Musicians gather at The Catalyst on Monday night to perform in the Jazz Jam. Billed performers and audience members collaborated and jammed together. Photo by Kyan Mahzouf

Members of the service industry crowd the bar, and a small group of metalheads shoot pool quietly in the game room. On the other side of the bar, chaos reigns supreme, as jazz musicians, both young and old, take part in the creation of what the players call “truly American music.”

Every Monday night through Oct. 17, The Catalyst is hosting Monday Night Jazz Jam, an old-school, open-mic style jazz odyssey in which audience members — whether they be vocalists, keyboardists, or horn players — sign up to play alongside the band. The results please the ear and embody the philosophy behind jazz.

Esoteric Collective, the quartet foundation upon which audience members construct their jazzy musings, kicks off with variations of works by some of America’s greatest jazz musicians. They cover everything from Miles Davis to Cole Porter.

“Jazz is America’s classical music,” said Esoteric Collective bassist Jamie Brudnick. “Each song exists as a template upon which we impose our improvisational skills.”

The knowing grin drawn across Brudnick’s face suggests the entire number is rehearsed — scripted — but he insists upon its spontaneity.

“We don’t use papers,” Brudnick said. “And we never play a song the same way twice. Hopefully we inspire other musicians to expand upon what they hear here tonight.”

Indeed, Brudnick makes it look easy, closing his eyes and plucking along in violent accord with a rapidly evolving rendition of George Gershwin’s “I Got Rhythm.” Such is the nature of Monday Night Jazz Jam. There are no auditions, no agendas, and no “papers.” The experience is entirely organic, with the musicians playing off each other, each sound expounding upon the last. One can’t help but feel that every aspect of the event is in perfect harmony with the impromptu, frenetic nature of jazz.

After a 30-minute set, Esoteric Collective opens up the stage to the audience for collaboration. A group of horn players joins the band, as well as a keyboardist, who relieves Esoteric Collective’s own Jon Dryden. Then, as if they had been playing together for months, the newly formed ensemble launches into a smooth, yet appropriately chaotic jam. Singers soon emerge from the audience, offering their vocal variations on the classics.

“It was like time travel,” vocalist Joy Rush said after her performance, “a real old-school jam session.” This sentiment was undoubtedly shared by the audience, which was warm and attentive throughout of the event, applauding each musician after their respective solos.

“This is the hottest thing going on,” said trumpet player Robert Reisman. “I’m coming back every chance I get.”

The event is limited to people ages 21 and up, but if you’re of age and looking for an inexpensive ($5 to $10 donation), intimate setting to listen or contribute to the sounds of a group of jazz sages paying homage to the chaos that surrounds them, then Monday Night Jazz Jam might just be your thing.