Luciano Pavarotti is in the other room singing to me, aided by my TV.
How loud, I’m wondring, can an opera singer sing (in db). I’ve been to Opera in the Park in Golden Gate Park, and they were quite loud. But I’ve never been to a professional opera in a good house.
Not amplified, of course.
Peace,
mangeorge, who didn’t go see The Three Tenors. 
According to the Metropolitan Opera Guild, up to 125 dB.
It’s a combination of the singer’s voice and the design of the house. The famous opera houses tend to be huge, and have to carry an unamplified voice up to the nosebleed section. I’ve never attended an outdoor concert that wasn’t amplified.
I thought maybe close to that.
See reply to panache45’s post, below.
The outdoor concerts I attended were amplified, but the speakers were situated oddly. If you were pretty close to the stage you could hear the singers more than the amped sound. Easily so. It was prety cool, and they were loud. I suspect they’d hold back on the volume to not risk their voices needlessly.
While I was in Manhattan the Italian (most were) cabbies actually would sing arias along with the radio. This was in the mid 60s, and 8 track was just coming into being. I loved that place. 
It may be 125 db if you put the mike right in front of the singer’s mouth. When people ask how loud something is and someone answers with a certain number of decibels, that doesn’t tell you anything unless they say what distance it was measured at. Db are a measure of loudness at one particular point, not a measure of total sound energy. The amount of sound that something puts out should be measured in acoustic watts, or in DB at some particular distance.