Why this cinematic sound trick makes your brain tingle and your skin crawl

WE'VE ALL FELT the sense of dread that comes with tension in a movie or television scene. A change is imminent. Even if we're unsure of what is tipping us off, our brains are aware of it. In these situations, sound designers—audio professionals who use tried-and-true strategies to keep us on the edge of our seats—often control our emotional cues.

The Shepard tone is one of their most popular methods for making us feel anxious. Invoked when sounds that resemble the pitch of a scale seem to continuously ascend or descend, getting ever higher or lower, this classic auditory illusion's signature psychoacoustic effect is known as the Roger Shepard effect, after cognitive scientist Roger Shepard, who pioneered graphical visualizations of complex statistical data. Film sound designers and composers frequently use this method to evoke a dramatic and menacing atmosphere, most notably Hans Zimmer for Christopher Nolan movies like Inception and Interstellar.

According to Andrew King, head of the Centre for Integrative Neuroscience at the University of Oxford, the illusion is a loop of many tones that are separated by exactly one octave. The midrange frequencies are stronger, while the lowest and highest frequencies are hardly discernible, according to him. The smooth succession of sounds in between, which "produces the illusion of an eternally increasing [or decreasing] pitch," is all that is actually audible when the softer, comparable sounds at the beginnings and ends of the scale fade into one another.

To enhance the mood of the scenario, audio professionals also riff on the tone. Al Nelson, the sound designer for Skywalker Sound, applied the Shepard principle and a variation known as the Shepard-Risset glissando in Top Gun: Maverick, in which the tones glide up or down endlessly. Nelson continued the jet sound effect with the repeating octave below as the movie's protagonist, Maverick, gradually increased the Darkstar experimental jet's speed near Mach 10. "It's quite suspenseful as we tick faster and faster," Nelson said.

As an alternative to static, noise-based sounds, Nelson loves employing tone in sound design. A faraway factory, cutting-edge gear like spaceships or other contemporary vehicles, even purely subjective phenomena like tinnitus (ear ringing) instances, can all be used to bring tone to a certain tale event. A tone is frequently added to ambiances to produce frightening, fantastical audio landscapes.

But why do these noises make people anxious? It's because they appear to change more than they actually do, claims Barbara Shinn-Cunningham, head of the Neuroscience Institute at Carnegie Mellon University. She observes that "the brain responds pretty strongly to change." While the growing scale shift (or "local change") is easily noticed, the overall content's repetition and similarity are not (there is no "global change"). There may be ambiguity and perceptual conflict as a result of the duality. Shinn-Cunningham compares the disconnect we perceive to the impossible Penrose steps in M. C. Escher's surrealist painting Ascending and Descending, which appear to rise or fall even though their height remains constant.

The brain underpinnings of this audio trick's emotional impact, in particular, remain a mystery, as with other excellent illusions. Musical anticipation, a psychological notion that motivates listeners to anticipate what will play next, can influence emotions. The tone may throw you off-balance, adds Oxford's King, "and it appears probable that the unending nature of the Shepard illusion is a crucial aspect in deciding how individuals react to it." This intricacy is echoed by King. He says that in addition to the auditory regions of the brain, listening to music also stimulates the reward, emotional, motivational, and movement centers.

Strong aural compositions go beyond basic effects, according to Randy Thom, head of sound design at Skywalker Sound: "Sound design in cinema, like visual design, is all about influencing the audience's perceptions and emotions." It may not always be as obvious when Maverick tests the boundaries of the Darkstar how the prickling rise and fall of the Shepard scale. Even if you can't hear some frequencies, they might nevertheless have an impact on your body and emotions. That's a deception for another day, though.