Starting in Thor: Ragnarok, Hulk can remain calm enough for long enough to carry on semi-intelligent conversation.
Is the voice acting done by Mark Ruffalo? Is it digitally or acoustically altered?
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According to Ruffalo, the entire acting piece (voice, facial motion capture and body motion capture) was done by him with no outside assistance.
The Hulk. That's all me that's me and and the animation team all working together but it's all I shot all. That's me acting, that's my voice, that's my face and it's my body moving around
and this video interview with Yahoo goes over much the same ground
“When Hulk is speaking, that’s me,” the actor says. “That’s me on the day, in my motion capture suit, and they’re capturing my voice.”
The key to doing the "hulk voice" is to
You can see him doing some of the mo-cap in these clips
And hear him doing the voice on the red carpet at the film's premiere
According to the following comicbook.com post.
Up until now, every version of MCU Hulk has been voiced by Incredible Hulk '70s TV series icon, Lou Ferrigno; however, Ferrigno wasn't brought back for Thor: Ragnarok. So, who is the new voice Hulk? Marvel fans will be proud to know that it's Bruce Banner himself, actor Mark Ruffalo!
For Thor: Ragnarok, Ruffalo took on all aspects of his character, playing Bruce Banner while doing both the motion-capture and vocal performances as The Hulk. That means Thor: Ragnarok's Hulk is the most complete version of the character we've ever seen, and as stated in our official Thor: Ragnarok review, the difference really shows onscreen!
comicbook.com, 'Thor: Ragnarok' - Who Is Hulk's New Voice?
The IMDB page for the cast of the film only shows Mark for the role of Hulk too for whatever that's worth.