Just as he has done nearly every warm day for the past quarter-century, Mohamed Gardian Jalloh turned onto Denise Drive in Capitol Heights on Wednesday afternoon and flipped a switch. What emerged from the speaker on the roof of his white van was the instantly distinctive soundtrack of summer: ice cream truck music.

Within 45 seconds, a door slammed, a mother shouted and a little boy in blue pants and a white shirt streaked to the corner.

“Hey, Mohamed, can I get an ice cream sandwich?” the boy said breathlessly, thrusting a wad of dollars through the window in the side of the truck.

“They do know my music,” Jalloh said with a laugh, rummaging in the box freezer as more kids lined up. “When they hear it, they come.”

Like hundreds of ice cream vendors in the area, Jalloh has been cleaning his truck and stocking his freezers in the run-up to Memorial Day weekend. There are new products to learn (the European Magnum bar is being heavily promoted this season), and new customers to meet on his route in Prince George’s County.

But the music never changes. When you’ve latched onto one of the most effective Pavlovian come-and-get-it gimmicks in marketing history, you stick with it. Over and over.

And over.

Jalloh has been locked on this 40-second loop for more than a decade. Sometime this weekend, he will mark, by rough calculation, his 13,500th repetition. It’s his constant companion. He hears it in his sleep.

But he has no idea what it is.

“The name?” he said with surprise, looking up at the Nichols Omni music box that is nearly identical to those in every other ice cream truck in America. “I don’t know what it is. It’s number four. It’s the only one I play.”

Vaguely familiar melody

Guy Berliner can’t name the song, either, and few people know more than he does about the ice cream truck business. The owner of Berliner Specialty Distributors, one of the country’s largest ice cream truck depots, was standing last week in a marina of white trucks. More than 200 vendors pay to park their vehicles overnight at his Hyattsville facility and plug their freezers into long banks of power lines.

Dozens of truck owners, most of them immigrants from Africa, waited to rush cartons of Creamsicles and Choco Taco across the 80-degree parking lot and into their humming freezers.

Davis Copperfield’s territory is Bethesda, and he was loading his truck with Haagen-Dazs bars and Starbucks coffee ice cream. He holds up a box of Ben & Jerry’s Cherry Garcia bars. “The liberal activists love these,” he said with a bellowing laugh.

Copperfield, who spends seven months a year driving an ice cream truck in Maryland but also owns a hotel in his native Nigeria and a bakery in Ireland, couldn’t name the tune he has played for years. “Pop Goes the Weasel, maybe?” he said.

Jalloh has been parking his truck here since coming from Sierra Leone in 1985. He’s on his third vehicle, a gleaming Grumman panel van that he is rolling out for the first time this season. New truck, same music.

Before heading on his rounds, he played the tune for Berliner, who nodded at the vaguely familiar melody.

“Oh, sure,” Berliner said. “That’s a popular one. But I have no idea what it is.”

Loops that lure them in

The earliest ice cream trucks issued their calls to the curb with jingling chimes, such as the set hanging from a vintage Good Humor truck being restored in Berliner’s workshop for the National Museum of American History.

The first melodic version appeared in 1929, when a California vendor bolted an amplified music box to the roof of his truck and brought the kiddies running with a Polish folk song called “The Farm Pump,” according to ethnomusicologist Daniel Neely.

It worked. Today, very few sounds command such instant recognition, Neely said, even when listeners don’t know the tune.

“I compare ice cream music to church bells,” he said. “Both are beacons. The sounds are really simple, but you know instantly what they are.”

But Neely, who wrote a chapter on ice cream truck music for an Oxford University book on sound studies, couldn’t peg Jalloh’s song.

“Of course, I’ve heard it, but I don’t know what it is,” Neely said after listening to a recording of the theme.

Today, the most popular ice cream truck song in the country is a half-minute loop of “The Entertainer,” according to Mark Nichols of Nichols Electronics, the Minnesota company that makes most of the nation’s ice cream truck sound systems. His father started the firm in 1957 after inventing a transistorized replacement for the clockwork-style machine that used to be the norm.

“The technology has changed, but the songs have hardly changed at all in a century,” Nichols said. “They’re all old, all simple and all in the public domain. We go out of our way to avoid violating anybody’s copyright.”

Also echoing around American cul-de-sacs each summer, he said, are “Turkey in the Straw,” “Sailing, Sailing,” “Little Brown Jug” and “Camptown Races.”

The music box man knows

Not much has broken into the repertoire since Nichols, chasing demographic trends, sent for the sheet music of “La Cucaracha” about 10 years ago.

“Every now and then, somebody will want to try some rap music,” he said. “The problem is that customers think it’s just somebody playing their radio too loud. When you hear one of these vending classics, they know exactly what it is.”

But none of those standards is the tune that Jalloh blasts every day. Nichols agreed to listen to the recording.

It took him three notes.

“That’s ‘The Picnic,’ ” he said.

The song is no song at all. Rather, it is a medley of tunes. An Asian manufacturer placed it on a computer chip to show off a new technology. It was just supposed to be a sample, but Nichols included it in the Omni music box. It became an ice cream standard.

Nichols has been able to identify a riff on “She’ll Be Coming ’Round the Mountain” in the latter measures but little else. A decade of nostalgia rests on a mystery programmer in Taiwan.

“I haven’t been able to trace it any more than that,” Nichols said. “But it sure got a lot of play.”