For Lee Walpole, sound mixing and editing for “The Aeronauts” was about cause and effect.

“If she (Felicity Jones) pulls a line in the basket, the line must affect the balloon and they get picked up by the wind. It’s cause and effect. You hear a wisp of wind, you hear a flutter of the balloon, you hear the line and the basket,” he said. “You have to create that sequence of events.”

Watching Tom Harper’s “The Aeronauts” in the theater is a completely immersive experience. As Felicity Jones and Eddie Redmayne take flight in their balloon to break the world record for flight altitude in 1862 and ascend to new heights, the sound wraps around the audience. From butterflies to the wind to the rigging poles in the balloon, Walpole worked on creating the sonic atmosphere of the film.

His first approach was to get the sound of the balloon right.

“We break the balloon down into four key components: the basket itself, the instruments suspended around the balloon, the rope riggings that tether the balloon to the basket and the balloon itself,” Walpole explains. “We had to create sound palettes for each of the four elements. We had to create bespoke recordings.”

When Walpole got into the production basket at the beginning of the shoot, “we realized it made an individual and complex sound, so we had to stop that sound so we could get the clean dialogue.” That meant in post-production, Walpole had, “nicely recorded dialogue and very little else.”

He created an entire world around balloon sounds, including commissioning a basket “from a Welsh coffin-maker.” Once they had their own basket, his team of foley artists stepped inside to record sounds. In addition, he bought sheets of silk and fabric, and recorded those to capture the flutter of the balloon skin.

Harper was keen to explore picture ratio for the film, shooting in scope on the ground, and opening up when the balloon takes flight.

“I thought it would be cool to do the same with sound,” Walpole explains. “We mix on 7:1 on the ground and open to Atmos when the balloon gets into the air. Tom was instantly on board with that, and Atmos became such a great tool for us. “ He adds, “We could take the four key components and put the balloon above the viewer’s head. The basket goes to the front, and it just gives this illusion of first-person sense for the viewer.”

As the balloon ascends and descends, that sound needed to be relayed to the viewer. Walpole says that was the toughest aspect to create since the balloon was a gas one and not hot air.

For him to tell the story, Walpole constructed the sound of air from “multiple layers of wind. It was constantly changing throughout the film.” He says, “For any given scene, there were four or five different winds being treated as objects in the Dolby Atmos world. They also played very quietly so as a viewer, you don’t latch into the cue, but you have a sense of continuous movement around you.”

The key, Walpole says was to be “delicate at all times.” Too much sound would capture the viewer’s attention. “You just wanted them to be aware.”