The Siri team, on the Apple Machine Learning Journal:
Siri is a personal assistant that communicates using speech synthesis. Starting in iOS 10 and continuing with new features in iOS 11, we base Siri voices on deep learning. The resulting voices are more natural, smoother, and allow Siri’s personality to shine through. This article presents more details about the deep learning based technology behind Siri’s voice.
Just scroll down to the bottom and listen to the progression between iOS 9, 10, and 11. It’s really something.
[via Josh Avant]