Sometimes, we forget the clue that's in the name. Our two most famous Requiems – by Mozart and Verdi – are dramatic works filled with dread: the terrifying judgements of the day of wrath, the potent sound of the last trumpet sounding alone in the depths. Tigran Mansurian's 2011 Requiem sees it differently, taking us back to the fact that the Latin word means “rest”. Despite this work having been written as a memorial to the Armenian genocide, Mansurian leaves the horrors behind and creates a contemplative, introspective work, music to soothe a troubled soul rather than to strike awe into a fearful one.

Before Mansurian's work, in last night's concert at their home in Katowice, Alexander Liebereich and the Polish National Radio Symphony Orchestra (NOSPR) played a work of a very different kind: Debussy's early cantata L'Enfant prodigue, composed in just thre weeks, which won its composer the Prix de Rome at the tender age of 22. The cantata tells the biblical tale of the Prodigal Son through just three voices: the son (tenor), his mother (soprano) and father (baritone), here given the names Azaël, Lia and Siméon. The music is lush, orientalist stuff in the mould of Délibes or Massenet: elegant woodwind arabesques swirl in overlapping lines, a tambourine in the Air de danse evokes the jangling of a belly dancer's costume, string phrases sweep.

NOSPR is a wonderful hall – one of a small number in the world with an acoustic that brings out every detail of the colours of individual instruments within an orchestral mix (Budapest's Müpa is another, as is Disney Hall in Los Angeles). This is an orchestra who know their home well, and those instrumental colours came through with rare vividness and beauty. Every woodwind instrument shone; horns were suffused with richness; harp notes were resonant; strings were silky smooth.

However, Debussy's inexperience at writing for voice and orchestra is evident. Virtuosic as the orchestral writing is, it doesn't leave space for the singers and our three soloists – Johanna Winkel, Christian Elsner and Stephan Genz – were continually struggling to stay above the orchestra. The principal casualty was diction: I made out very few words of French in the whole work. There was plenty of nice timbre from Winkel as Lia and well turned, urgent phrasing from Elsner as Azaël, but this is a narrative piece, and shorn of its text, the story wasn't being told.

For the second half of the concert, a pared down NOSPR string section was joined by the 34-strong RIAS Kammerchor in the Mansurian Requiem. This was music making of a different order, music to fill one heart with the composer's emotion.

There is magic in the human voice, more magic when several voices share a line in perfect unison, more still when several of those lines interweave and pass themes back and forth between them, and unbelievable, astounding magic when all this happens in a hall which amplifies every detail of the timbre and the diction. This is a set of singers who are so good technically and have such perfect understanding of each other that the artistic expression can simply flow.

Mansurian employs many different string textures, from fluid lines to pizzicato to whisper quiet tapping of bows on strings. Under the energetic podium presence of Alexander Liebereich, NOSPR played them all persuasively. The opening Requiem aeternam set the mood of meditative calm. The Kyrie blazed into more urgent pleading, then quietened with a more pizzicato accompaniment before the series of waves of sound that is the Christe eleison. The Dies irae is more frantic before giving the lead to two soloists: baritone Andrew Redmond sang with authority, to be capped by soprano Anja Petersen who produced the creamiest of timbre for her Mors stupebit. The devout Lachrymosa was notable for its quiet glissandi.

Influences from Middle Eastern music abound, and it's a tribute to all the singers that they seemed to have no difficulty whatsoever in singing perfectly accurate counterpoint with a lot of intervals and melodic shapes that you just wouldn't normally find in the Western choral canon.

But much as I could go on about the technical qualities of this performance: it's the resulting spirituality of the music that mattered, the ability of this group of musicians to create an overwhelming sense of rest and calm. Requiem aeternam indeed.