In a review of Sacrificium in Opera News by Stephen Francis Vasta I read "One might point out that metered 'shakes,' and the faster bits of trills, sound more like flutters on a single tone, rather than two distinct notes."
Now I admit I can't tell if this is a simple statement of fact rather than a criticism. Nevertheless I feel obligated to explain.
When a pianist or a violinist plays a trill it's two different notes. When I write one for my midi player, it's two different notes. There are these insane arguments over whether the triller should start on the upper or the lower note which I will not pretend interests me. I know on my midi player it made no difference.
This has nothing to do with singing. All opera singing is a flutter. This is called vibrato, and consists of a consistent fluctuation in pitch generally about a half tone wide. Patty Page had absolutely no vibrato [play a 45 record at 33 speed]. Violinists imitate this by wiggling their wrists. The brain hears this as a pulsing single note. One achieves a trill by widening the vibrato until the brain no longer integrates the flutter into one note. A trill is a fast very wide vibrato. A wobble is a very slow, wide vibrato.
Or yes. So?
Don't tell Cecilia she's supposed to sing two separate notes, or she might start trying to do it.