Language Log (my new favorite RSS feed) points to a Physics Today article that explains why you can never understand the words an operatic soprano sings—even if she is singing in your language. The story discusses a study conducted at the University of New South Wales in Sydney, Australia, which experimentally shows through acoustics that different vowel sounds are almost impossible to differentiate.
The issue appears to be resonance frequencies. The one that helps distinguish vowel sounds is the first resonance frequency, R1. And for really high range singing, a soprano’s fundamental frequency (f0) is actually above the first resonance frequency. Vocal practice (such as forward placement through opening the mouth wide and smiling) helps raise the resonance frequency, but not enough. Fabulous experimental data rounds out the picture.
This also confirms customary directorial practice which tells sopranos just to sing “aah” on particularly high passages, and compositional practice which avoids difficult rounded vowels on high notes (unless, of course, the composer was Beethoven).
By contrast, of course, a tenor’s high range is well below his first resonant frequency pretty much all the way. Yet another reason that tenors are in demand: you can actually understand us.