Imagine you had invented oil painting, from scratch -- and then the first pictures you made turned out kind of hit-or-miss. After all, as a pioneer, you'd have no one else's successes, or mistakes, to work from. It could take a while -- even a whole career -- before you fully got the knack of your invention.

That's seems to be the predicament facing Janet Cardiff, a Canadian artist who's lately been working mostly from Berlin.

For more than 10 years, Cardiff has been acclaimed for working out an art form with few precedents. She has her audience put on headphones, then guides a walk through the real world. An audio track provides convincing sound effects, contemplative ruminations and bits of fantastical storytelling that mix with the actual noises the listeners hear around them. Imagine an old-fashioned Acoustiguide, but with an experimental artist in control and a big chunk of the world as the exhibition to be annotated.

Cardiff's latest art walk, "Words Drawn in Water," commissioned by the Smithsonian's Hirshhorn Museum and Sculpture Garden, was launched there yesterday.

Your first contact with Cardiff's work comes at a kiosk in the Hirshhorn lobby, where you surrender a credit card or photo ID and claim a tiny iPod music player and pair of headphones (the players were a gift from Apple). Then you sit on a bench by the lobby's rear doors and wait in silence for your ride to begin.

The sound of a bass voice singing "Ol' Man River" gets things started, mixing with the lobby's other noises, which the earpieces don't keep out. The tune fades, and a throaty woman's voice -- it's the artist's, but she sounds like a torch singer -- starts to speak to you.

Cardiff muses about a series of paintings of the river Thames, painted in the 1870s by James McNeill Whistler. She contemplates the fact that a drop of flowing water once seen by Whistler could now be running through the fountain in the Hirshhorn's courtyard, or could be in that apple that you'll eat next year. Thoughts and memories, she says, can be like that, "connecting from one time to another, and from one person to another."

Hence the snippet of "Ol' Man River," which Cardiff now explains reminds her of her mother, who heard and met Paul Robeson as a little girl and counts him as her hero. A song about a river becomes a drop in the bucket of memory, flowing from Cardiff's mom, through her to us.

But that's enough sitting in a museum. Cardiff's work is about getting beyond such traditional art spaces. Cardiff's tone shifts from a poetic lilt to a businesslike clip, and the voice in your ears tells you to get up, walk through the Hirshhorn doors and out into the fountain courtyard.

Things look normal enough, but the sounds you hear tell a different story. That voice, poetical again, tells you there are people watching from the windows up above, and you hear them clapping and whooping as you walk by -- though there's no sign of them if you glance up.

Cardiff and her husband, George Bures Miller -- an artist who sometimes co-authors works with her, and almost always helps with technical details -- have developed stunning tricks for recording sounds that seem absolutely present when played back through headphones. If trompe-l'oeil pictures fool your eyes into seeing painted things as real, this sound art is trompe l'ear.

As instructed by Cardiff -- now back to her sober, how-to voice -- you head out of the courtyard, toward the open spaces of the Mall and the Hirshhorn's sculpture garden. According to your ears, at least, a helicopter passes overhead, and Cardiff's voice, again in full melodrama mode, whispers: "I think they're tracking me," only to then downplay that idea as a figment of her own imagination, brought on by paranoia bred of her visit to Washington.

As Cardiff's voice leads you into the sculpture garden, among its Rodin bronzes, snippets of other talk enter your ears. You hear Cardiff in conversation about art with someone who your eyes tell you is not there. You catch a fragment of anecdote told by an older woman who says she's a fifth-generation Washingtonian. There's the sound of a museum guide giving a standard tour of nearby sculptures -- also a fiction, or at least recorded sometime in the past.

Occasionally Cardiff speaks directly to you, telling you about what's going on in the alternate world she seems to be in. As you walk down a narrow path beside a reflecting pool, she tells you that she's "walking along a ledge 20 stories high," and then goes on to comment, cryptically, "Sometimes I'm not sure what's real, but they said that this might happen -- a side effect." Or she launches into more generally meditative observations: The pipes and wiring under the Mall, she says, are like "the hidden layers of history in this city . . . the voices that are lost in the air."

This, more or less, is how things continue for the rest of the piece, as Cardiff leads her audience along the Mall, past the Smithsonian's vintage merry-go-round, into its "Castle" -- with a meditative stop at the tomb of founder James Smithson -- out again onto the Mall and then into and through the Freer Gallery of Art, and finally into that museum's elaborately decorated Peacock Room, painted by Whistler in the 1870s -- at the same time, that is, as the river pictures that started Cardiff talking as we began our walk.

This partial description points to some of the obvious virtues, and failings, of Cardiff's work.

The basic premise of her novel art form is engaging. It's interesting to find yourself led by the ear, as it were, as an artist gets you to experience the world in a new way.

Cardiff's illusionistic sound effects can be captivating, too. Sometimes they lead to stunning coincidences that link the real world all around you and the sonic fictions she provides. As I was taking in Cardiff's latest art walk, a jogger happened to run by, followed by a family crossing my path, at precisely the moment that Cardiff narrated the passing of an imaginary jogger and family in her fantasy world, and just as her headphones fed me the sound of running shoes and stroller. Every time I've done one of Cardiff's walks -- this must be the fourth different one I've come across, in places as varied as Muenster, Germany, and Oakville, Ontario -- there's been some moment where reality and fiction intersect, or collide, or merge in interesting ways.

The best bits of Cardiff's new piece are the most straightforward: They happen when her businesslike voice tells us where to walk, and then lets her sound effects subtly comment on the real world all around us.

It is the audio equivalent of how great landscape painters choose an actual scene, and then give it suggestive tweaks so that it subtly changes meaning. That was the crisp mode that Cardiff used for her most famous piece, called "Forty Part Motet." She taped the 40 separate singers in a cathedral choir, both as they sang and as they chatted between takes, then played them back through 40 separate speakers pointing inward in a ring. By inviting us to wander freely in that circle, Cardiff let us become intimate with both singers and their song, and with how a bunch of quirky individuals, and their peculiar vocal parts, combine to form a whole. (Someone ought to bring this great work to Washington.)

The worst bits of "Words Drawn in Water" are those in which Cardiff puts on her storytelling voice and goes all poetical and pseudo-philosophical on us. At those points, the piece recalls an overwrought dream sequence from Hollywood. It doesn't really capture how the unconscious works, or even how a glimpse into an alternate reality might feel; it just regurgitates received ideas about how to construct way weird fantastic worlds.

Cardiff is surely onto something in her art. Over the next few decades, and with a few dozen more walks under her belt, she's likely to sort out what works best in her new medium, and where it tends to fail. Or, short of that, maybe a dozen other artists will take up where Cardiff chooses to leave off and realize the full potential of this novel form.

Janet Cardiff's "Words Drawn in Water," commissioned by the Hirshhorn, will be available free of charge Wednesday through Sunday 11 a.m.-3 p.m. through Oct. 30 at the museum, on the Mall at Seventh Street SW. Call 202-633-1000 or visit www.hirshhorn.si.edu.

Janet Cardiff at the Hirshhorn, working on her audio installation: "Connecting from one time to another, and from one person to another."Visitors to the Hirshhorn experience the audio walk "Words Drawn in Water" using feet, ears, eyes and iPod.