It seems to me that, when singing a linear three-note figure with a quick passing note, it is much easier to articulate the passing note when the figure is falling than rising, and I'd like to know why.
To see what I mean, try these at 90 BPM, 120 BPM, and 140 BPM, with legato in the voice:
In Early Medieval chant manuscripts (semi-improvised vocal music), it happens more often than not that the passing note is replaced by a slide or Schleifer or glissando. Rhythmically it is the same performance, except the passing note is not distinct.
But this happens only when ascending, never when descending: Not only is the slide symbol (the medieval quilisma) found almost exclusively in rising passages (except in a handful of manuscripts like the Montpellier H codex), but in other ornament-less manuscripts those same rising passages often have no passing note, while the corresponding falling passages almost always do.
This makes sense to me, for it feels a bit harder for the human voice to articulate a passing note when ascending, and easier to blur it into a portamento-like glide.
This difference in ease is perhaps clearer in rapid arpeggios. Try singing these between 180 BPM and 200 BPM with legato in the voice, and you'll see that falling is slightly easier to pull off than rising, without blurring the passing notes:
But why is this? Is there some acoustico-physical difference concerning tightening the vocal cords or sharpening the soundwave that translates well to the practicality of distinguishing the passing notes versus eliding them?