There are 72 guys in the Arlingtones Barbershop Chorus, and only one of them sports an oversized, yellow, foam-rubber cowboy hat. Then again, he is not unrepresentative.

There's the smiling man with the funny moustache who looks like a J.R. Ewing clone, the 14-year-old with the red hair, the PhD, the banker--and even a barber. But to Phil Davenport, the director, they all have at least one trait in common.

"I tried to do a sociological study once of the kind of people who become barbershoppers," says Davenport, a grizzled ex-Navy pilot with a habit of doing Al Jolson-style endings to songs. "They run the full range of careers, but a lot of them are in the military--and there's not one of them that's not a ham."

Getting men to sing in public is not an easy thing in America, he points out--particularly singers with no training. And this applies to most barbershoppers, he adds, noting that a good half of them hold no music at rehearsals.

That's okay, because their director doesn't read the music, either. "Oh, I can read it passably if I have to," he says, "but you don't really need to in barbershop."

The 19th century singing form, indigenous to America, is actually a recreation of a four-part chord. Tenor, baritone and bass singers, by listening carefully to the lead, maintain that chord throughout a song "wherever feasible" says the Arlingtones' vice president, Lou Perticari.

The chord, which barbershoppers call "elusive," ideally is sung to a perfect scale. If this is achieved, and if all the singers use the same pronunciation (so "where" comes out as "ware," not "whir"), the three lowest voices should "generate the sound of the tenor part," says Perticari.

"Theoretically, the tenor doesn't even have to sing--he just echoes the overtones," he explains. In practice, however, no quartet dares to sing without its tenor--and that goes double for a barbershop chorus such as the Arlingtones.

Such a larger group, described by one singer as "just like a quartet, only fatter," is the beginning point for most barbershop singers. "We'll take a guy who might not even be able to sing too good," says Mike Gorham, the Arlingtones president, "and we'll hook him up with a more experienced singer, called a 'leaner,' so the beginner has somebody to lean on, see?"

Davenport says a man who may be "too shy to stand up in public and sing doesn't mind so much when he's with a big group. Then the next step is to get him into a quartet." The Arlingtones officially includes four quartets, although there will be seven combinations in their show this weekend.

The Arlingtones' quartets are registered with the national organization, The Society for the Preservation and Encouragement of Barber Shop Quartet Singing in America, or S.P.E.B.S.Q.S.A. ("If you can pronounce that, you can sing barbershop!" says Perticari.) S.P.E.B.S.Q.S.A. picks up the bulk of the Arlingtones' annual $48 dues, and uses the money to arrange contests, concerts and "lots of interchapter activity," says Davenport.

Davenport himself is a one-man interchapter activity. A barbershop director for 20 years ("I was singing with a group that didn't have a director, so I thought I'd give it a try,") he heads both the Arlingtones and the Sweet Adelines, the women's branch of the Alexandria barbershop group.

Barbershoppers are a sexually segregated lot, it seems, though the Arlingtones deny any male chauvinism. "This isn't just a place for guys to get away from women," says Davenport. He explains that the female voice range is a good two octaves higher than the male parts in barbershop, "and a mixed chorus comes out with a very neutral sound."

The Sweet Adelines have chapters throughout the metropolitan area, and Davenport says the women "tend to be more dynamic than the men's groups. If you can convince them that, by wearing costumes and a lot of makeup, they become somebody else, then they'll stand up there on stage and really put out," he says.

Davenport calls the Alexandria male barbershoppers "the eminent chorus in this area--they're the Mid-Atlantic District champions of S.P.E.B.S.Q.S.A." He assesses the Arlingtones, now celebrating their 25th anniversary, as "a good group--middle of the pack."

Although there are those in every barbershop group who take the competition seriously, the art form is mostly "for those who love harmony and just want to have a good time," he says.

The group gives monthly free concerts for nursing homes and community groups, and is getting ready for its annual sing-out. This year, the format is a scripted play with a western music theme, called "Sing-out at the O.K. Chorale," or "How the West Was Fun."

Here, little dogies get along, cowboys are not fenced in and discouraging words are scarce in songs that are "strictly family entertainment," says Perticari. And oversized, yellow, foam-rubber cowboy hats are as welcome as the hams who wear them.

"Sing-Out at the O.K. Chorale," by the Arlingtones, 8:15 p.m. March 5 and 6 at the Thomas Jefferson Community Theatre, 125 South Old Glebe Road, Arlington. Admission is $5 for adults, $2.50 for students and senior citizens. For more information on the Arlington chorus, call 527-6195. The two other Northern Virginia barbershop chapters are the Alexandria Harmonizers (248-8383), and the Fairfax Jubil-Aires (860-8803).