An immense sense of isolation underscores Ian Tuason’s feature debut, the audio-driven horror smorgasbord “Undertone.” Following an exhausted horror podcast host watching over her mother’s deathbed, the film is initially terrifying thanks to its undercurrent of pulsing grief, rendered with subtlety and sleight of hand. Although it eventually leans into traditional genre hallmarks, its introductory musings are novel, taking the form of a one-woman performance showcase that makes ingenious use of visual and auditory negative space.

Sound is a vital tool in Tuason’s film, which begins with a children’s lullaby interspersed with the belabored breathing of a woman on the verge of death. The cycle of life is captured even before we meet our protagonist, a young and troubled woman named Evy Babic (Nina Kiri), who waits patiently for her comatose mother (Michèle Duquet) to pass in her sleep, a story drawn from the director’s own experience caring for his aging parents. The family’s rickety, suburban duplex — Tuason’s own childhood home in Toronto — also plays host to 3:00 a.m. podcast recordings, which Evy coordinates with her caring co-host in London, Justin (Adam DiMarco), one of several characters whose voice is heard, but who we never physically see.

In “Undertone,” the very idea of presence goes beyond the physical, drawing not only skillful sound design, but on gestures towards the spiritual — and eventually, the demonic. Evy and Justin’s show, “The Undertone,” focuses on urban legends, and their latest episode concerns a series of bizarre audio files of a husband and his sleep-talking wife, sent to them by an anonymous source, whose contents slowly begin to reflect Evy’s growing anxieties. It’s the acoustic version of found footage.

The crux of Evy and Justin’s co-host dynamic involves Justin playing a staunch believer while Evy plays a skeptic, though her domestic predicament has begun to challenge that status quo. Sitting by her mother’s bedside, she often searches for answers, and seems to regret turning her back on religion (their home is filled with Christian iconography), which, in Evy’s insomniac state, causes her to search for clues within audio recordings and children’s songs, which she starts to play backwards while desperately jotting down any hidden messages she can find.

Whenever Evy sits down to record in her darkened living room, she places noise-cancelling headphones over her ears, causing the soundscape to shift noticeably in texture, isolating her image in the process. As the audio calls with Justin and the eerie recordings dominate the soundtrack, Tuason’s camera remains tethered to either closeups or wide shots of Evy for lengthy periods, allowing our eyes to drift around the screen. During these moments, cinematographer Graham Beasley makes tremendous use of light, shadow, and stillness, drawing our gaze back and forth between the character and her darkened surroundings without doing very much at all.

At times, as the recordings grow more disturbing, gradual zooms and pans across space invite anticipation (as do mysterious bumping sounds from the top of the staircase, and bedroom lights mysteriously turning on.) As the silence and emptiness begins to fill with shadowy figures, we’re left to agonize over Evy’s safety and state of mind, especially as she starts drinking again to cope with the stress. However, the repeated buildup of these techniques has an expiration date. There are only so many times something spooky can occur without Evy noticing it, and only so many instances the film can reveal something eerie was a dream sequence, or hard cut to black before taking us to the following day, before an audience gets frustrated and impatient.

There’s scary, and there’s startling, and while “Undertone” starts out as the former — its terrors are realistic and unnerving, thanks to its proximity to death — it ends up using the latter as a crutch. Before long, its unique setting and aesthetic approach give way to all the things even semi-seasoned horror fans will have seen a hundred times before, from creepy crayon sketches, to paranoid montages of occult research, and even a deep-cut demonic presence from ancient folklore that may or may not have a hand in things. The movie is at its strongest when dealing with spiritual and emotional mysteries in the dead of night, driven by Kiri’s fearlessly troubled performance that turns slowly inward. The story’s emotional undertones, if you will, are far more affecting when they aren’t bogged down by cheap thrills, which only act as opaque filters for its tale of a woman dealing with horrors that will, eventually, come for us all.

To pivot away from such rigorous drama, crafted with such hair-raising audio-visual dexterity, swiftly lets the air out of the room, as the film’s technical acumen becomes entirely geared towards the rote and familiar. Tuason, however, proves himself a deft enough tonal tinkerer at times, marking a notable arrival in the world of independent tech horror, regardless of where things end up. If nothing else, “Undertone” may be a promise of greater things to come.