12. The system of claim 4 , comprising: determining a loudness of the audio input representing the utterance based on a sound pressure level of the audio input, wherein (i) placing the mobile device into the sleep mode, (ii) bypassing, by the mobile computing device, further processing of the voice command, (iii) bypassing, by the mobile computing device, emitting the ultrasonic signal, and (iv) bypassing, by the mobile computing device, outputting the visual indication that the mobile computing device is processing the voice command is based on the loudness of the audio input representing the utterance.
4. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving, by a mobile computing device that is (i) configured to process voice commands that are preceded by a predefined hotword, and (ii) is in proximity to another mobile computing device that is also configured to process voice commands that are preceded by the same, predefined hotword, an audio input representing an utterance by the speaker of a voice command that is preceded by the predefined hotword; while receiving the audio input representing the utterance by the speaker of the voice command that is preceded by the predefined hotword, performing, by the mobile computing device, an operation; after receiving the audio input representing the utterance by the speaker of the voice command that is preceded by the predefined hotword, receiving an ultrasonic signal from the other mobile computing device; in response to receiving the ultrasonic signal from the other mobile computing device, (i) placing the mobile device into a sleep mode, (ii) bypassing, by the mobile computing device, further processing of the voice command, (iii) bypassing, by the mobile computing device, emitting an ultrasonic signal, and (iv) bypassing, by the mobile computing device, outputting a visual indication that the mobile computing device is processing the voice command; and while receiving the ultrasonic signal from the other mobile computing device and bypassing processing of the voice command, continuing, by the mobile computing device, to perform the operation without interruption.