Cattle Decapitation headlines the California Blood Tour at Volume 11 Tavern Wednesday, Oct. 13. Devourment, Knights of the Abyss and Burning the Masses open the 7 p.m. show. Tickets are $15.

“It’s just like, ‘blaaaggghhh gaagggghh rrrrrrggggghh,’ ” a co-worker of mine recently intoned, mimicking a generalized heavy metal vocalist spewing into a microphone. He stifled a smirk and continued. “The music’s OK, but, man, the vocals are just, ugh.”

His son had recently discovered an appreciation for some sort of metal. I couldn’t tell by the descriptionloud, heavy, screamywhich of metal’s many subgenres the kid had taken to, but it hardly matters: Convincing a naysayer that metal vocals are more than just rote, mindless screaming is like trying to convince a tea partier that the president isn’t a socialist. Good luck.

As one fond of the louder end of popular music’s spectrum, I often find myself in these sorts of exchanges, trying to defend my affinity for singers who don’t exactly sing. I’m not entirely sure why I bother, but I’m also not sure why it’s such an issue. Nobody seems to mind when James Brown or Janis Joplin unfurls a throaty howl that launches the VU meter into the red. But these screams are the exception, not the rule, like Nels Cline’s feedback in milder-by-the-album Wilco songs. Still, the timbres are there, and they’re powerful. In metal, I suppose it’s just too much.

But that screaming is often a key element to the music’s appeal, not just some annoying, unnecessary surface. Vocalists like Slayer’s Tom Arayaa bona fide screamerdeliver a sense of urgency unknown in other genres, one that only heightens his band’s aural violence. His screams are those of the damned souls and war victims in Slayer songs.

In other cases, the vocals are buried in the mix and beyond incomprehensible, but their effect on the music’s texture is profound. The harsh shrieks of Washington state black metal hippies Wolves in the Throne Room chill like freezing rain, spotting the band’s expansive atmospheres with dark, frigid squalls. The harsh howl Chapel Hill’s Jenks Miller used to drive Horseback’s 2009 album, The Invisible Mountain, is a similarly crucial textural element, from which the album derives much of its dramatic conflict.

But there is perhaps no vocal metalist more inimitable and essential to his band’s mission than Travis Ryan, founder and frontman of San Diego grindcore masters Cattle Decapitation. One of metal’s most dynamic and versatile voices, Ryan is no bipolar sing-scream diva, nor a one-note Cookie Monster growler. Actually, he rarely, if ever, screamsat least not in the facile human way. Instead, he wields an arsenal of grunts and growls, squeals and spews.

See, Cattle Decapitation is, aside from its name, best known for its advocacy of animal rights and condemnation of the damage and waste humans create. Their latest album, 2009’s The Harvest Floor, is a 10-track tour of a hellish slaughterhouse. In delivering the gruesomely descriptive lyrics, Ryan mimics the wretched squeals of murdered livestock, the gurgle of viscera unspooling from carcasses. As with all great vocalists, Ryan has a voice that is the perfect match to Cattle Decapitation’s songs. He pairs a low, guttural growl with a ghastly shriek and more than lives up to the (in)famous Cannibal Corpse review, “If vomit were a movie, this would be the soundtrack.”

But in songs that aim to instill a sense of horror and disgust, these sounds are made to measure. Even without visiting the liner notes to check the lyric, one instinctively feels the plight of the songs’ victims and the outrage of the narrator. What’s more, we’re treated to a singular vocal talent whose knack for exploiting timbre and rhythm to affect the outcome of a song is easily the band’s greatest attribute.

This is cost of admission for metal’s extremities. The first complaint is never the squealing guitars or abrupt rhythms; the vocals are always the obstacle, and one often worth overcoming.