known as an unreleased stop or an applosive, is a stop consonant with no release burst: no audible indication of the end of its occlusion (hold). In the International Phonetic Alphabet, lack of an audible release is denoted with an upper-right corner diacritic (U+031A ◌̚ COMBINING LEFT ANGLE ABOVE (HTML
̚)) after the consonant letter: [p̚], [t̚], [k̚].
Audibly released stops, on the other hand, are not normally indicated. If a final stop is aspirated, the aspiration diacritic ⟨◌ʰ⟩ is sufficient to indicate the release. Otherwise, the "unaspirated" diacritic of the Extended IPA may be employed for this: apt [ˈæp̚t˭].
In most dialects of English, the first stop of a cluster has no audible release, as in apt [ˈæp̚t], doctor [ˈdɒk̚təɹ], or logged on [ˌlɒɡ̚dˈɒn]. Although such sounds are frequently described as "unreleased", the reality is that since the two consonants overlap, the release of the former takes place during the hold of the latter, masking the former's release and making it inaudible. That can lead to cross-articulations that seem very much like deletions or complete assimilation.
For example, hundred pounds may sound like [ˈhʌndɹɨb ˈpʰaundz] but X-ray and electropalatographic studies demonstrate that since inaudible and possibly-weakened contacts may still be made, the second /d/ in hundred pounds does not entirely assimilate a labial place of articulation but co-occurs with it.
In American English, a stop in syllable-final position is typically realized as an unreleased stop; that is especially the case for /t/,, but in that position, it is also analyzed as experiencing glottal reinforcement.
Such sounds may occur between vowels, as in some pronunciations of out a lot. The overlap there appears to be with a glottal stop, [t̚ʔ]: the /t/ is pronounced, and since it is between vowels, it must be released. However, its release is masked by the glottal stop. (See: T-glottalization, in some dialects).
The term "unreleased" is also used for a stop before a homorganic nasal, as in catnip. In such cases, however, the stop is released as a nasal in a nasal release and so it would be more precisely transcribed [ˈkætⁿnɪp].
In most languages in East and Southeast Asia with final stops, such as Cantonese, Hokkien, Korean, Malay, Thai, and West Coast Bajau, the stops are not audibly released: mak [mak̚]. That is true even between vowels. That is thought to be caused by an overlapping glottal stop and is more precisely transcribed [mak̚ʔ]. A consequence of an inaudible release is that any aspirated–unaspirated distinction is neutralized. Some languages, such as Vietnamese, which are reported to have unreleased final stops, turn out to have short voiceless nasal releases instead. The excess pressure is released (voicelessly) through the nose and so there is no audible release to the stop.
The Formosan languages of Taiwan, such as Tsou and Amis, are unusual in that all obstruents are released but not aspirated, as in Tsou [ˈsip˹tɨ] "four" and [smuˈjuʔ˹tsu] "to pierce", or Amis [tsᵊtsaj] "one" and [sᵊpat˹] "four". (The symbol for a release burst, ⟨˹⟩, is acknowledged but not supported by the IPA.)
In Pirahã, the only surviving dialect of the Mura language, there is a special register of speech using solely humming, which doesn't involve an audible release and may be transcribed as [m̚] of different length and tone.