Change Accessibility Siri preferences on Mac
On your Mac, use the Siri pane of Accessibility System Preferences to type your requests to Siri instead of speaking them. This feature can be useful if speech is difficult or you don’t want others to hear your Siri requests.
To change these preferences, choose Apple menu 
> System Preferences, click Accessibility, then click Siri.
Enable Type to Siri | Type your requests to Siri instead of speaking them. If a headset with a microphone is connected to your Mac, Siri will speak your typed request. |
If you prefer to read responses from Siri in the Siri window instead of having Siri speak them, turn off voice feedback in Siri preferences.
See alsoUse Siri on your Mac