https://techcrunch.com/2017/09/19/does-siri-sound-different-today-heres-why/

Did you notice that Siri sounds a little more sprightly today? Apple’s ubiquitous virtual assistant has had a little virtual work done on her virtual vocal cords, and her newly dulcet-ized tones went live today as part of iOS 11. (Check out a few more lesser-known iOS 11 features here.)

It turns out a lot of work went into this little upgrade. The old methods of creating speech from text produced the familiar but stilted voices we’re all familiar with from the last decade or two. Basically you took a big library of voice sounds — “ah,” “ess,” etc. — and stuck them together to make words.