This. It would be a tremendous battery drain if your phone had the microphone on at all times, with a constant connection to the server, sending all audio over

This. It would be a tremendous battery drain if your phone had the microphone on at all times, with a constant connection to the server, sending all audio over it at all times, just so it can detect when somebody says "hey siri". It's a custom low-power chip they added on the 6S and later to allow a local tight loop that only listens for that utterance using a hardware-assisted neural net, and only activates the rest of the software stack if it detected it with reasonable confidence. There's a nice blog post about it here from the apple engineers (see the "Two-pass detection" section): https://machinelearning.apple.com/2017/10/01/hey-siri.html https://machinelearning.apple.com/2017/10/01/hey-siri.html