Even by the extraordinary standards of Cuban music, Vocal Sampling are a pretty extraordinary bunch of musicians. First, they create the sound of a salsa band without the use of a single instrument. And second, despite the all-pervasive influence of the Buena Vista Social Club phenomenon, which manages to give the impression that Cuban musicians only spring into action at the age of 80, they are young.

To appreciate what Vocal Sampling's music is all about, you have to see them perform live. Let's face it, unless you're talking about 16th-century church music or are too young to remember The Flying Pickets, the phrase a cappella is somewhat shudder-inducing. This, however, is alt. a cappella: intricate arrangements, precise execution, superb ensemble musicianship. Six funky guys producing an astonishing array of sounds woven, not into a tricksy circus act, but into a seamless musical whole.

The initial reaction of audiences tends to be disbelief. Next comes scepticism, swiftly followed by close questioning of the sound engineer to see where the background tapes have been hidden. By the time it dawns on people that there are no background tapes, no technical wizardry - no tricks except the obvious one - everybody has forgotten that there are no instruments anyhow, and the place is rocking.

Despite the show's surface spontaneity and exuberance, it's obvious that these are highly accomplished musicians who know what they're about. So it is no surprise to discover that it all started at the Instituto Superior de Arte in Havana, when a group of friends and fellow music students began showing off at parties.

"It was just a game at first," says singer and bandleader Rene Banos. "Sometimes we'd go to a party and there'd be a couple of instruments; sometimes there'd be none; so we'd improvise. Then we began experimenting with different styles, and gradually we realised we were on to something that could be developed. It wasn't just a viable performance style in its own right, but it was great fun to do. And so we started doing more and more of it."

Banos began playing violin when he was eight, then switched to piano. "But when we do the instruments in concert, what counts is the individual's vocal register and not which instrument they actually play," he says. "See the guy who does bass in the band? He actually studied the trombone - but he has a deep voice, so he's the ideal bass for the band." When they work on recreating an instrument, do they concentrate on reproducing the actual sound, or on mimicking the actions used to play it? "A bit of both," he says. "On a surface level we just imitate the instrument, but realistically it's impossible to sound like an instrument does. So in a way, we're playing the role of the instrument rather than trying to be the instrument itself - sort of acting it out."

It's often impossible for one performer to recreate an instrument by himself; the piano, for instance, requires the use of several voices at once, to convey the feel of both melody and accompaniment. In some ways, though, unpitched percussion is the trickiest. "Synchronicity between the singers is much more difficult; you can't rely on the usual scale of do-re-mi." Much of Vocal Sampling's success derives from the group's clever arrangements, for which Banos is chiefly responsible. "With each new song we add to our repertoire," he says.

"First of all we sort out the arrangements. With standards such as Guantanamera, for example, I break the orchestral arrangement into six parts, then sort it out for each different voice. Once that's done we progress to the stage of spontaneous embellishment." It is essential to get the balance right: everything is tightly choreographed so nothing can get out of synch, but it needs to look casual and uncontrived. "We try to make the audience think they're at a big party."

One big party: that's the image of Cuban music which the Buena Vista Social Club has conveyed to the world. But is that how it really is? Banos frowns. "In Cuba," he says carefully, "there has always been a great appreciation of music - not just of Cuban music, but of all musics. The fact that there is such an international interest obviously helps the home base in a big way; but for musicians in Cuba things are basically the same as they've always been."

"Those Cuban musicians who wanted to make money went to north America. Those who wanted to make music stayed in Cuba. They have had no commercial pressure to speak of for generations. You are not tempted to sell out where there is no currency to earn."

A musical world where commercial pressures just do not count is almost impossible to imagine. But it is this, more than any other factor, which has helped Cuban music develop in so many directions simultaneously. "In Europe and America, the word `folk' means something in the past: something just about dead. In Cuba it is not unusual to find musicians who play both jazz and folk professionally. All music is mixed in the same bowl."

Or as he puts it in his one of his own songs, Cantando a Coro, (Singing in the Chorus), "Cuba is a land of soneros, rumbers and troubadours, of machetes and drums, of guitars and rockers. The postmen sing; little girls and farmers sing; the palm trees and the fields; here come the Cubans, singing."

Vocal Sampling play at Vicar Street on Sunday May 13th as part of the ESB Routes in Rhythm series 2001