There's no shortage of passion in this musical celebration of the Sufi poet Rumi, but passion doesn't always make for easy listening.

Don't get me wrong: the nine tracks on The Passion of Rumi by Shahram and Hafez Nazeri are eloquent blends of classical and modern Persian music boasting crisp musicianship and appropriately intense vocals for the transcendent subject matter.

For those unfamiliar with Persian vocal stylings, however, Shahram Nazeri's throaty undulations may be as much of an acquired taste as Wagnerian opera. A renowned vocalist, Shahram possesses a rich, resonant voice that conveys a wide spectrum of emotions, but every now and then, as on the sparsely accompanied "Fanayam Man II," it's hard to describe his sound as other than a prolonged yodel.

Alas, the liner notes lack comprehensive lyrics and translations, making Shahram's vocalisations not only alien to western ears, but also incomprehensible. Ironically, Rumi may be more accessible in translation than through the Nazeris' music.

But if you get past that point, you'll find some truly beautiful music on The Passion of Rumi. Hafez Nazeri's compositions, played solely upon traditional instruments (the lute-like setar and barbat, the violin-like kamancheh, the percussive daf and tombak) have a unique, almost timeless sound.

The mood is predominantly solemn and meditative, rising at times into spiritual ecstasy. The opener "Mystic" is a slow, enigmatic piece that starts with the plaintive sound of the kamancheh, which is more akin to the erhu than the classical violin, and is gradually joined by lute and voice. Shahram's sonorous vocals work splendidly when balanced by the ensemble, as on the uptempo "In Solitude" and the final nine-minute epic from which the CD takes its name.

More typically western in its harmonies and structure is Hafez's long setar solo on the instrumental "Journey to Eternity," which adroitly showcases his skill as a composer and player. The setar has an unusual, slightly metallic sound, and Hafez plays masterfully.

As might be expected, The Passion of Rumi is not for the faint of heart. The modern Persian ensemble Axiom of Choice makes a gentler introduction to the genre, but those wishing to explore Persian music further may well find that passion with which Shahram and Hafez Naferi infuse their music is worth the universality it loses.