Culturally, Back's primary claim to fame is as the chief remaining bastion of Gaelic psalm singing, a strictly Presbyterian mode of musical worship unique to the Western Isles, while Chennai, whose inhabitants are predominantly Hindu, is a major centre of Carnatic music, reflecting the Persian-influenced heritage of India's southern states.
Chennai is also, however, home to the KM College of Music and Technology (KMCMT), a pioneering institution founded by the Oscar-winning Bollywood/Hollywood film composer (and multi-instrumentalist, producer, singer-songwriter and philanthropist) AR Rahman, best known for soundtracking Danny Boyle's 2008 smash Slumdog Millionaire, and named by Time magazine among the world's 100 most influential people.
Rahman - who curates a concert of his compositions at Celtic Connections this week performed by the BBC Scottish Symphony Orchestra - is by some computations the world's best-selling recording artist, with his movie scores and other projects generating total album sales of around 300 million - more than Elvis, The Beatles and all five Jacksons combined. With its combined programme of Indian and western classical music, together with a strong emphasis on contemporary technology, his Chennai college stands distinct from all other Indian conservatoires and, in terms of indigenous forms, also gives equal weight to the north and central subcontinent's Hindustani traditions.
All of which goes to explain how a troupe of musicians and singers, the KM Hindustani Ensemble, will fly in from Chennai to team up for a separate concert with the Lewis Psalm Singers at Celtic Connections this week, in perhaps the festival's boldest and furthest-reaching international collaboration to date.
With Rahman's presence headlining a substantial Indian contingent among this year's Celtic Connections line-up, as part of the festival's Commonwealth focus ahead of this summer's Games, he was very keen to involve some KMCMT students in his own concert - to the point of offering to pay for their flights - a desire that happened to chime with an item on Celtic Connections artistic director Donald Shaw's long-time wish list.
"Ever since I took on the job, I've been wanting to do something with the Lewis Psalm Singers, as they hadn't been at the festival since 2005," Shaw says, referring to the group's celebrated Salm & Soul collaboration that year with an African-American choir from Alabama.
"But it had to be absolutely the right thing, especially in terms of putting them together with other musicians, and as we discussed bringing over the Hindustani Ensemble, the two things just came together in my head. Harmonically, melodically and rhythmically, there are a lot of similarities between Indian and Gaelic music, and strong parallels too between both traditions' spiritual dimension."
In the case of Gaelic psalm singing, the spiritual isn't so much a dimension as the music's entire essence and reason for being. "It's not a choral tradition as such," says Calum Martin, an elder of Back Free Church who is also precentor of the Lewis group, calling out the psalms line by line for the rest to sing back. "Originally, of course, it's a congregational form of singing, but within that congregation each individual is making their own act of worship by singing the words and melody in their own way, adding their own grace notes, and with all those individuals together, there's really no other sound like it in the world."
"Free heterophony" is the technical term for the protean mesh of solo responses, with their myriad micro-harmonies, that generate Gaelic psalms' extraordinarily primal yet ecstatic, oceanic surge and swell: if you've never experienced it, do yourself a favour and get down to Saturday's show at Kelvingrove, where the main hall's soaring arched heights will likely redouble it to positively mind-blowing proportions - perhaps reminiscent of the 1000-strong congregations Calum remembers from his youth, whose sound now survives only on a few archive recordings.
While Calum explains that the Gaelic words are simply translations from the standard Scottish psalter, and the main melodies all of English and Lowland stock, he is aware of the theories that the Hebrideans' distinctive mode of delivery dates from much further back, an argument supported by striking similarities with Ethiopian Coptic singing, which is recognised as one of the oldest extant forms of Christian music.
Rather than any direct interaction, though, he suggests that any such ancient connection exists more at the local, human level. "I think what it boils down to is that this is above all a natural, instinctual way for people to sing together, to give thanks or to worship or to gather communal strength, and these are just two of the places where it's been preserved."
One acknowledged influence on psalm singing - as with Gaelic music as a whole - is the music of the bagpipes, and in particular the classical tradition of ceol mor or piobaireachd, in which Shaw also identifies some of those parallels with Indian music. "Both piobaireachd and ragas share the same essential theme-and-variations structure," he explains, "with both of them always coming back to a single, memorable, modal melody. And despite all the evolution and adaptation both traditions have undergone over hundreds or thousands of years, those basic structures still form the bedrock beneath a lot of today's music."
Certainly Calum, in exchanging material via email with his Indian counterpart, for rehearsal ahead of the Lewis and Chennai groups' first actual meeting later this week, has been struck by how readily their two sounds work together, despite their apparently vast disparity. "We're going to do a couple of songs together, one from each of our repertoires, as well as our own sections of the concert, and while it's been a challenge, not least in terms of language, on a musical level there are actually lots of things that come together really easily," he says. "I hear huge similarities between ours styles of singing, the way our singers use their voices, how they ornament a melody: there really is a lot of commonality there."
Gaelic psalm-singing's spiritual origins and essence, however, do raise some delicate but thorny issues when it comes to taking them out of the kirk and into the concert hall - let alone bringing them into communion with other religious cultures. While the Lewis Psalm Singers group deals with the first by donating all concert fees to the Bethesda Hospice on Lewis (as well as choosing their bookings very carefully), Calum reports matter-of-factly that some of the island's faithful would actually prefer that the tradition die out as congregations dwindle than see it transplanted to a secular context.
India's musical traditions too, however, are fundamentally devotional in origin, with much of their core material traceable back to the ancient Vedic scriptures, while AR Rahman, himself a devout Muslim, conflates the spiritual and the creative in a way with which the psalm singers will surely empathise - regardless of Rahman's Hollywood stardom and globetrotting schedule.
"Every time I sit for a song," he has said, "I feel I am finished. It's like a beggar sitting waiting for God to fill your bowl with the right thought. In every song, I ask help from Him. To create music that will connect with so many people is not humanly possible without inspiration."
KM Hindustani Ensemble with Lewis Psalm Singers are at Kelvingrove Art Gallery and Museum on February 1 at 8pm. The Music Of AR Rahman, with the BBC SSO, is at the Glasgow Royal Concert Hall on January 30 at 7.30pm