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A capella awe: How are their voices doing that?!

Lewis Jacobs / Lewis Jacobs/NBC

How do a capella singers, like those on the NBC show "The Sing Off," manage those vocal gymnastics, anyway?

When watching the NBC reality show "The Sing-Off," a singing competition featuring a cappella groups, it's incredible to hear how full a sound the singers can produce with just their pure voices and no musical accompaniment. The human voice is the only "instrument" used. 

With a cappella singing "you're putting everything out in the open with nothing else but the voice box, lips, teeth, and tongue to shape the music being made," says Dr. Thomas Carroll, a voice specialist and director of the center for voice and swallowing at Tufts Medical Center in Boston. Carroll should know. Before attending medical school, he majored in music and sang high tenor in the Oberlin Obertones, an all-male collegiate a cappella group.

According to Carroll, a cappella singing is not necessarily more demanding on the voice than singing with musical accompaniment. But, he says, it may take more athleticism from a vocal standpoint, especially to beat box, also known as vocal percussion, or mimicking the sounds of drum beats, rhythms, and other percussion instruments.

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Making those explosive drum set sounds is a unique skill, explains Carroll. You have to breathe more often than other kinds of singing and support the breath for a two- to three-minute song. "It's almost like running a marathon," he points out.

Endurance is one of the unique challenges of a cappella singing and another is top-notch technique, says Jodi Jenkins, an associate professor in the voice department of the Berklee School of Music in Boston, who has sung soprano in the a cappella quintet Vox One for two decades. Since singers are not supported by music, she explains, you need really good ears to keep everybody on key and together rhythmically.

And you need a good understanding of musical instruments to figure out how to mimic them without sounding cheesy, Jenkins suggests. She considers the electric guitar one of the harder instruments to imitate because making the sounds is rough on the voice.

A cappella, meaning "in the church style" in Italian, originated when musical instruments were not allowed at religious services. It has since evolved to include barbershop quartets and doo-wop groups, and the genre, which now includes everything from rock to jazz to gospel, has been enthusiastically embraced at the collegiate level and in some high schools.

"The Sing-Off" introduces the musical form to an even larger audience. But of the 16 groups competing this season, would coed groups have any physiological edge over single-sex groups?

Both Carroll and Jenkins believe that an all-female group might be at a slight disadvantage because they would not have a deep bass voice so they could be limited in lower vocal ranges. An all-male group without a falsetto voice might run into similar difficulties in the higher octaves.

Although a coed group gives you all the voice options, ultimately it comes down to talent, sound, and performance. During the show's first two seasons both winners have been all-male groups with six people.

While larger groups have more energy from more participants, Jenkins says they may also have too much going on and lose the music. She favors smaller groups with at least five people for all the vocal parts, which she considers more intimate so audiences can get to know the performers better.

Readers, have you tuned in and become a new fan of a cappella music? And a cappella singers, tell us how you make those instrumental sounds seem so authentic.