The NYTimes reports:
"But during rehearsals for the Metropolitan Opera’s production of Wagner’s 'Tristan und Isolde,' which opens on Friday night, the German tenor singing Tristan, Peter Seiffert,
has been using a personal electronic prompter: an earpiece through
which he can hear the cues directly. The only previous time a performer
at the Met has used such a device came last season in a speaking role
for the new production of Donizetti’s 'Fille du Régiment.' Mr. Seiffert
has sung several major Wagner roles to acclaim, including Tannhäuser
for his Met debut in 2004. When he sang Tristan in Berlin two years ago
he also used an earpiece. A Met official said yesterday that Mr.
Seiffert was still deciding whether to use the earpiece on Friday, when
the conductor Daniel Barenboim would be making his Met debut."
Orchestras playing too loud are one thing; singers going deaf for it are entirely another.