Shimmering guitars like waves on a vast ocean? Check. Throbbing basslines? You bet. Vocals like a trapped bear’s howl? Oh yeah, albeit leavened this time out with somewhat monotone crooning. (In any case, the lyrics remain mostly indecipherable.) Despite some very pretty organ work on “Holy Tears” and some occasional electronic trickery, In the Absence of Truth is recognizably an Isis album, from the slowly rising feedback that opens “Wrists of Kings” to the last jangling chord of the closing “Garden of Light.” So is it really necessary? To many ears, the band peaked with 2002’s Oceanic and its associated remixes; 2004’s Panopticon was monochromatic, beloved mostly by folks late to the party. This, the Boston quintet’s fourth full-length, is their most sonically diverse album to date. But in the process of adding new facets to their sound, Truth winds up reinforcing self-imposed limitations. Isis’s music is not about catharsis, which is a big problem for metalheads or anybody else looking for a reason to get worked up. No matter how loud the guitars get, they always sway gently, never crashing or exploding. Isis have already toured with Tool; if they keep pushing this stoic we’re-above-rocking-out shtick, they’ll be opening for Coldplay soon.