“A frustrated listener might well define grand opera as musical theatre 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.”
Note: The article was originally published in Physics Today, March 2004, under the title “Acoustics Experiment Shows Why It’s So Hard to Make Out the Heroine’s Words at the Opera”.
BONUS: An opera that some may enjoy finding difficult to understand.