Nylons Have Been Through Wringer : Music: Frequent touring, shifts in style and the death of a member have left the a cappella group grittier than before.

To fully appreciate that the unaccompanied human voice is a perfectly self-contained vehicle for musical expression, one need only absorb the lessons of the better exponents of a cappella singing.

Current exemplars include the doo-wop derived work of the Persuasions, the complex gospel-jazz arrangements of Take 6, the sophisticated mbube choral styles of South Africa’s Ladysmith Black Mambazo, the soulful street-pop of the Mint Juleps and True Image, the eccentric theatricality of the Bobs and the Edlos.

If enthusiastic execution were the sole prerequisite to fame, the Nylons might be the best known of these elite specialists. The soul-pop quartet performs two shows tonight at Humphrey’s.

The Nylons were formed in 1979 by struggling Toronto actors Marc Connors, Paul Cooper and Claude Morrison, and the group established its four-part style with the addition of Arnold Robinson, a former member of the Platters. The singers merged elements of doo-wop with R&B; and pop and delivered the package tightly bound in articulate arrangements that favored simple tempos and straightforward tonalities.

The Nylons wrote much of their own material, and they reinvented pop chestnuts by the likes of Sam Cooke (“Chain Gang”), Gene Pitney (“Town Without Pity”) and the Turtles (“Happy Together”) with arrangements that made them seem as though they had always been intended for unadulterated vocals. In concert, the quartet generated an infectious energy that conquered the most passive of audiences.

After several albums, innumerable concerts, and key personnel changes, the refortified Nylons arrive in San Diego after having survived some tough challenges to continuity. About 18 months ago, baritone Micah Barnes replaced Cooper. Last March, tenor Connors died of AIDS and was replaced by Billy Newton-Davis. Countertenor Morrison and bass Robinson remain.

Speaking by phone from a recording studio in Los Angeles on Monday, Barnes discussed how the changes have affected the Nylons’ approach to their music.

“When Marc died, we knew we had to dramatically shift the nature of our sound,” he said. “We didn’t specifically try to replace his voice, which was a very clear, cutting, bell-like tenor. Instead, we concentrated on looking for someone whose essential qualities as a singer and a personality were that of a Nylon.”

According to Barnes, being a Nylon requires a willingness to throw one’s lot in with the group, to adapt to a group dynamic, and to share.

“We knew that Marc’s replacement had to be able to accept input from his brothers, he had to have a sense of playfulness, and a deep love of and commitment to entertaining,” Barnes said. “You see, what we do is a high-wire act, in a sense. It’s very difficult, and when people watch us perform, they can’t quite believe that we’re pulling it off. It’s important that we remain relaxed in the face of that challenge because we have to deliver night after night.”

Newton-Davis possessed the requisite qualities, and contributed something more.

“What Billy brings to the group is a more contemporary, R&B-oriented; approach. As a result, the nature of our sound has changed from that beautiful vocal sheen we used to get, to one that’s grittier, earthier and more rhythmic.”

Barnes could also claim a major role in that change. Before becoming a Nylon, he fronted and composed for an eight-piece soul band in Toronto. And while a background in pop and soul was good preparation for the Nylons’ repertoire, Barnes’ experience as a bandleader made the adjustment difficult in unforeseen ways.

“Yes, I went through ‘instrument withdrawal,’ ” he said, laughing. “At first, not having instrumental support was a real shock, like standing naked on stage. It took me about four or five months to reach the point where I could completely relax, because, you know, there’s no tonal reference. You’re on your own!”

Barnes’ singing mates recognized the difficulties of acclimation, but from the opposite perspective. Robinson and Morrison had been vocalizing without accompaniment for so long that they imagined being thrust into the reverse situation, and it enabled them to focus on Barnes’ psychological needs.

“They really dedicated a lot of energy to making me as comfortable as I possibly could be,” Barnes said. “They could appreciate the experience of being a fish out of water, and as a result they were incredibly supportive.”

With several months of hard work behind them, the new Nylons are taking their reworked show on the road. Their Humphrey’s concerts will showcase material from their albums and new tunes written by Barnes and Newton-Davis. Several of the newer arrangements in their set can be found on the Nylons’ current live album, “Four on the Floor.” Barnes said one of these is a potential show-stopper.

“Our version of (Laura Nyro’s) ‘Eli’s Coming’ is what you critics might call a tour de force--I’m not allowed to call it that,” he said, laughing. “And we do a fun, flip version of Randy Newman’s ‘Lonely at the Top.’ In a sense, we’re moving forward as we look back, by introducing audiences to material that’s slightly more contemporary, and that reflects our personnel changes, without leaving them disappointed at not hearing a favorite song.”

The Nylons will perform at 7 and 9 p.m. today at Humphrey’s, 2241 Shelter Island Drive. Tickets can be charged by calling 278-TIXS ($19 plus service fee) or purchased at the door.