A method and processing system for validating an audio signal is disclosed. A valid audio signal is an audio signal that has been determined to originate from a microphone and has not been provided by a digital audio source. In some implementations, one or more speakers may generate a sequence of one or more subaudible tones. Simultaneously, a microphone near the one or more speakers captures an audio signal. If the captured audio signal includes the one or more subaudible tones from the speakers, then the audio signal is a validated.