So much music is implied by the visual artsthat it's no wonder cutting-edge installations include an essential musical element composed or compiled from preexisting pieces for the occasion.

Audiences are galvanized by the special-event status. Museums and musicians find audiences that don't typically come their way. Is this a brave new niche in the performance world?

We're not talking about the Philadelphia Museum of Art's "Art After 5" concert series. Or music that's a last-minute accompaniment to something that's pretty much finished. Conceptually, this recent wrinkle crosses a few extra bridges: The three near-daily performances of David Lang's Lifespan at the Philadelphia's Fabric Workshop and Museum through April 4 have three singers from the Crossing choir breathing and whistling to a four billion-year-old rock that's said to predate life of any sort.

More expansive in scope is New York City's forward-looking Park Avenue Armory, where pianist Helene Grimaud, typically a concerto soloist at the Kimmel and Mann centers, played near-daily hour-long solo recitals amid water in a flooded drill hall, with all manner of meditative lighting effects. Conceived by visual artist Douglas Gordon and Grimaud over two years, tears become . . . streams become . . . is on display at the Armory  through Jan. 4 (sans Grimaud, who has been replaced by a player piano with moving keys but no sound).

If nothing else, installations address the challenge of translating serious music events to the visual age. The San Francisco Symphony's recently opened SoundBox venue is equipped with atmospheric effects that can accommodate installation artists in a high-concept nightclubbish atmosphere. The Kimmel Center's SEI Innovation Studio, which just hosted an immersive performance of Terry Riley's In C with artwork projected onto the walls, is headed in similar, medium-fusing directions.

The biggest barrier is going down the rabbit hole with the conceptual artist.

Classical-music audiences are used to having everything spelled out for them, often while renewing their acquaintance with Beethoven symphonies or hearing new works with roots in the past. But if gallery-goers are asked to apprehend something so obvious, the artist is laughed out of the room. Installations are environments where individual components don't add up - in an experience that may be too personal to articulate. This is an inner journey without a designated destination.

It's a hugely different playing field. At the Fabric Workshop and Museum (where an audience of 20 is considered large), are those who giggle at the singers missing the point? Or seeing the situation in the cold light of day? At the Armory, those who paid $90 a ticket only to hear an extremely attractive but one-hour Grimaud recital - taking place in a very large puddle - may leave disappointed.

At the Grimaud performance I caught recently, I had to give up any sense of what comes next. The first half-hour at the Park Avenue Armory was spent watching water bubble up from holes in the floor in various evocative shapes, but at such a gradual pace that I felt under-stimulated. Grimaud, who made her way to the piano wearing Doc Martens, performed a preset program, but with unfamiliar works such as Luciano Berio's Wasser Klavier and Toru Takemitsu's Rain Tree Sketch II, you didn't always know whether she was playing the music in its preestablished order, or when one piece stopped and another began. I loved that.

The floor's wading pool - less than an inch deep, with the piano on a small island - reflected the grand, curved ceiling (resembling a European train station) so seamlessly you could wonder which was the real object and which the reflection. Lighting effects sometimes gave the impression of beams coming up from unfathomable depths. The acoustical properties of the water gave Grimaud's sound an alluring exterior gloss. A separate piano stood on a separate island. I kept expecting her to migrate but she didn't.

Size counts here. Interior spaces this large and relatively empty are seldom found in urban areas. The environment literally felt out of this world. But were my inner defenses removed to the extent that I revisited any personal rivers of tears that the title suggest? Well, no. But this is one installation I'm not likely to forget.

The Fabric Workshop experience is more about what is not there. The performance room is plain, white, unfurnished, with fluorescent light. The four billion-year-old rock, suspended by a string, is tiny and unimposing. The music by the Pulitzer Prize-winning Lang contains no traditional singing, and is only 15 minutes long.

Yet Lifespan contains a compressed lifespan, starting with bleak-sounding winds suggesting a barren rock flying through space, slowly taking on minute stirrings of activity that almost seem trivial, and then becoming snuffed out by the violent contractions of the universe. Only later did I find out that Lang wrote on the score, "This is not a gentle piece."

Such observations are one part of the conceptual journey that is continued in Allora & Calzadilla: Intervals, a larger show on the upper floors of the Fabric Workshop and the Philadelphia Museum of Art that contemplates the past-and-future nature of the world's creations. Mysteries aren't out to be solved. Crossing choir singer Steven Bradshaw, who is also an accomplished visual artist, was led to the conclusion that "the universe does not owe us an explanation." For anything.

How's that for humility? But 200-plus performances of Lifespan down the road, how might conclusions morph? Maybe that journey will end with a giggle, after all.

The Douglas Gordon installation tears become . . . streams become . . . is available for public viewing through Jan. 4 during the usual open hours of the Park Avenue Armory, 643 Park Ave. in New York City. Admission is $8. Information: www.armoryonpark.org.

The Fabric Workshop and Museum, 1214 Arch St., has multiple performances of David Lang's Lifespan through April 5. Information: 215-561-8888, www.fabricworkshopandmuseum.org.