The IPA articulatory-oriented description of a trill implies repeated 
contact between an active articulator (like the tongue tip) and a passive 
articulator (like the alveolar ridge) - this description may need some 
modification applied to bilabial trills, and for uvular trills the tongue 
dorsum is the passive articulator. Note that contact does not imply complete 
closure. From an acoustic point of view, the important thing is a series of 
successive rapid decreases and increases in amplitude.

I'm completing an acoustic study of trills in 18 languages, and very often 
only the first contact shows signs of complete closure - successive contacts 
are weaker, with a more approximant-like acoustic structure, occasionally in 
a strong-weak pattern if more than 2 occur. I think that incompletely 
occluded trills are not only possible, they occur quite commonly in speech, 
even relatively carefully articulated word-lists. Overall, trills occur 
about 30% of the time in non-spontaneous samples in languages said to have a 
trilled /r/. Other realisations are taps (most common), approximants, and 

To transcribe this more open trill sound, you could use the IPA symbol for 
the apical trill, i.s. [r], with the subscript 'more open' diacritic (looks 
like a small capital T).

One final thing: research suggests that apical trills are made in the same 
way as the vibration of the vocal folds - not individual neuromuscular 
commands for each contact, but an essentially aerodynamically driven 
vibration which is helped along by uneven mechanical displacements in the 
vibrating structures (so the vocal folds 'wobble' a bit, with some bits 
moving faster than others, generating a rhythm due to the internal 
vibration), by the elasticity of the articulators, and by the tension in 
e.g. the vocal tract walls, which stores energy. Having an incomplete 
closure means that the pressure behind the constriction will reach a lower 
maximum, so the elastic return forces in the tongue tip would have to play a 
bigger role in maintaining vibration (or you up the oral pressure by 
contracting the lungs more).



Mark J. Jones
British Academy Post-doctoral Research Fellow
Department of Linguistics
University of Cambridge
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