No matter how supple or powerful her voice is, an interpretive singer is only as good as her material and her producer. Four female soul singers with luscious vocal chords illustrate this point with their newest albums. Thanks to crucial navigating by their producers, Cheryl Lynn and Deniece Williams sail to the best albums of their careers. By contrast, Randy Crawford and Roberta Flack are set adrift in a sea of bland material.

Cheryl Lynn's voice has been recognized as an instrument of immense potential ever since her 1978 debut. Lynn has a breathy, confessional quality that establishes intimacy even as it glides through tricky note leaps and rhythm shifts. Her potential has finally been unleashed on her new album, "Instant Love" (Columbia FC 38057), by producer Luther Vandross. Vandross--once a featured singer with Chic, Change and Roberta Flack--is the hottest soul talent in New York right now. He combines the modern street sound of Chic's disco-funk rhythms and full, bright sound with the traditional pop-soul melodies and harmonies of Dionne Warwick and Aretha Franklin. Lynn's voice has Warwick's silky sense of unperturbed uncontrol, but can also break up a phrase into sharp stop-and-go rhythms. Thus Lynn is the perfect vehicle for Vandross (whose favorite singer is Warwick).

"Instant Love" begins with two uptempo dance tunes penned by Vandross with Miles Davis' bassist Marcus Miller. The title tune is pushed by Miller's churning bass figure answered by clipped guitar and piano chords. Lynn pops out the syllables with a choppy beat and maintains a humming tone. "Sleepwalkin' " features a clever Vandross lyric about a love-dazed Lynn finding out in the morning where she was the night before. She sends syllables spinning with the same shimmer as Miller's synthesizer. Lynn proves she can caress slow romantic ballads too as she swims across Paul Riser's string arrangements on Tawatha Agee's "Day After Day" and Ashford and Simpson's "Believe In Me." The record is crowned by a dramatically escalating duet between Lynn's wispy soprano and Vandross' rumbling baritone on Marvin Gaye's ballad, "If This World Were Mine."

Since she left Stevie Wonder's Wonderlove in 1976, Deniece Williams' career has grown steadily both commercially and artistically. It has reached full blossom with "Niecy" (Columbia FC 37952), her second collaboration with Philly soul master Thom Bell. Her church-trained voice has never been in better control as she builds the verses with confidential quivers and then sends out the choruses with trembling, almost desperate hope. Moreover, Williams' lyric-writing has improved dramatically, and she has co-written seven of this album's eight songs. Her songs accurately describe the tight corners that love can paint one into. Her lyrics supply plenty of substance for Williams' confessional vocal style.

"How Does It Feel" neatly catalogues the unspoken signs that a lover is leaving and poses the title question without supplying easy answers. Co-author Thom Bell applies the famous Philadelphia soul sounds to bring out the gradual realization of the verses and the urgent questions of the chorus. Williams and Bell have written three other songs about impatiently waiting for uncooperative love to arrive. Williams sustains notes tantalizingly as her eagerness audibly squirms against her patience. "A Part of Love" is more an art song than a soul ballad, as Williams carefully explores the melody around Bob Babbitt's bowed piccolo bass solo. The album's highlight, though, is Williams' recent hit single: an irresistible remake of "It's Gonna Take a Miracle," first issued by the Royalettes in 1965 and revived by Laura Nyro and LaBelle in 1972. Williams gives the verses a relaxed reading that exudes contentment but builds the choruses to falsetto squeals of undeniable joy.

Randy Crawford, who first emerged as a guest singer with the Crusaders, has always had a spectacular voice with Aretha Franklin-like potential. None of her albums has yet captured that potential. Her latest attempt, "Windsong" (Warner Bros. 23687-1), fails to provide the songs or the backing to push her voice into high gear. Tommy LiPuma--George Benson's former producer--chooses schmaltzy midtempo ballads by Marvin Hamlisch, Leon Russell and Bill LaBounty, all too mushy for Crawford to sink her teeth into. The rhythm section--featuring two Toto members--plays it quite safe and Nick DeCaro's string arrangement erase the edge from everything. The overall effect is that of throwing water-logged timbers on a promising fire. The two songs with real soul--Stevie Wonder's "We Had a Love So Strong" and Don Covay's "Letter Full of Tears"--feature gutsy vocals by Crawford way out in front of the straggling band.

Robert Flack's problem is not unrealized potential, but the high standard set by her own past performances. "I'm the One" (Atlantic SD 19354), the Washington native's latest record, falls well short of that standard. The record was produced by a committee of Flack, percussionist Ralph MacDonald, William Eaton and William Salter, and the latter three wrote four of the nine songs. Neither the four producers nor the all-star band (Stuff plus Marcus Miller and Grover Washington Jr.) can inject much life into the steady diet of ballads that ooze along with no sense of urgency. Flack seems to have fallen prey to the mistaken notion that the slower and dreamier the song, the more profound it is. The worst example of this approach is Flack's current single, "Making Love," written by Carole Bayer Sager and Burt Bacharach. Flack whispers the maudlin lyrics "meaningfully" and the syrupy melody drags interminably. There is little on this album to hold one's attention, much less rouse one's spirits, as Flack once did.