ERIC B. & RAKIM ‘Let the Rhythm Hit ‘Em’ MCA: Albums are rated on a scale of one star (poor) to five (a classic).

This duo’s genius lies less in innovation than in mastery of form, the full realization of what is called the “old school” style of rap. Rakim’s voice, some marvelous horn suddenly given the power of speech, is like Miles’ fluegelhorn, Cecil Taylor’s rhythmic midrange, Coltrane’s whispering honk: deadly because quiet, oddly compelling because low in the mix.

What he says--basically, variations on the theme of his own excellence, hip-hop’s subject A--is less important than how he says it. His smoldering rage is subsidiary to its barely controlled expression.

Eric B. takes simple snatches of beat and melody, shifts them 10 degrees, and fills the spaces with grunts, saxophone wails and washes of sound. He keeps it on the brink of anarchy, just unpredictable enough to rivet your attention, regular enough to dance to.

They scratch in beats, they repeat the old rap formula, they incorporate jazz and soul songs and surface noise . . . they do what everybody else does, but better. Call them hip-hop classicists. This, their third LP, might be the first mature rap masterwork.