An ultrasonic proximity sensor can determine one or more characteristics of a local environment. For example, the sensor can emit an ultrasonic signal into a local environment and can receive an ultrasonic signal from the local environment. From a measure of correlation between the emitted and the received signals, the sensor can classify the local environment. By way of example, such a sensor can assess whether an in-ear earphone is positioned in a user's ear. A media device in communication with the sensor can transmit an audio signal to the earphone for audio playback responsive to a determination by the sensor that the earphone is in the user's ear and can redirect the audio signal to another playback device responsive to a determination by the sensor that the earphone is not in the user's ear. Related and other aspects also are described.