An adaptive echo cancellation system introduces an acoustic reference signal to audio content being transmitted to the speaker for playback. The acoustic reference signal is an out-of-band signal, such as an ultrasonic signal, which is typically not audible to humans. The microphone of the mobile device receives the audio content played back by the speaker as well as audio content introduced by the user (e.g., the speech of the user). The adaptive echo cancellation system detects the acoustic reference signal and determines a time delay between when the acoustic reference signal was introduced to the audio content and when the audio content including the acoustic reference signal was received by the mobile device. Echo is cancelled from the received audio content based on this determined time delay.