Aerodynamic constraints on language history: The ease of bilabial trills

Abstract

Bilabial trills, transcribed [B], are markedly rare as speech sounds in the world's languages, even though Catford (1977) claims that they are the easiest type of trill to produce. With one exception (Liangshang Yi), all languages known to have bilabial trills developed them in a highly restricted environment consisting (historically) of a sequence of a voiced bilabial nasal, a voiced bilabial stop, and a high rounded vowel, e.g., [mbu] → [mBu]. In many cases, the trill remains allophonic (e.g., Na?ahai, Windua) but some languages have undergone a restructuring resulting in phonemic bilabial trills (e.g., Kurti, Atchin). The limitation to this environment suggests that aerodynamic conditions normally only satisfied in speech by this sequence are required for labial trilling to develop. The essentials would seem to be: (1) transglottal airflow without intraoral pressure buildup, permitted by the nasal escape in [m] and not significantly affected by the very brief closure typically found for a homorganic post-nasal stop; (2) subsequent maintenance of bilabial closure without nasal flow, providing for an oral release; and (3) a following vowel with a target position for the lips of narrow aperture. Since this target lip position requires only a small movement from closure, articulatot movement is slow [Kent and Moll (1972)]; hence, there is a period of time during which the lips remain close enough together for Bernouilli forces, in the absence of increased intraoral pressure, to reclose them and initiate trilling. A simplified quantitative model of this process will be presented, together with speculations on why bilabial trills do not otherwise occur linguistically. [Research supported by NSF Grant BNS 87-20098.]