Pop singers, by and large, are specialists, tending to inhabit a single style and sounding ill at ease elsewhere. Not that such discomfort ever stops a pop star from suddenly deciding to try on a new sound, but the bad fit that almost inevitably results keeps such changes from becoming habitual.

Some actually manage to pull off such transformations, though, and two recent quick-change artists -- Police-man Sting and part-time Honeydripper Robert Plant -- have managed fairly radical shifts in direction with admirable ease and success.

Of course, Sting's widely reported shift to jazz hardly constitutes new surroundings for him, as Police drummer Stewart Copeland discovered the singer playing bass with a jazz group. What is surprising about Sting's first solo album, "The Dream of the Blue Turtles" (A&M SP-3750), is the direction this jazz infusion takes, for by recruiting jazz musicians, Sting wasn't assembling a band to play be-bop. Instead, what he has attempted is a sort of hybrid that filters pop through a bop perspective, offering an almost inverse vision of the Police's equally able, rock-informed sound.

It isn't the easiest of compromises. Where rock makes much of minimal music, turning simplicity into a cardinal virtue, the jazz instincts of Sting's new playmates tend toward far more involved playing. As a result, the driving Motown beat that fuels "If You Love Somebody Set Them Free" is almost overwhelmed by Kenny Kirkland's chattering organ and Branford Marsalis' impassioned tenor sax counterline. Taken as pure music, it's tremendously energizing, because various voices mesh so synchroniously. But in pop terms, it's a maddening muddle, because each element seems to fight for the listener's attention, rather than focusing it on the melody.

Fortunately, that's the exception, not the rule. On "Moon Over Bourbon Street," the jazz approach works wonderfully. There, though, the dynamic is different, for not only has Sting elongated the verse and chorus structure, but he uses the additional musical action in the song strategically, to foster an air of suspense befitting the lyrics' mysterious protagonist. By contrast, "Shadows in the Rain" comes off well because it completely recasts that Police chestnut as a hard-bop workout driven by Darryl Jones' walking bass.

Sting's greatest strength overall, though, is that he tailors everything to fit his own familiar vocal style. By refusing to affect jazz mannerisms, he holds the music in check, maintaining the album's balance. In fact, his main vocal failing is lyrical, as he too frequently pops up in the pulpit during the likes of "Russians" and "Love Is the Seventh Wave." Even this, though, is tempered with humor, as when "Seventh Wave" ends by spoofing "Every Breath You Take."

Where for Sting vocal mannerisms serve as a sort of defense against stylistic excess, Robert Plant's idiosyncrasies miraculously anchor his new album, "Shaken 'N Stirred" (Es Paranza 90265-1-E).

Considering how extreme Plant's vocal devices are -- for example, his habit of leaning into a blue note, twisting it into another key in the manner of a sanctified gospel singer, then somehow slithering into an Arabian flourish to end the phrase -- it's a wonder they can support a single performance, much less an entire album. But "Shaken 'N Stirred" makes a virtue of its tangents, for by keeping everything off balance, Plant somehow exposes the central energy of his music.

In "Too Loud," for instance, a sing-song vocal line is spun out against an increasingly disjointed dance beat. By rights, the song should tumble into confusion, but the circularity of Plant's vocal ideas establishes an underlying, unifying logic. Similarly, "Doo Doo a Do Do" chases Plant's mad-dog-in-heat moans with Roni Halliday's girl-group backing vocals, establishing a tension that unfolds into an almost palpable atmosphere of frustrated desire.

All of which makes for a quite startling departure from Plant's previous work. That's not to say that there aren't songs here that fit comfortably within the Plant canon, for "Trouble Your Money" sounds very much like something from "The Principle of Moments" album, while "Sixes and Sevens," despite its textural trappings, is typical of the type of ballad Plant has sung for years.

In essence, though, what Plant has done is ignore stylistic concerns and instead emphasize his own vocal identity. Thus, "Little by Little" uses the same bent-note howls that exemplified Plant's work with Led Zeppelin, but applies them in a new context. As a result, instead of his singing simply descending from the heat of the band, he conveys a sense of distance and alienation as his voice pushes against the cool, hard sound of Jezz Woodroffe's synthesizers. Even when Robbie Blunt's guitar solos against Plant's exhortations in the final chorus, the sense is less of empathy than sympathy.

That ability to find your singular identity in new musical surroundings is, in the long run, what reinvention is all about. That's why "Shaken 'N Stirred" is one of the most important, and heartening, albums of the year so far.