WHEN you walk to your seat in a movie theater for one of the “Live in HD” broadcasts of the Metropolitan Opera, your experience begins with the sound: the instantly recognizable, immediately comforting hum of instruments tuning and the audience stirring, piped in live from the Met itself.


I heard it when I stepped into the Murdock Theater in Wichita, Kan., last November to attend the screening of Philip Glass’s “Satyagraha,” part of a season in which I traveled throughout the country, attending the Met’s 11 HD broadcasts. It was the same reassuring bustle, whether I was about to see Verdi’s “Traviata” in a theater over a casino in Las Vegas or Mozart’s “Don Giovanni” in the middle of a snowstorm in Boston.


Then the lights go down and the opera begins, the surround-sound quality in the theaters loud and clear. As the Met proudly says on its Web site, “It is the next best audio experience to being in the opera house itself.”


The question is whether “the next best” is good enough when it comes to the complete opera experience.


It is a question with real urgency as the broadcasts become the way more and more people experience opera. The Met is leading a revolution, albeit one that has less to do with what it’s putting onstage than with how it’s sending it into the world.


None of which is to diminish the significance of the HD series. Fundamentally changing the way the performing arts can be delivered to audiences, the broadcasts are the most important thing to happen in opera since the advent of translated supertitles in the early 1980s. The “Live in HD” series, which began six years ago, now reaches 1,700 theaters in 54 countries. And audiences are turning out.


If all 17 performances of this season’s new production of “Don Giovanni” had sold out the 4,000-seat Met, it would have accounted for a total audience of 68,000. The broadcast of the opera in October, by contrast, reached 216,000 worldwide in one fell swoop, and the Met expected 50,000 more to see it in delayed showings in Asia, Australia, New Zealand and South Africa, and in “encore” broadcasts in North America and Europe. (Next month the company will similarly repeat its broadcasts of the four operas in Wagner’s “Ring” cycle.)


The number of broadcasts now encompasses about a third of the Met’s Saturday matinee performances; the number is limited by the series’s cost and logistical challenges, as well as by the company’s reluctance to repeat operas year to year.


While the audience numbers can’t match the millions that tune in for the Met’s radio or television broadcasts, the HD performances, with tickets costing about $20, have begun turning a profit for the company. And they have the added intangible benefit of associating the Met with something more innovative than the stale media of public television and classical radio.


The program speaks to the vision of Peter Gelb, the Met’s general manager, as a promoter. The rest of the arts world was unprepared for the program’s success. Other opera companies have only haltingly begun to explore a similar approach, with theater companies (like the National Theater in London), dance troupes (the Bolshoi Ballet in Moscow) and symphony orchestras (the Los Angeles Philharmonic) following suit.


But what actually happens when the lights go down? How does the movie theater experience compare with the live — really live, not live in HD — production in the house?


While the sound quality in the theaters is exceptional, what the microphones don’t, and can’t, capture are the differences in size of voices. In December, when I saw the broadcast of Handel’s “Rodelinda” on a balmy day in New Orleans, the countertenor Andreas Scholl seemed to have as big a voice as the mezzo-soprano Stephanie Blythe, a jarring experience, since Mr. Scholl was nearly inaudible in the house a month earlier.


Even as the vocal performances are homogenized, the visuals are often thrown into higher relief. In getting so close to the performers the broadcasts can create remarkably strong moments.