It's a sound that's smoother than butter and sweeter than ... well, honey.

The Grammy-award winning a cappella group Sweet Honey in the Rock — the all-women, African-American vocal ensemble that blends styles from gospel to jazz to Motown, from Miles Davis to Jimi Hendrix — returns to Madison and plays the Wisconsin Union Theater on Friday, Oct. 7.

"We all have experience with the music of the '60s," Sweet Honey in the Rock member Carol Maillard said. "That range of music is embedded in us."

For Friday's show, "there will be some songs that people have never heard, and some songs they'll be familiar with," Maillard said. "Hopefully they'll sing along, and enjoy the music as much as they did the last time we were there."

Vocalists Aisha Kahlil, Louise Robinson, Nitanju Casel and Ysaye Barnwell perform using a wide variety of pitch, range and timbre. The group, which also includes American Sign Language interpreter Shirley Saxton, often uses vocal percussion and the imitation of other musical instruments.

Some a cappella groups focus on technical aspects like precise diction and a completely unified sound, and can end up sounding mechanical and superficial (think "jazz hands"). But Sweet Honey in the Rock creates a smooth blend of rich textures while maintaining personality within each voicing; it's impossible to put the group in the same league as most a cappella performers.

Part of this unique sound can be credited to the group's sense of musical construction. Instead of adhering to the typical soprano/alto/tenor/bass distribution, the group makes decisions about arrangements together.

"When a person writes a song or puts an arrangement together, someone might say, ‘I hear Nitanju on the top on this one,' but Nitanju may say, ‘Oh, I can learn the bass part for this song,' and someone else will take the part," Maillard said.

While they like to experiment with their voices, their collectively unique sound comes from individual members' talents. Casel's strength is singing high in different musical qualities (timbres) and jazz improvisation in her solos. "She can surprise you onstage," Maillard said.

Robinson and Kahlil are strong imitators of musical instruments.

"Some of (Kahlil's) songs sound like tone poems — there aren't words, just different sounds she's able to layer, and imitate things like birds," Maillard said.

Maillard, who is also an actress, has experience in shifting genres among gospel, jazz, R&B, hip-hop, pop and Broadway.

"Some people have strengths in different parts in their voices, but we all have to be able to at least try and work different parts of our voices, depending on how the composer hears the arrangement," she said.

Civil rights activist Bernice Johnson Reagon formed the group in 1973 from the D.C. Black Repertory Theater Company. Four women, including Reagon, Maillard and Robinson, comprised the original group.

"Everything worked," Maillard said. "I think that's the best way to put it — it all clicked."

While their signature sound is pure a cappella, the group doesn't confine themselves to this style. They'll debut a contemporary classical piece, "Affirmations for a New World," with the National Symphony in 2012.

"We're basically an a cappella group, but we're not opposed to working with instruments. When we show people that we have the sensibility and ability to do different kinds of things, we open up our opportunities — you can't as an artist stay closed off just to you and what it is you do," she said. "We have to be open or we die."