Wake word detection (listening for a specific phrase, such as "Hey Siri") typically happens using a completely different subsystem from active voice recognition

Wake word detection (listening for a specific phrase, such as "Hey Siri") typically happens using a completely different subsystem from active voice recognition, since it's a much more bounded problem that also needs to run with far lower power consumption. (in the case of Siri specifically, Apple actually has a pretty nifty whitepaper: https://machinelearning.apple.com/2017/10/01/hey-siri.html https://machinelearning.apple.com/2017/10/01/hey-siri.html) That said: right now, common sense would dictate that your phone's battery really can't handle recording full audio at all times, but once that's no longer the case, it does seem problematic that we don't yet have the audio equivalent of putting a piece of tape over your laptop webcam.