Head to St George's Terrace in Perth's CBD at close-of-business, and you can expect to see power suits heading home or to one of the chic drinking holes.

But one balmy evening in early February it was the setting for something a little more surreal; a commune of strangers coalescing on the footpaths and staring skywards to hear the world's biggest surround-sound system.

Individually, in pairs, clusters or family groups, they had come to hear Siren Song, the centrepiece of Perth Festival. If they looked uncertain, it was because there was no "venue", but rather a zone spanning from Barracks Arch to William Street (four blocks). It was hard for the audience to be sure they had "arrived" — until the music started.

When the first strains of Noongar singer Karla Hart's chants of praise were heard, people started to smile. Seven minutes later, as the Siren Song faded out and the city sounds reasserted themselves, clusters of people dispersed as if in a daze.

A helicopter fitted with speakers is the roving eighth channel of this multi-channel sound installation. ( Supplied: Perth Festival/Toni Wilkinson )

How do you play buildings like they're instruments?

Wendy Martin, Perth Festival's artistic director, first heard Siren Song at Hobart's Dark Mofo festival, in June 2017, when the unearthly voices of singers Tanya Tagaq, Deborah Cheetham and Carolyn Connors sounded out over Hobart's waterfront daily at dawn and dusk.

So when Martin invited producers Supple Fox (Hannah Fox and Tom Supple) and sound artist Byron J Scullin to bring their work to Perth Festival, she was imagining sunset views over the Indian Ocean — one of the iconic Australian vistas, surely.

But it turns out that setting up an eight-channel sound installation requires more than a view. The acoustic properties of the location are essential.

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"The sound needs things to reflect off," Fox says. "And Perth's quite a windy city, so we had to consider that as well."

The audio installation comprises banks of speakers affixed to seven different skyscrapers along St George's Terrace, each its own "channel".

"Every set of speakers is playing back a different voice," Scullin explains, "and the chord is formed in the air around you."

The end result, for each listener, is the result of "the way the sound hits all the buildings and travels down the canyon of that street, and the way the sound reflects [off surfaces] and is thrown around."

The coup de theatre in Perth, as in Hobart, is a "roving" eighth channel, affixed to a helicopter — a touch of Apocalypse Now, minus the napalm.

Getting the location and balance of the channels right is quite the logistical feat — accomplished with a lot of trial and error — let alone composing for that kind of audio set-up.

As if that was not challenging enough, the composition is rewritten every day "based on what we're hearing, and to change the mood," Scullin explains. As the ten-day cycle of the piece progresses, the soundscape becomes less purely seductive, and more challenging and experimental.

A communal, secular ritual

Crowds gathered at the top of St George's Terrace to hear Siren Song, part of Perth Festival's opening night. ( Supplied: Perth Festival/Jasmine Koong )

On opening night of Perth Festival, the chopper swung into sight slowly, engine purring, speakers in full song, before hovering centre stage above Barracks Arch just in time to segue almost seamlessly into the Noongar "cleansing and clearing" ceremony (Gnarnk-Ba Waarnginy, or Speaking Fires of our Mother) below.

Voices on stage blended with the recorded vocals above in a polyphonic flurry featuring just one word: "Woola" — a Noongar word translated as "a cry of praise".

Perth Festival drew an audience of 40,000 for its 2016 opening night event Home; only a fraction of that turned out for this year's opening night.

But impact is measured in more than numbers, and an event that rejects Insta-frenzy, and requires no jostling for the best view, has its charms. We were strangers gathering at dusk to experience something together. It was a "you had to be there" moment.

And this communal ritual was repeated every dawn and dusk for the next 10 days (Perth Festival estimates that 200,000 people experienced the work) — even more magical for the lack of overt ceremony or signposting, and the tension between the concrete setting and ethereal soundscape.

"We wanted it to be a sonic intervention as opposed to a destination event," Fox says. "We wanted to find a place where people naturally are or naturally travel through."

Sounds for the city

Scullin composed new material for Perth, incorporating recordings of local singers Kristel Kickett, Karla Hart and Tara Tiba with vocal recordings from the Hobart edition of Siren Song.

Kickett sings soul, and her mother Denise is a Noongar Wadjuk cultural adviser. Hart, 2017 Perth NAIDOC Artist of the Year, is a filmmaker (NITV's Family Rules), dancer and writer, in addition to being a singer. She wrote and sung the Woola chant featured in Siren Song.

"It was important for the festival and for us to have Noongar voices," Fox says. "This is their land."

Tiba, who performed as part of Perth Festival's 2016 opening night, Home, fit the bill of "interesting-sounding voices" the artists were looking for. Having emigrated six years ago from Iran, where women are forbidden to sing in public, she says: "Being considered as a Perth singer means a lot to me. The first time I sung on stage was in Perth."

The creative process was very collaborative: "[The singers] came to us with some ideas of the kind of material they want to do, and then Byron improvised and extrapolated out and found the character and story and history in their voice, and tried to capture that," Fox says.

Fox, Supple and Scullin are based in Melbourne, and previously collaborated on the sound installation Bass Bath (which also premiered at Dark Mofo, in 2015).

Fox previously told ABC that Siren Song was exploring sound as a form of authoritarian control, and partly inspired by the use of public speaker systems in North Korea.

While the effect of the work is more uplifting and wondrous than threatening, Scullin says the group are very conscious of the potential for unease, and aim to make a gentle breach of the psychological space of passers-by.

"People's sound space is quite a personal space, especially these days when everyone can manicure their own soundscape by popping in a pair of earbuds and playing their own a la carte soundtrack," he says.

Perth Festival runs until March 4, 2018.