Most music fans are free to have a drink while they listen to live performances. Jazz-clubbers routinely have to buy — though not necessarily drink — two cocktails as part of the price of entry. Indie rockers don’t feel dressed without a can of Pabst in one hand. But classical music fans can’t so much as bring a bottle of water with them to their seats.

Not so at Opera on Tap, a raucous and sublime opera recital held each month in the back room of the Park Slope bar Freddy’s. Its next performance — themed “Hot and Steamy” in honor of the season — is this Thursday.

“Opera performances always have to be so perfect. That’s what theaters require, what you have to do for auditions,” Opera on Tap’s founder and “general managing diva,” Anne Ricci, said. “I just want to sing and have fun and recover some sense of spontaneity and creative energy.”

“Somehow pairing opera and alcohol — or just a comfortable setting — gets people not to have that stick-in-the-butt feeling you get sometimes with classical music,” a co-founder and co-managing diva, Jessica Miller, said.

Ms. Ricci, 30, is an emerging opera singer who has performed with the Chicago Humanities Festival and at Lincoln Center a few times with American Opera Projects. She got the idea for Opera on Tap in a conversation with Donald O’Finn, who books the acts for Freddy’s back room. It was there that she once complained that opera singers have fewer opportunities to perform than instrumentalists, let alone rock musicians. Mr. O’Finn offered her a gig at Freddy’s. She brought in fellow songstresses Ms. Miller, 31, and Carla Roberts, 32, as co-managing divas, and her college friend Elizabeth Gibbs, 30, as orchestral manager.

Last year, they gave their first performance, assuming it would be a oneoff, but the audience — and Mr. O’Finn — loved it, as did the performers. Two months later they had a second show, and since then, they have performed each month to a full house.

“We didn’t realize that there was this whole group of people who had never heard classical music or an operatic voice — and not this close up, in this kind of atmosphere,” which she describes as noisy, un-elitist, imperfect, and fun, Ms. Miller said.

Most audience members learned about Opera on Tap by word of mouth, though the shows have been featured in local listings. “The beauty of Freddy’s is that the bathroom is in the back room. We garnered a lot of audience members that way.”

Most of the audience is young, though a group of 50-somethings now attend every show, getting there early to make sure they get a seat at a table. Last Passover, two 80-year-olds who knew opera well stayed past midnight.

The singers — mostly opera students and emerging professionals — love the performances, too. And not just because it gives them a coveted chance to perform before an audience.

Ms. Ricci recounted watching one performer sing for the first time at Freddy’s. “For her first aria, she looked like an opera singer. A pint later, she settled into her own skin and sang from the heart. The whole energy in the room was different,” she said.

It’s wonderful, too, Ms. Miller said, to have creative control over your own performance, as opposed to feeling like a cog in someone else’s machine. The singers are accompanied by a piano player and sometimes by the ad-hoc ensembles Ms. Gibbs organizes, which perform several pieces on their own at each show while the singers take a break. Since instrumentalists have a much easier time finding freelance work, Ms. Gibbs said she had a much harder time finding musicians who are free to perform.

Not so with the singers. At first, they all were friends of the founders. Now the 30-strong ensemble includes friends of friends, as well. This month, Opera on Tap will hold auditions — especially for male singers.

Because scheduling is difficult, the performers rarely have a chance to rehearse. Ensemble pieces usually get a run-through in the hour before the show, but for solos and duets, the singers are on their own. As a result, standards from “La Bohème,” “Carmen,” and various Mozart operas show up quite often. More obscure pieces, often by contemporary American composers, make it into the repertoire quite a bit because singers can have trouble finding venues in which to perform them. Occasionally, a number from musical theater will sneak in; this Thursday’s “Hot and Steamy” theme naturally suggests “Summertime” from “Porgy and Bess,” for example.

Most songs go off quite well, though it can be often especially fun to watch singers exuberantly forget their lines, cut their duet partner off, and still scramble to make the song work. “Sometimes I think that the train wrecks are as popular as the successes,” Ms. Ricci said. “Part of the point is to show that opera singing is human.” In that spirit, she said, Mr. O’Finn once came up to her while she was practicing before a performance and reminded her not to rehearse too much.

Whatever it is, it’s working. To accommodate its growing audience, Opera on Tap is expanding to new venues. On July 27, it will perform at Barbès, another bar with a back room in Park Slope. And the group is talking to the Parkside Lounge on the Lower East Side for a show in August. Finding other venues has taken on some urgency: Freddy’s — a beloved neighborhood bar — is slated to be condemned as part of Bruce Ratner’s Atlantic Yards development plan. Ms. Ricci said she would be in close touch with Mr. O’Finn to find out if the bar will have to move, and if it can find a new location.

The divas are seeking to make Opera on Tap a nonprofit. They expect to be incorporated in the next few months, in time to make whatever donations they receive at their fall fund-raiser tax deductible.They are upgrading their Web site www.operaontap.com, so that performers can link to audio recordings of their singing — and post their résumés.

July 13, Freddy’s Bar (485 Dean Street at Sixth Avenue, Brooklyn, 718-622-7035).