These intrepid early music singers compare their sound to a string quartet’s – a match made in heaven for the ACO.

The Hilliard Ensemble sing Byzantine chant and thirteenth-century polyphony with otherworldly purity and precision that astounds audiences around the world. But they know their limits. “Presentation has never been a strong point of the Hilliard. We wouldn’t win any beauty competitions standing up there,” says countertenor David James, somewhat sheepishly.

Since 1974, the all-male English group’s core repertoire has been sacred a cappella fare from the Middle Ages and Renaissance and new music specially composed for the foursome, but over the past two decades they have brought collaborators into the mix to liven up their own performing tradition. It’s a gamble that’s paid off: Officium, their 1994 album of ancient hymns and chants accompanied by saxophonist Jan Garbarek was a surprise sensation, selling more than 1.5 million copies.

“We’ve since branched out in more collaborations with instrumentalists,” says James. “It’s amazing how you can generate a special electricity between two groups with similar ideas but who come from a different angle.”

It is in this spirit of shared creative ideals that the compelled the Hilliard Ensemble to join forces with the Australian Chamber Orchestra for a national tour in March. Although an unusual and seemingly unlikely match for a band of string players accustomed to performing with guest soloists, James explains that the two groups could in fact be kindred spirits. “We’re a very small ensemble of four voicces and we don’t have a conductor, so we feel very much like a string quartet.”

The countertenor, two tenors and baritone “adapt to the colour in one another’s voices to create a unified sound – it’s a very internal and democratic approach to performing.” Similarly, he says, “the ACO perform as one; when you hear them it’s incredible.”

With both groups known for adventurous programming, the joint concerts offered tantalising opportunities for their worlds to collide. In one masterstroke of connectivity, The Hilliard Ensemble performs two centuries-old versions of the haunting chant Veni Creator Spiritus, and then invite the ACO to respond with Australian composer Ross Edwards’s exuberant string orchestra setting of the same melody. They come together for the Australian premiere of Obikhod, a substantial Russian work for vocal quartet and strings composed for the Hilliards by Alexander Raskatov.

The quartet sans orchestra also sings one of their signature works, Most Holy Mother of God by Arvo Pärt. James is full of praise for the reclusive Estonian composer with a cult following, who has written extensively for the ensemble since the 1980s. “Arvo is our god… But perhaps that sounds too grand! His music is so special to us. The nuances and inflections – it’s as if he’s singing with us.”

The rarefied worlds of early and contemporary music go hand in hand for The Hilliard Ensemble, whose unusual blend of voices means that “there is an enormous amount of medieval and Renaissance music for us, but then we’re rather stuck. From the moment we started, we realised the only other thing we could do was create our own repertoire of contemporary music”.

Links between the very old and the very new are most compelling in sacred music like Arvo Pärt’s (whose distinctive style is referred to as “Holy Minimalism”), says James, since “so many of the composers living now are influenced by and go back to the earliest time to explore the simplicity and purity of that style.

“We like to mix early and contemporary music within our concerts, like we’re doing with the ACO. It’s surprising but with some of this very early music, people from the audience come up to us after the concert and they’re confused about which was the new piece and which was the old piece. Whether it was written in 1480 or 2010, in many ways it’s all new music because it’s the first time many in the audience have heard it.”

James is the only founding member who has remained in the group for almost four decades, but three of the members have been singing together for 23 years. In that time it’s become a family of sorts – with a schedule that crams in more than 100 concerts a year, he points out that they’ve “spent more time together than the average person would spend with their partners and family.” They even go on holidays together, rather than to get away from each other. And in that time the four men have come to know one another’s voices as if each was an extension of their own.

Despite the excitement of collaboration, there’s not much The Hilliard Ensemble can do about looking like undertakers onstage: it comes with the territory of solemn sacred music. Unlike their fellow a cappella Englishmen The King’s Singers, who recently toured Australia, you’ll never catch the Hilliards doing Beatles arrangements. “We love singing the heavier stuff. It gives us the most pleasure and challenge to keep the atmosphere and intensity going. It’s not an easy ride for the audience; they have to be drawn in and come with us on the journey.”

The Hilliard Ensemble tours nationally with the Australian Chamber Orchestra March 10 – 22. The vocal quartet performs its Hilliard Songbook at the Adelaide Festival on March 14.