On/off head detection of personal acoustic device using an earpiece microphone

A personal acoustic device comprising:

• an earpiece having a microphone and configured for attachment to a head of a user or an ear of the user, the microphone disposed at a location on the earpiece such that the microphone is acoustically coupled to an environment external to the earpiece, the microphone configured to generate a first electrical signal responsive to an acoustic signal incident at the microphone, the earpiece having a speaker configured to generate an audio signal in response to a second electrical signal;
• and
• a control circuit in communication with the microphone to receive the first electrical signal and in communication with the speaker for providing the second electrical signal, the control circuit configured to:
• determine a characteristic of a transfer function based on the first electrical signal and the second electrical signal;
• and
• determine an operating state of the personal acoustic device based on the characteristic of the transfer function, the operating state comprising at least a first state in which the earpiece is positioned in the vicinity of the ear and a second state in which the earpiece absent from the vicinity of the ear.