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Acoustics Experiment Shows Why It's So Hard to Make Out the Heroine's Words at the Opera (Bertram Schwarzschild, PhysicsToday)

Vocal-tract resonances enhance the output of the vocal cords. They also create the distinctions between different vowels sounds. For sopranos singing high notes, the two functions come into conflict.

A frustrated listener might well define grand opera as musical theater where you have a hard time making out the words even when they're being sung in your own language. Conceding the point, many opera houses nowadays always flash surtitles above the proscenium. Comprehension is particularly difficult in the higher reaches of the soprano register. Hector Berlioz long ago warned composers not to put crucial words in the soprano's mouth at high notes.

A recent study at the University of New South Wales in Sydney, Australia, lays most of the blame on an inescapable tradeoff dictated by the physical acoustics of vowel differentiation and singing very high notes. Acoustical physicists John Smith and Joe Wolfe, working with physics undergraduate Elodie Joliveau, have carried out an experiment that demonstrates why different vowel sounds are almost impossible to distinguish when sopranos are singing in the highest octave of their range.