On Thursday night, Philadelphia's highest elevated concert unfolded. More than 800 feet above street level, three bands took to a pop-up stage on the 57th floor One Liberty Place for the latest Sofar Sounds show.

"Thank you all for coming out — and coming up — tonight," the night's emcee, Davis Howley, said to a mostly seated audience just before the start of the first act. "Welcome to Sofar Sounds. We bring music and poetry to nontraditional performance spaces so that you can experience artists  stripped down and away from distractions."

After reminding everyone to silence their phones, Howley introduced the first duo, hip-hop artist Kuf Knotz and harpist Christine Elise. With an orange-kissed city skyline as a backdrop,  Elise began strumming the first sounds of the night. Soon after, Knotz interjected a series of upbeat and raw lyrics, launching a sky-high concert experience that quickly drew its audience into complete silence.

Thursday's sold-out show was one of more than 100 Sofar Sounds concerts held over the last seven years in Philadelphia. Since its humble start in 2011, the scene has grown extensively, expanding from free shows in living rooms to 150-person concerts atop major attractions like One Liberty.

Kuf Knotz performs with Christine Elise (in background with harp) during a Sofar Sounds show at the One Liberty Place observation deck.
CHARLES FOX / Staff Photographer
Kuf Knotz performs with Christine Elise (in background with harp) during a Sofar Sounds show at the One Liberty Place observation deck.

Sofar Sounds, a music start-up from London, curates "secret" shows in 411 cities across the world, on every continent except Antarctica. Each production features three live acts, none of whom are announced until the audience shows up at a venue unknown  until the day before.

To get a ticket to a Sofar event, you must apply online. The only thing you know about a show ahead of time is what neighborhood it's in. If it sells out, as often happens, tickets are distributed through a lottery system and spots are announced by email a few days before the event. You can request up to six tickets per person (newly raised from two), and if you get in, organizers will email you the address of the venue one day before the scheduled date. Once you arrive, the artists are announced.

Philadelphia was among the first 10 cities to host Sofar events, and as of three months ago, it became one of only eight locations that Sofar  considers "full-time," meaning the city curates at least eight shows a month, with a set amount of money guaranteed to each artist up front. Sofar Sounds Philadelphia even has a full-time in employee, Carolyn Lederach, the city director. The rest is run by 35 volunteers.

"It's crazy — we went from one show every month, sometimes every couple of months, to multiple shows every week, and they're almost always packed," said Howley, who has been volunteering as an emcee for Sofar Sounds since 2012.

The increased frequency developed over time as shows began selling out as though Beyoncé were coming to town. Lederach said 300 people were applying to see a band in a small living room, which was the driving factor for creating more shows.

"There are people who come out every month who've been with us from the start," said Lederach. "You get to be really intimate with the artist and experience music that isn't in a crowded venue or noisy bar full of distractions."

Arjun Dube, drummer for the duo Trap Rabbit, performs in a Sofar Sounds show at One Liberty Place.
CHARLES FOX / Staff Photographer
Arjun Dube, drummer for the duo Trap Rabbit, performs in a Sofar Sounds show at One Liberty Place.

"People show up purely to discover music," said 25-year-old pop songstress Elizabeth Mencel, who performed as ROZES on Thursday night. "It doesn't matter who's playing — no one knows who's going to be there. As an artist, it sets you up to be more vulnerable, but [it] gives you this really intimate setting with your audience."

Most recently known for her collaboration with the Chainsmokers on their song "Roses," ROZES has collaborated with artists inclucing Alicia Keys and Galantis. She's among the better-known artists — including the Districts and Grammy winner Esperanza Spalding — who've been showcased at Philly shows.

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A majority of the bands, however, aren't what most would consider famous. That doesn't seem to deter the loyal following, even as the cost to attend a show has continuously increased.

Sofar Sounds recently began producing eight shows a month; its bimonthly shows were selling out almost immediately.
CHARLES FOX / Staff Photographer
Sofar Sounds recently began producing eight shows a month; its bimonthly shows were selling out almost immediately.

At the start, all performances were free, and a hat was passed around for people to donate money to the bands. About two years ago, Sofar started pricing tickets at $10 a person; in May, the price went up, doubling to $20. The new ticketing structure enables Sofar Sounds to guarantee the artists a $100 paycheck each in addition to a video recording of a song of their choosing.

As its following  grows, Sofar Sounds hopes to expand on relationships with corporate partners. Shows have been held at places including fast-casual salad chain Sweetgreen, the rooftop of the  Free Library of Philadelphia Central Parkway Library, the  Fleisher Art Memorial, the graveyard of Old Swedes Church, and WeWork coworking space.

But Lederach believes the performances held in South Philly rowhouses, Fishtown lofts, and West Philly living rooms will always be a part of the Sofar experience.

"We simply want to get more involved with the community," she said.  "If someone has a business that they think might be a good fit for a show, and we can help bring light to what they're doing, then we're all for that."

Sofar Sounds has also recently been trying out monthly comedy nights, featuring three comedians per show. Other new trends include themed shows, like '80s nights, where bands play two covers from the 1980s followed by two originals.

Those interested can create an account at sofarsounds.com and sign up for biweekly newsletters that announce all forthcoming shows.

Sofar Sounds also recently launched an app available for  Apple and Android phones that allows users to bypass the waiting list and receive instant tickets.