SANDY’S SESSIONS

Blocks away, where Central Square’s nightclubs dominate and black-clad youth throng sidewalks to smoke, the muffled sound of amplified rock has become mere background. The lively, crisp sound leaking onto the street from Sandy’s Music, though, still startles, even after a dozen years of Monday night jam sessions.

Passers-by glance into the shop in surprise, drawn by the counterintuitive twang of banjo, guitar, mandolin and fiddle, as though seeing Sandy’s — now deep in its third decade — for the first time. Indeed, it is exactly the kind of shop that fades into obscurity but is always around for those who need it, and it is largely the same for the old-time music played there.

“Some nights nobody will show up. Other nights there’ll be one or two. Six or eight these days is most common,” says Sandy Sheehan, owner of the shop and overseer of the event. “We’ve had people from teenagers up to people in their 60s or 70s. It’s totally informal, totally unstructured. It starts when people show up and ends when they leave.”

There is one rule: “We do not want singer-songwriters. This is specifically old-time music.”

Sheehan, also the host of WUMB-FM’s late Saturday “Traditional Folk” show, fits his part: He is scraggly and lined, a bit reclusive, the perfect mountain man, laconic and blunt, and his narrow shop is similarly well cast, crammed full of slide whistles, “authentic Ozark harps,” sheet music, packets of string, straps, light-absorbing black matte instrument cases and, hanging everywhere, those lustrous guitar and banjo bodies. The only bow to modernity is the computer, already yellowed although Sheehan claims it’s new, and a rack of compact discs.

The musicians wedge themselves into the aisle, showing up in profusion on a recent Monday that would seem to overwhelm the store’s resources — but somehow don’t. Folding chairs are produced from odd crevices as players arrive.

“We always seem to have enough,” Sheehan says.

The sessions are reputed to have drawn such names as Peter Wolf (“once in a great while,” Sheehan says), John Hartford, David Bromberg (“when he was in town”) and Mark Sandman and others from Morphine. The players that show up regularly are retirees, homemakers, software engineers, market researchers. They are grizzled old-timers with ponytails, such as Len Katz, or buttoned-down types such as Bob Miller, celebrating time away from his job at Procter & Gamble. Somewhere between the two is young John Ostwald, the software engineer from Cambridge’s BBN Technologies, who shows up with long hair, a button-front shirt and, this night, a homemade mandolin, its body made of a Coleman oil can. A candolin. The players marvel and crack jokes. Even Sheehan, who is dismissive of his own banjo playing and stays grimly behind his computer while the others zip through “Bumblebee in a Jug” and “Sandy River Belle,” gets up to see the thing.

“How do you manage up by the 12th fret,” he asks, looking dubiously at the instrument’s neck.

“I usually don’t,” Ostwald replies. He offers Rebecca Bearden, a mandolin player, a chance to try it.

“I wouldn’t know where to begin,” she says.

“It’s a mandolin!” he tells her, passing the instrument and taking out his fiddle. “Try cello fingering. You’ll be much happier.”

The group plays companionably, speaking occasionally, comparing notes between songs, some concentrating, some in that magical zone where they seem to watch in pleasant surprise as their fingers play for themselves. The songs are short. Most are played with a jaunty, seamless elegance, even as more and more musicians take their place and join in. Once or twice a song is started and abandoned — experiments that the group kindly, and without formality, votes down.

Some players, such as Katz, have been coming since the beginning. Barkev Kaligian, 69, of Lexington, recently stopped in for the first time, drawn by a calendar of events in a bluegrass newsletter. The musicians play in peace and apparent perfection without benefit of long experience with each other.

“After every session I go home and my wife says, ‘Did you have a great session?’ Every session is a great session,” Miller says. “It’s always a lot of fun, that’s for sure.”

Bearden sets aside the oilcan instrument and goes back to her own mandolin, which has an old-time appropriate thin rope in place of a strap. More people arrive. More chairs are magically produced.

“It’s totally unstructured, informal and unpredictable,” Sheehan says again.

“Just like the music itself,” Katz says.

The old-time jam sessions start at about 7 p.m. on Mondays at Sandy’s Music, 896A Massachusetts Ave., Central Square. There is competitive parking on the avenue at metered spaces (free at night). Call (617) 491-2812 for more information, or go to sandysmusic.com. The sessions are free.