Since then, Yat Po have staged three full-length productions and collaborated with local dance and stage veterans, as well as emerging Cantonese opera talents.

The collaborations continue this weekend at performances in combination with the Hong Kong Chinese Orchestra. Titled it is a light-hearted exploration of concepts in traditional Chinese medicine mixing storytelling, movement and fresh arrangements of familiar tunes.

"It'll certainly be an unusual concert experience," says an enthusiastic Yan Huichang, artistic director of the Chinese Orchestra.

Seeds for their collaboration were planted two years ago after he saw a performance of Yat Po's second full-length production. He was impressed by the holistic theatrical experience and how a cappella arrangements were combined with the sheng, a traditional reed instrument.

But as the title suggests, whether they can successfully meld the many different instruments of the orchestra within the upcoming show with Yat Po is a question mark.

"It does sound like mission impossible to match an a cappella group with a full orchestra," Yan says. "There's indeed a risk of failure."

Then again, Yat Po, literally "one-shot" in Cantonese, was always a gamble. It started to coalesce in 2007 when its creative trio turned Yuri Ng's initial idea for a ballet featuring an ensemble of singing men into a piece of choral theatre.

The concept had been on the choreographer's mind for nearly a decade, but it began to take shape only when he met Ng Cheuk-yin, the frontman of fusion band SIU2 and a founding member of one of Hong Kong's earliest a cappella groups. The composer-musician had long been interested in how the human voice could be used to produce different qualities while keeping a uniform tone.

"With the voice we can create elemental sounds one moment and then disco beats the next. Yet there's a kind of musical consistency because all the sounds are produced from one source and that is the mouth," Ng Cheuk-yin says. That differentiates it from the piano and other instruments, which have fixed range and tone although they can be played in diverse styles.

To give vocal direction to what was meant to be dance music, the two Ngs eventually brought in Chiu, who trained in choral conducting in the US. But with directors from different disciplines steering the project, evolved into a hybrid of a cappella and contemporary theatre instead of a ballet. Commissioned for the 2008 New Vision Arts Festival, the production was such a hit it went on to represent the city at the 2010 Shanghai Expo and the 2012 Macau Arts Festival.

Given the continued performances, the three directors decided to formally establish Yat Po Singers and secured a government grant that enabled them to employ four full-time performers.

"We want to raise the bar and keep developing the art form of a cappella theatre. There's no way amateur performers can do this," Ng Cheuk-yin says.

A cappella, a style of vocal performance without the backing of musical instruments, began in Europe as a form of early religious music. Today the term embraces all forms of unaccompanied singing in genres from doo-wop to rock. Still, a cappella performances are rarely coupled with theatre.

"Because of our own backgrounds, we're interested in exploring the vocal and the physical in theatre," says Chiu, who is also artistic director of the annual Hong Kong International A Cappella Festival. "When these two things come together, our team becomes unique."

But there was no full-time a cappella ensemble in Hong Kong, let alone professional a cappella theatre. How would the Yat Po directors recruit the singers and turn them into versatile performers?

"Ours is an experimental group, not just artistically but also practically speaking. How can they do everything? How can they be everything? The audience might well wonder," says Ng Cheuk-yin.

One way they have been training the artists is to have them climb up the stairs from their studio in Quarry Bay. The exercise isn't simply a matter of building stamina; the four singers must finish a song at the precise moment they reach the roof of the building, 13 floors above.

"It's about pacing and balancing themselves," says Yuri Ng.

He had received ballet training since he was a child, "but I thought, for these boys who were already in their 20s, are there other ways to make them understand their bodies and be sensitive to the others on stage?"

Despite formal training (opera in the case of Raoul Chan), the Yat Po quartet had to learn to constantly adjust to each other's voices while singing their own parts. However gifted the singer, a cappella performances require a lot of listening, Chan says.

Keith Wong, who often takes the falsetto roles even though he is a natural baritone, adds: "I must pay attention to the others' voices - their pitch and texture - as I keep tuning mine. Our voices also change from day to day. When I sound more nasal one day, the other three have to adjust to achieve better harmonies."

After three years together, the ensemble members have grown as individuals, too.

Besides coming up with new musical arrangements, they have staged small theatrical productions of their own, among them a series under "a cappella-la-la", a project involving tenants at Taikoo Place earlier this year.

"When I first joined the Yat Po Singers, I thought I was merely a performer. Having created my own pieces, I've come to understand what an artist should be and develop my views on theatre," says Sam Lau, a surveyor-turned-vocal percussionist.

Cabaret singer and actor Rick Lau Wing-fung, who recently collaborated with Yat Po on one of their productions, has come to appreciate the challenges of a capella theatre.

"In a typical musical, the lead actor may be put in the spotlight for a scene, then leave to get changed before returning to the stage again for another scene … But in a cappella theatre, because it's singing without instruments, even if you're not the lead singer of the scene you still have to focus on your parts to produce background music as an ensemble."

While he learned to rein in his resonant voice for the show, Lau says a cappella performers need to step forward: "They tend to focus on blending sounds. But stage presence is something else. They have to feel comfortable to take centre stage, to grab the audience's attention."

If a cappella theatre is to develop as a genre, performers need to present themselves as actors as well as singers, he says.

Yat Po are still feeling their way in uncharted territory and the trio of directors have had their differences over how the ensemble and a cappella theatre should develop, Chiu admits.

At the same time, they see plenty of room for improvement in their vocals. "I don't know yet whether we're going in the right direction." After a long pause, Chiu adds: "Why didn't we fall out? Because unlike a soloist playing the piano, or an artist working on his paintings, ours is an art of teamwork."

May 8-9, 8pm, Cultural Centre Concert Hall, TST, HK$100-HK$380 Urbtix

Four to explore

With the film and US groups such as Pentatonix (more than seven million subscribers on their YouTube channel) a cappella is going mainstream. Here are the Yat Po Singers' favourite groups.

a Finnish ensemble whose repertoire ranges from sacred music to the Beatles and Europop. Their name means boundless in the local language. "Their unique musical style comes with richly layered arrangements and harmonising voices."

the outfit from Germany perform a range of jazz, funk and world music as well as something they refer to as vokal bastard. "The stunning sextet makes a wide range of sounds through cutting-edge interpretation."

this Australian group took their name from a radio documentary by pianist Glenn Gould. They perform a variety of genres including classical, R&B and soul. "Four unique voices blend harmoniously in innovative arrangements and spontaneous performances."

this Swedish ensemble favours mainly jazzy arrangements, with occasional tangents towards folk and pop. "There are stories behind each of their songs, which inspires the way I create my work."