1. A computer-implemented method comprising: receiving audio data; determining, by a component of a mobile computing device that remains activated when the mobile computing device is in a low power state, that the audio data encodes a likely utterance of a prefix of a predetermined activation term that initiates activation of a hardware component of the mobile computing device that (i) would otherwise be deactivated when the mobile computing device is in the low power state and (ii) is otherwise activated when the mobile computing device is in a wakened state; and (a) in response to determining, by the component of a mobile computing device that remains activated when the mobile computing device is in a low power state, that the audio data encodes the likely utterance of a prefix of a predetermined activation term that initiates activation of a hardware component of the mobile computing device that (i) would otherwise be deactivated when the mobile computing device is in the low power state and (ii) is otherwise activated when the mobile computing device is in the wakened state, and (b) before the component of the mobile computing device or any other component of the mobile computing device determines that the audio data or other audio data encodes a likely utterance of the entire predetermined activation term, activating a hardware component of the mobile device that (i) would otherwise be deactivated when the mobile computing device is in the low power state and (ii) is otherwise activated when the mobile computing device is in the wakened state.