Voiceless dental and alveolar trills are a type of consonantal sound. They differ from their cognate /r/ only by the vibrations of the vocal cord. It occurs in a few languages, usually alongside the voiced version, as a similar phoneme or an allophone.
Proto-Indo-European*sr developed into a sound written as ⟨ῥ⟩, with the letter for /r/ and the diacritic for /h/, in Ancient Greek. It was probably a voiceless alveolar trill and became the regular word-initial allophone of /r/ in standard Attic Greek that has disappeared in Modern Greek.
*Proto-Indo-European *srew- > Ancient Greek ῥέω "flow", possibly [r̥é.ɔː]
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 that air is not allowed to escape through the nose.
It is a median consonant, which means it is produced by directing the airstream down the midline of the tongue, rather than to the sides.
Contrasts with /r/: нарня[ˈnarnʲæ] "short grass". It has the palatalized counterpart /r̥ʲ/: марьхне[ˈmar̥ʲnʲæ] "these apples", but марьня[ˈmarʲnʲæ] "little apple"
A voiceless alveolar fricative trill is not known to occur as a phoneme in any language, except possibly the East Sakhalin dialect of Nivkh. It occurs allophonically in Czech.
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 that air is not allowed to escape through the nose.
It is a median consonant, which means it is produced by directing the airstream down the midline of the tongue, rather than to the sides.
Allophone of /r̝/ after voiceless consonants for speakers that do not merge it with /ʐ/. Present in areas from Starogard Gdański to Malbork and those south, west and northwest of them, area from Lubawa to Olsztyn to Olecko to Działdowo, south and east from Wieleń, around Wołomin, southeast from Ostrów Mazowiecka and west from Siedlce, from Brzeg to Opole and those north of them, and roughly from Racibórz to Nowy Targ. Most speakers, including speakers of standard Polish, pronounce it the same as /ʂ/, and speakers maintaining the distinction (which is mostly the elderly) sporadically do so too.
Asu, Eva Liina; Teras, Pire (2009), "Estonian", Journal of the International Phonetic Association, 39 (3): 367–372, doi:10.1017/s002510030999017x
Dankovičová, Jana (1999), "Czech", Handbook of the International Phonetic Association: A guide to the use of the International Phonetic Alphabet, Cambridge: Cambridge University Press, pp.70–74, ISBN0-521-65236-7