Imagine a giant, two-story barn, with nothing on the walls and little furniture. Music so loud that one can shout in someone's face and not be heard. Diversions and drinks no more exotic than pinball and Pepsi.

Perhaps only teen-agers could appreciate such a mix. But that is the point, and they are the customers, at Three Macs' Disco Fire, a discotheque that opened in Novemeber in downtown Silver Spring. It is the area's only night spot expressly for, and usually jammed with, teen-agers.

These are the no-nos at Disco Fire: no alcohol, drugs, profanity, loitering, bare feet, T-shirts, shorts or fighting.

These are the constants one policeman at the door. At least six members of the Mackel family running the various rooms and operations. An average of 300 customers each weekend night.

These are the demographic: about 75 per cent of the crowd is 18 or younger. They come in about equal numbers from the District and Montrgomery County. About 90 per cent are black.

And this is the scene: noisy; cheery, rompin' and stompin'. If you like your reggae and your rock without the tinkle of glasses or the turmoil of concerts, this is the place.

Tony Mackel Sr., a 49-year-old former "lifer" in the Army, is in charge of the club. It is located at 1100 Bonifant Street, three blocks west of Georgia Avenue and one south of Colesville Road. The premises used to be a carpet factory and indoor miniature golf course. The total floor space is more than 4,300 square feet, enough for more than 600 revelers.

The other two Macs on the marquee are Mackel's two oldest sons, Kenneth and Rodney. But "Big Mac's" four other sons and two daughters, as well as his wife, also work at Disco Fire. The only employee who is paid is the Montgomery County policeman Mackel hires to keep the lid on things.

Disco Fire is open only on weekends, holidays, and evenings before school holidays. The festivities begin at 9 p.m., and usually run until about 2 a.m. Admission is usually $3, although a one-time membership fee of $3 entitles customers to cheaper admission and door prizes on subsequence visits.

Disco Fire bills itself as "a private club for the young set," but it is private only in the sense that the Mackels reserve the right to turn away anyone who won't abide by the rules.

And it is for the young set only in the sense that teen-agers are the majority. All ages are welcome. Indeed, Mackel tells of a 60ish couple who wandered in one recent night, explaining that DF was the only place they could do the hustle.

That is the main reason the young folk like DF, too. "I think it's real nice," said Cookie Manning, 18, who, like many customers, lives at the Summit Hill Apartments on nearby 16th Street. "Everybody's dancing. There's no fussing, no fighting. It's not like a bar."

"If I weren't here, I'd be home watching TV," said Michael McMiller, also 18. "And I'd much rather be here."

Small wonder, for few homes have two video ping pong games, two pinball machines, sandwiches at reasonable rates and - on most weekends - a live band rather than records.

Nor is there a more relaxing place to meet many friends at once. Patricia Gibson, 16, a sophomore at Montgomery Blair High School, said she used to attend weekend dances at Summit Hill. At Disco Fire, however, she said, she can mix not only with her neighbors and schoolmates but with students from Roosevelt and Coolidge High Schools in the District.

Mackel said the club has had little trouble, and only one arrest, since it opened Nov. 26. The arrest was of a gatecrasher. "But the most surprising thing is that the kids police themselves," said mackel, a short, stout man who usually sports a long cigar. "They want to keep this a nice place."

Mackel invested $70,000 in the niceness of the place, much of which went for a sound system that could probably project as far as Baltimore. He said the club is doing "well" financially, although "the first few weeks were difficult." in the future, Mackel said, he plans kiddie disco afternoons and senior citizens nights.

"But I'm interested in more than the almighty dollar," Mackel said. "If that was all, I would have said, 'Hey, let's get some booze in here.' I just want to give the kids a place that's theirs. Just want it to be the place where you hear good music."