In case you didn't know, the average opera singer has a vibrato that causes the pitch to waver for about a half step, or the distance between c and c# if you don't know what a half step is. It waves half of this pitch above and the other half below the intended pitch. Listeners generally imagine the pitch to be somewhere in the middle of the wavering sound. It is only your imagination that makes this a precise pitch. So making comments about the singer being sharp for the whole aria may only indicate that your ear is interpreting the vibrato sharp. Is she sharp? Yes. Is the exact same note also flat? Yes. Some singers push energy to the upper part of the vibrato or the lower part. Pitch wavering is the same but energy is unbalanced. Maybe your ear hears this as sharp or flat. They aren't giving up their vibratos.
A vibrato becomes a wobble when the speed of wavering slows down.
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