Many speakers turn voicing on and off irregularly at the beginnings and ends of sentences, which gives a querulous tone to the voice. This departure from perfect periodicity is called laryngealization or creaky voice quality.
The Laryngealization (la) option controls the amount of laryngealization, in the voice. A value of 0 results in no laryngealized irregularity, and a value of 100 (the maximum) produces laryngealization at all times. For example, to make Betty moderately laryngealized, type the following command:
[:nb][:dv la 20]
The la option creates a noticeable difference in the voice, although it is not altogether a pleasant change.