The voices of Trio Mediaeval take you by surprise. Essentially, they take French, English and Norwegian polyphony and give it an electronically amplified, spaced-out twist. Much of Norway’s early musical history is unknown, the largely Christian folk tradition passed down orally rather than in writing, although monasteries from the 12th century onwards played a large role. On tour around the UK in May, they have still to perform at the Sage Gateshead and Manchester’s Victoria Baths. LSO St Luke’s provided an appropriate setting for their music: a mixture of old and new, sacred and secular, relaxed yet formal.

Trio Mediaeval, now in their fifth year together, blend their Norwegian heritage with pan-European influences. They possess freedom in several ways, and it’s an element of their music-making that they prize. Firstly, they are free from genre. They do not sound particularly mediaeval, and do not intend to. But they do retain traces of early Western sacred music, for example in their use of parallel fourths and sacred-sounding chants. Their style is also distinctly folky; deceptively simple, earthy and full-bodied, like ballad-singing. Secondly, they are free in performance. They stood at three microphones on the LSO St Luke’s stage, and in the second half they even dispersed around the hall at one point. Thirdly, they are free in diction, moving effortlessly from Latin to Norwegian to Italian to ‘tralling’ (made-up sounds or words) and treating them all equally as tools for the all-important notes. Only the Latin sounded oddly stilted against the calm evolution of the music.

The three female vocalists blend well – one breathy, one dark, one golden in tone. Tonight they used their voices like instruments, largely without vibrato and perfectly in sync. The first six numbers blended seamlessly between traditional songs and 13th-century English sacred chant, with Arve Henriksen’s quivering pocket trumpet providing the stimulus for many of the melodies. Constant electronic amplification shrouded the sound in an echoey glow, very different to the echo produced in a stone church surrounding. It was the element that really brought the performance into the 21st century and accentuated the group’s imagination. There was a sense of humour here too, in the spontaneous ornamentations and perfectly judged, almost ritualistic timing.

The second half contained more of the Norwegian traditionals in which the group seemed most at home, including the gorgeous Till, Till Tove. The brief addition of the Harbinger fiddle during Ave donna Santissima added a breath of fresh air, as did Henriksen’s trumpet solos. Henriksen is caught somewhere between modern jazz improvisation and experimental contemporary classical. His abstract interludes employed his own strained vocals, a synth machine and his trumpet with its unique tone, as he created sounds with his lips more reminiscent of beatboxing than anything else. Played breathily into a microphone, the trumpet sounded more like pan pipes or flutes, as if evoking a dramatic Nordic mountain landscape. This should have been incongruous with Trio Mediaeval’s stronger, more emotional core, but it lent tension and an unexpected edginess to the concert that was much needed.

Taken as a whole, the evening was an intense series of reverent and joyful numbers, full of life and disregarding the need to fall into any particular genre. Perhaps there are limits to how long this same slow-moving style can sustain a concert audience. But as an overall experience, Trio Mediaeval is quite, quite beautiful.