You can only pound your head against a wall for so long before you get a headache. On their self-titled new album, Metallica, rock’s pre-eminent speed-metal cyclone seems to realize this. Up to this point, the Bay Area band had accomplished the unthinkable: They wrote scathing diatribes about such topics as our desensitized society and the horrors of drug addiction, signed with a major record label, and then watched millions of kids buy these spewings, all without the benefit of one hummable melody. Horatio Alger would be proud, if not deaf.

Still, setting yourself up as the soul-searching conscience of male teens has its price. With each of their six previous releases, Metallica have grown sludgier and more self-important. The culmination of this slide was their overcooked 1988 album …And Justice for All, a breakthrough that went double platinum, inspired the release of their first video, and earned them a 1989 Grammy nomination. On that record, they made their credo graphically clear: The earth is on its way to becoming a postnuclear cinder, those who survive will wind up embittered quadriplegics — and this might be a good thing, since all shards of decency and individuality are dead anyway, and humanity itself has been reduced to a mass of cowering hypocrites.

Life can be like that, but this doesn’t make the message or the relentless music any less of a dead end. To the rescue comes Metallica. The band still roars and lumbers like Godzilla partying in Tokyo, but the lyrics are, well, introspective. It has dawned on the band that military-industrial complexes and twisted governments aren’t the only seeds of evil — that negativity all too often starts at home. So in ”Sad but True,” grunting singer James Hetfield confesses his devotion to his partner while chastising her for not taking responsibility for her day-to-day life. Granted, it’s not ”Feelings,” but huge leaps in emotional expression may be too much to expect from a band that titled its first album Kill ‘Em All.

The music, too, has taken a leap beyond speed metal. The songs are tighter (the nine-minute behemoths of Justice are gone), the arrangements more concise, and the band plays actual hooks. The album’s centerpiece, a sulking rumination on one man’s ruined life called ”The Unforgiven,” starts slowly, with crystalline overlays of acoustic and electric guitars, and builds to a cruncher with Kirk Hammett’s emotional, vibrato-drenched guitar solo. By welding the jackhammer attack of thrash to the complexities of old-fangled art rock, Metallica may have invented a new genre: progressive thrash.

Metallica still don’t seem to have a sense of humor, and by taking full advantage of the extended length of CDs, Metallica is far too long (12 tracks, over 60 minutes). But by the last song, the band has made up for any excess. ”So many things you don’t want to do…What have you got to lose?” Hetfield sings in ”The Struggle Within,” which commands an emotional basket case to reach out and touch someone. By next year, Metallica may have decided that such positivity is for naught. Judging from the near-idealism of Metallica, though, chances are they’ll at least bring along a pillow before they start banging their heads against the wall. B+