Abstract: An audio system has a speaker port defining a speaker port cavity, which has a speaker port outlet communicating acoustically with a user's ear canal. A speaker generates output sound in the speaker port cavity based on a speaker signal received at the speaker. The output sound travels along an acoustic path extending from the speaker through the speaker port outlet to an ear canal. A speaker port microphone in acoustic communication with the speaker port cavity produces a speaker port microphone signal in response to input sound. An analysis unit receives the speaker signal and the speaker port microphone signal, uses the speaker signal and the speaker port microphone signal to determine a change in acoustic properties of the acoustic path, and uses the determined change in acoustic properties of the acoustic path to determine a change in physical properties of the audio system along the acoustic path.