LAS VEGAS -- One of the more difficult sounds to create in Marvel and Disney's Captain America: The Winter Soldier was the Winter Soldier's arm, said the film's supervising sound editor and designer, Skywalker Sound's Shannon J. Mills, during a panel at the NAB Show.

Saying that directors Anthony and Joe Russo wanted it to sound "vintage," he explained that in the story, the arm was installed in 1950's Russia and made out of scrap metal. "It was built in a time before there were advanced robotics, so they wanted it to sound primitive."

Mills (Avatar) was joined by the film's supervising sound editor, Daniel Laurie of Skywalker Sound (Harry Potter and the Deathly Hallows Parts 1 and 2), and Bruce Markoe, senior vp postproduction at Marvel Studios, in a session presented by Motion Picture Sound Editors (MPSE).

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Screening an action sequence that takes place on a highway, Mills explained, "They wanted it to be gritty and real. We recorded sound for the vehicles out in the desert. For the guns, we had a hard time finding guns that sounded specific to a city environment, so we recorded those in places that were safe and also had concrete, in order to sound real."

Captain America: The Winter Soldier also received a mix in Dolby's immersive Atmos sound format, which involves placement of speakers around the auditorium as well as across the ceiling.

"Sound is such a critical part of the moviegoers' experience," Markoe said. "[New options include] Dolby Atmos, and Imax has a new sound system that is coming out with next year. They are significant leaps forward in what we can do. We hope people experience it in the theater."

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Mills related that the sound team took advantage of the Atmos format -- which allowed them to place sound in more specific locations -- for scenes with Falcon (Anthony Mackie) in flight. "We were able to hear him flying above us and around us. It was fun to use Atmos to position him."

But the speakers warned that the competing sound brands can be challenging. "The more formats we have to complete back our schedule up," relates Mills. "It impacts our time to do the creative." (An effort to create a standard for a single immersive sound format is in the works).

Tight schedules add to the challenge. "We have so little time to finish these movies," Markoe said. "We have to be careful not to shortchange sound. It's still a small percentage of the budget, but its awfully important. We have to spend more time to make sure each format gets the best possible quality."

Added the Marvel exec, "We are also taking steps to try to optimize sound for smaller devices; we'll do a trim pass if it will help."

E-mail: Carolyn.Giardina@THR.com

Twitter: @CGinLA