I love Siri. I use it all the time. But there are some occasions where having that voice coming out of my phone is just really inconvenient. Fortunately, Apple designed Siri so that you can shut off the vocal responses and just see replies in text.
Pop into Settings > General > Siri, and then tap on Voice Feedback:
Tapping on Voice Feedback provides two choices -- Always and Handsfree Only (see below). Select Handsfree Only, and now when Siri answers a question or does something for you, the response is text only.
If "Hey Siri" happens to be turned on and you invoke Apple's digital assistant by using the phrase, Siri assumes that you're unable to touch the iOS device screen and still provides a voice response. By the way, if you ask Siri to shut up, it won't respond verbally... but you may hurt its feelings (see image at top of this post).