Vivaldi's Four Seasons echoes through the Second Empire architecture of City Hall. Tourists and commuters are surprised to find that the nearly 300-year-old piece is being played by a youth of 18.

Sean Bennett is dressed in a gray hoodie and jeans. Attire is one of the few casual things about him. He's selectively social, but he feels most open and vulnerable with his violin.

"I find it really hard to fake emotions in a piece," says Bennett, seated on a chair in the central courtyard of City Hall, near his favorite spot for busking. "You have to trust yourself and trust your emotions."

For six years, Bennett has been playing on the street four times a week. He practices a minimum of three hours a day. He is his own harshest critic.

"I messed up," he says in the middle of one piece. He stops playing for a moment. A voice from across the courtyard yells, "No, continue!"

Born in South Philadelphia but residing in West Oak Lane, Bennett was 10 years old when his teacher passed around a sheet for violin lessons, and he signed up. He struggled with reading music, but he kept practicing.

"Nothing is easy," says Bennett. "If it's easy, you're doing it wrong."

Now, with eight years of experience on the violin, Bennett is in his fourth year as a member of the Philadelphia Youth Orchestra.

Daniela Pierson, artistic director of the music-advocacy group Musicopia, noticed Bennett's dedication early, when he took several buses to make rehearsals for the Musicopia String Orchestra.

"He was engaged more intensely than most students are at that age and skill level," says Pierson.

At first, Bennett took the classes because his friends were taking them, but as the friends began to lose interest, his interest began to peak. It wasn't until he "auditioned for basketball" and scored in his own team's basket that he decided to stick to the violin.

One day, as his older brother played basketball on a nearby court, Bennett took up his violin and began to play, with his case open on the floor. Someone dropped money in it. A lightbulb came on.

The 12-year-old Bennett began to take the subway to the corner of 40th and Market Streets to play. After an hour and a half on his first day, he had made $200. These days, he says, he makes on average $50 an hour. Maybe (he guesses) he was cuter then.

As he plays at City Hall, many walk by, some make eye contact, and a few leave money.

Whether he knows it or not, Bennett has a fan base, and it's growing.

Stephen Flemming, 32, remembers the first time he heard the then-12-year-old play. "I was astounded," says Flemming. "I don't know many adolescents pining over classical music."

Flemming, who teaches third grade at John B. Kelly Elementary School, has seen Bennett both playing the violin and commuting by bus.

"I have never ever seen him apart from that violin," says Flemming. "When I look at him, I think that he could be one of my students, and his future looks so bright."

James Rout, visiting the city from Tennessee, stops in his tracks at the sound of Bennett's playing. "Seeing someone so young play without an audience and an ego," says Rout, "shows that he's playing for the art."

Bruce Potter rummages in his pockets for a few bills to show support. "It sounded like an orchestra," says Potter. "I thought it was coming through speakers."

After every contribution, Bennett pauses and says, "Thank you."

Playing for the public, Bennett says, has thickened his skin. He remembers playing at 15th and Walnut Streets when an older man asked, "How does a black man play the violin?"

Bennett was baffled. And angry. He kept playing.

Flemming, who is black, took to his blog to showcase Bennett's playing and urged others to support him. It was an act that was very important to him, he says: "Black boys get a lot of bad press in a heartbeat."

Bennett has a diverse circle of classical-musician friends, yet he's aware people don't think of such musicians as being diverse in age or ethnicity. So when, on separate occasions, R&B/neo-soul singer Jill Scott and Ahmir "Questlove" Thompson, famed drummer for the Roots, walked by, acknowledged him, and contributed money, he was floored.

"It was great," says Bennett. "I feel like, if I were to play for some artists, they wouldn't really care."

Bennett likes many music genres, with classical music preferred. Brahms, Mendelssohn, and Bach are among his favorite classical musicians. "A lot of today's music has no thought or emotional depth," said Bennett. "If you take away all the words and really go into the music, most of it is simple."

At 14, Bennett cowrote a piece called Blue Evening for a Musicopia concert. It was presented to a very impressed faculty. "It's a great piece. Music is totally in his blood," says Pierson. "He learns so beautifully."

In 2012, Bennett won the All City Philadelphia Concerto Competition, and he spent the summer training in Michigan at the Interlochen Summer Arts Camp.

Back at the City Hall courtyard, Terumi Takeuchi has been listening, eyes closed, for the last 10 minutes.

"He's giving a little bit of happiness and peace to everyone," she says. "He needs to know that."

Bennett knows the city's routine by heart. From rush hour to lunch time, he examines pedestrian patterns. When he was younger, he used his earnings to buy scooters and shoes, and to treat his friends to lunch. Now, he pays for everything he wears and eats - a decision he didn't have to make, but chose to. The second of six children, he doesn't want to be a burden.

His career trajectory is as improvisational as his street performances. As for continuing his musical education, Bennett has numerous performing-arts colleges on his wish list, with the Curtis Institute of Music as his top choice. Whether he decides to teach or join an orchestra, Bennett vows to put his art and happiness first.

In the meantime, he reaps his chief reward: to see people "just stopping and telling me I'm doing great."

"I'd rather that," says Bennett, "than money."

Hear Sean Bennett playing the violin outside City Hall in Philadelphia.

215-854-5054 @sofiyaballin