Voiceless alveolar tap and flap

The voiceless alveolar tap or flap is rare as a phoneme. The symbol in the International Phonetic Alphabet that represents this sound is ⟨ɾ̥⟩, a combination of the letter for the voiced alveolar tap/flap and a diacritic indicating voicelessness. The equivalent X-SAMPA symbol is 4_0.

The voiceless alveolar tapped fricative reported from some languages is actually a very brief voiceless alveolar non-sibilant fricative.

Features

Features of the voiceless alveolar tap or flap:

Its manner of articulation is tap or flap, which means it is produced with a single contraction of the muscles so that the tongue makes very brief contact.
Its place of articulation is dental or alveolar, which means it is articulated behind upper front teeth or at the alveolar ridge. It is most often apical, which means that it is pronounced with the tip of the tongue.
Its phonation is voiceless, which means it is produced without vibrations of the vocal cords. In some languages the vocal cords are actively separated, so it is always voiceless; in others the cords are lax, so that it may take on the voicing of adjacent sounds.
It is an oral consonant, which means air is allowed to escape through the mouth only.
It is a central consonant, which means it is produced by directing the airstream along the center of the tongue, rather than to the sides.
Its airstream mechanism is pulmonic, which means it is articulated by pushing air solely with the intercostal muscles and abdominal muscles, as in most sounds.

Language		Word	IPA	Meaning	Notes
Bengali^[1]		আবার	[ˈäbäɾ̥]	'again'	Possible allophone of /ɹ/ in the syllable coda.^[1] See Bengali phonology
English		throw	[θɾ̪̊oʊ]	'throw'	Allophone of /ɹ/ after /θ/.
Greek	Cypriot	αρφός	[ɐɾ̥ˈfo̞s]	'brother'	Allophone of /ɾ/ before voiceless consonants. May be a voiceless alveolar trill instead
Icelandic		hrafn	[ˈɾ̥apn̪̊]	'raven'	Realization of /r̥/ for some speakers. Also illustrates /n̥/. See Icelandic phonology
Portuguese	European^[2]	assar	[əˈsäɾ̥]	'to bake'	Apparent allophone of /ɾ/; distribution unclear, but common in the coda in Jesus (2001)'s corpus. See Portuguese phonology
Turkish		bir	[biɾ̝̊]	'one'	/ɾ/ is frequently devoiced word-finally and before a voiceless consonant. See Turkish phonology

Jesus, Luis Miguel Teixeira (2001), Acoustic Phonetics of European Portuguese Fricative Consonant (Ph.D.), University of Southampton
Khan, Sameer ud Dowla (2010), "Bengali (Bangladeshi Standard)" (PDF), Journal of the International Phonetic Association, 40 (2): 221–225, doi:10.1017/S0025100310000071