1) The obvious: Poor technique. Let us discuss the technical goal of an opera singer at its most practical and fundamental. The first goal of operatic vocal technique is that the singer should have no problem being heard in the presence of an orchestra. It is not a matter of loudness but of "presence"!
Fact: The smallest orchestra will always sound louder than the singer. In terms of decibels, a singer cannot compete with an orchestra!
Fact: A singer producing a strong resonance in the 2000-3000Hz range will always be heard better than an orchestra!
Are these two statements contradictory? Without an understanding of the acoustics of the human voice and its specific relationship to the human ear, the statements might indeed appear to be contradictory. However science makes it a little clearer.
It is remarkable how similar the high side of the spectrums are between Pavarotti and the Baby's cry. The lower side of the spectrum shows the baby in first formant dominance and Pavarotti in a tenor's proper second formant dominance. This difference can be explained relative to the undeveloped larynx of the baby vs. Pavarotti's adult larynx. The presence of the Singer's Formant (between the two red markers) in both spectra (almost identical) shows how very closely the relevant acoustic elements resemble each other.
When this acoustic element, the Singer's Formant, is present it causes a ringing sensation in the ear as if someone had rung a bell very close to the ear. As many coach-pianists can attest, it can be uncomfortable when a singer sings with this ring at close proximity to another human being.
In essence, we are not trying to be louder than the orchestra but rather creating an acoustic effect that the human ear picks up better than anything else, giving the impression that the voice is louder.
Good technique is not about loudness, but about presence.
2) The less obvious: Artsy-fartsy Music making! This has become a plague on the operatic world. So many top singers are concerned with making vocal effects that are supposed to reflect their understanding of poetry and important harmonic changes. That is superficial!
Quite humbly, the singers job is to make a consistent sound throughout the contour of the vocal line, making sure that each note is resonating properly for technical accuracy and for the musical environment. A truly musical singer does not try to make a color "over the music" but makes acoustic choices instinctively that puts him/her in "consonance" with the musical texture. The human voice is not simply a melodic instrument. Every note is a series of overtones. In truth the voice is a harmonic instrument. Tuning it to the musical environment requires incredible aural sensitivity.
To create sounds that are rich in harmonics and are consistent with the musical environment, the singer must sing a strong tone. One must not sing "off of the voice" (a weak, unsupported tone). Too often singers make unsupported sounds and call it musical sensitivity. Unsupported sounds are not musical. They lack the harmonic complexity to truly participate in the musical environment. Such artsy-fartsy singing is aurally unsatisfying and inferior. Not only do many important people in our business not know the difference, but they (aurally trained by hi-fi, over-produced modern recordings) even promote that weak sound.
A singer recently sang an unsupported tone and claim that it is in keeping with Mozart's style. This was a very musical singer who has allowed himself to be corrupted over many years of bad coaching. Mozart did not write for small voices. His orchestra is sizable! There was a time when substantial voices were expect to sing Mozart along with Verdi or Wagner such as the legendary Eleanor Stever here.
We are lucky in our time to witness the rise of a great Mozartian/Verdian. The very promising, Angela Meade singing Fiordiligi here:
or Mozartian/Wagnerian, René Pape:
Elegant and substantial of voice. Mr. Pape often speaks of returning to a time when Mozart is sung by substantial voices.
And one of my favorite Netrebko moments:
Not the largest voice to ever sing Mozart's Elettra, but the voice is thoroughly substantial and indeed present in every sense through the contours of Mozart's tour-de-force aria. That Ms. Netrebko does not always pick repertoire that suits her magnificent voice is another story entirely, but the rebuke she suffers from the blogosphere about her lack of talent is totally unfounded.
Whether with Mozart or Puccini or Verdi or Wagner, the issue of orchestra balance is the same. Until the singer is generating substance and squillo, do not blame the conductor and/or orchestra for playing with passion. At every operatic performance I have attended when it was said the orchestra was too loud, there was always a singer or two who carried through with great presence and ease. And not always the biggest voices!
Nature gives no unsubstantial voices. Man, on the other hand creates plenty of unsubstantial vocal techniques!