An audio signature is generated by a user device by generating an audio signal indicative of the user device, outputting the audio signal indicative of the user device through a speaker of the user device, using a microphone of the user device to sense audio, generating an audio signature of the audio sensed by the microphone in the user device, and transmitting the audio signature from the user device to a communication channel. The audio signal may be used by another user device which generates its own audio signature, and which can be compared to the audio signature of the user device that originally generated the audio signal to determine if the user devices are co-located.