A computer-implemented method comprising: receiving, by a first computing device, first audio data of a first utterance; determining, by the first computing device, that the first utterance includes a particular predefined hotword; based on determining that the first utterance includes the particular predefined hotword, determining, by the first computing device, whether the first audio data includes a unique ultrasonic audio signal previously outputted by a second computing device while the second computing device received second audio data of a second utterance that included the particular predefined hotword; and based on determining whether the first audio data includes the unique ultrasonic audio signal previously outputted by the second computing device, determining, by the first computing device, whether to perform a command that is included in the first utterance and that follows the particular predefined hotword, wherein determining whether to perform a command comprises: based on determining that the first audio data includes the unique ultrasonic audio signal previously outputted by the second computing device, rejecting the hotword or command and disabling access to one or more resources of the first computing device or preventing the device from unlocking or waking; and based on determining that the first audio data does not include the unique ultrasonic audio signal previously outputted by the second computing device, determining to accept a hotword or command and enable access to the device or unlock or wake the device. receiving, by a first computing device, first audio data of a first utterance; determining, by the first computing device, that the first utterance includes a particular predefined hotword; based on determining that the first utterance includes the particular predefined hotword, determining, by the first computing device, whether the first audio data includes a unique ultrasonic audio signal previously outputted by a second computing device while the second computing device received second audio data of a second utterance that included the particular predefined hotword; and based on determining whether the first audio data includes the unique ultrasonic audio signal previously outputted by the second computing device, determining, by the first computing device, whether to perform a command that is included in the first utterance and that follows the particular predefined hotword, wherein determining whether to perform a command comprises: based on determining that the first audio data includes the unique ultrasonic audio signal previously outputted by the second computing device, rejecting the hotword or command and disabling access to one or more resources of the first computing device or preventing the device from unlocking or waking; and based on determining that the first audio data does not include the unique ultrasonic audio signal previously outputted by the second computing device, determining to accept a hotword or command and enable access to the device or unlock or wake the device. based on determining that the first audio data includes the unique ultrasonic audio signal previously outputted by the second computing device, rejecting the hotword or command and disabling access to one or more resources of the first computing device or preventing the device from unlocking or waking; and based on determining that the first audio data does not include the unique ultrasonic audio signal previously outputted by the second computing device, determining to accept a hotword or command and enable access to the device or unlock or wake the device.