Superfans really don’t enjoy it when you mess with their favorite characters or storylines—just ask George Lucas. Not only do fans of his Star Wars franchise continue to hold grudges over characters like Jar Jar Binks and the controversial reversal of Han’s shooting first, but they scrutinize and dissect every minute detail, from lightsaber design to the strength of Boba Fett’s armor.

That’s exactly why the sound engineers behind The Avengers: Age of Ultron employed a team of no less than thirty individuals to get the sound just right, all the way down to the sound of Captain America’s shield.

The film’s producers were well aware their sound department was standing on the shoulders of giants for their newest endeavor. With Iron Man, Captain America, Hulk, and Thor each having their own preexisting popular film franchises, they faced challenges they wouldn’t have had they been working on something brand new. As with the powers the film’s heroes must wield, this is both a blessing and a curse and comes with enormous responsibilities to the franchise and fans alike. Preexisting sound libraries of characters’ voices, weapons, and actions had to be consistent, all while growing and evolving with the lifecycle of each character.

In the original Iron Man alone, there were three different iron suits, and the sound of each had to advance with the new features of each added invention. The first suit was a clunky, slow prototype that Tony Stark created while being held by terrorists. Compare that to the Mark 3, which was lighter and more nimble, had a new and improved Heads-Up Display (which might sound familiar, since it was created by repurposing the effect of the laser cannon from the vintage video game Space Invaders), and was blessed with the perfected power of flight. It’s a huge transition, and each suit gets the same treatment.

If the issues surrounding a character’s evolution aren’t challenging enough, then there is the added pressure of finding balance among multiple layers of effects, a problem The Avengers sound editor Christopher Boyes can attest to. Since this was the first time the Hulk was played by Mark Ruffalo, Boyes’s challenge was to create a sound that felt both human and sympathetic in association with Bruce Banner while also connoting the animalistic rage and fury of the massive green beast. After playing around with the growls of various wild animals to no avail, he blended his own voice with those of Mark Ruffalo, Lou Ferrigno (TV’s original Hulk), and “two people from New Zealand” to create the final product. With Ferrigno, Ruffalo, and the other three voices in the final product, he was able to honor both the original and more recent Hulk sounds while also adding his own notes to match the new film’s aesthetic.

These superhuman problems were resolved one at a time, often through trial and error and a bit of luck—for example, using ball bearings to create, by hand, the more than 100 sounds behind hulking metal antagonist Ultron.

Check out the film’s sound profile from the SoundWorks Collection for a more in-depth look at the franchise’s sound design.

Brian Platt writes about superheroes, sound design, and stock footage for VideoBlocks.

Image: Marvel Studios