The bass line, deep and thrumming, comes in, followed by thumping drums. The vocals and other background sounds blend in with the rest and, suddenly, a full band fills your ears. Except that's not what it is – not technically. It's actually a quintet of male voices, their vocals representing those big instruments you thought you heard at first. It's a cappella, and it's a growing trend in the music industry these days thanks to movies like “Pitch Perfect” and shows like “Glee.” However, Rockapella has been in the business of strictly vocals for years now, practically perfecting the art. You may know them as the guys from TV game show “Where in the World is Carmen Sandiego?” Their most recent album, “Bang,” is their 20th and showcases 13 original songs, as well as a bonus cover of Vampire Weekend's “A-Punk.” The group, which currently consists of Scott Leonard (since 1991, high tenor), Jeff Thacher (1993, vocal percussionist), George Baldi III (2002, bass), John K. Brown (2004, tenor), and Steven Dorian (2010, tenor), will be at Penn's Peak on June 7. We had a chat with Thacher, whose mouth drumming has changed the game for the guys that have been making mouth music for years. THE WEEKENDER: When you joined Rockapella in 1993, you brought your human beatbox skills with you. Was that a new thing? JEFF THACHER: It was. It was a very new experience for everybody. Nowadays, a cappella groups, it's kind of a given that they'll go seek out mouth drums. It's a no-brainer, but nobody had really done it that whole time, until the early '90s. I was one of those people dabbling with it in another group, and Rockapella said, “Hey, let's make this for real; let's make this full time.” They had recordings that had individual, sequenced vocal percussion sounds in their albums, and they weren't able to perform those live. They finally said, “Hey, let's figure out a way to do this live.” W: You can obviously take lessons for instruments like guitar and drums, but how do you learn how to do something like mouth music? JT: I always imitated sounds; when I was a kid, I made sound effects with my LEGO spaceships. I was part of a very musical family, so it was all around me. When I heard drum sounds on songs on the radio, imitating them seemed like a natural next step; I just kind of did it. I think the first song I ever did was “Billie Jean” by Michael Jackson. I would do the drum line, which is an iconic one, and my brother would do the bass. That is what showed me it could be done. I just never thought I'd be paid for it eventually. W: Does something like that hurt to do? JT: I like to describe it as playing a brass instrument. It's very similar; some of the techniques are the same. You're not hurting your voice, but it does take muscles. It's an athletic activity like any other that just gets better with time. W: Rockapella touts a “full band sound.” How do you ensure you have such a sound when there are only five of you? JT: The true art of professional a cappella, in other words with a fewer number of members, is in the arrangements. When you have quintet or something like that, what you do with your backup vocals is really the key to your success in many ways. You can have an amazing lead, but you need something going on in the background that glues it all together in an intelligent way. W: Members have rotated over the years of Rockapella's run – do you think that helps or hurts? JT: The trick is, when you have to replace someone, you don't take a backward step. You either evolve in a positive way or keep the quality just as good. We've been very fortunate that we've been able to, whenever someone leaves, say, “How can we evolve now?” We've been able to dip into a substantial talent pool. People are becoming better and better singers at a younger age. W: What do you believe is the appeal of a cappella music? What has kept you guys and the genre going for so long? JT: The reason is just as true as it was years and years ago: there's nothing between you and the music. People listening get it immediately. They don't have to look at a guitar and go, “Oh, I can't play that. I wish I knew how,” or “What is that keyboard doing?” You know immediately what's going on, and you can kind of relax because of that.