An electronic device such as a pair of headphones may be provided with ear cups having speakers for playing audio to a user. Capacitive proximity sensor electrodes having acoustic openings may overlap the speakers. The capacitive proximity sensor electrodes may include electrodes that are arranged in a ring. Control circuitry in the electronic device may use the capacitive proximity sensor electrodes to measure ear patterns of a user when the headphones are being worn on the head of the user. The control circuitry may include switching circuitry that allows the electrodes to be dynamically combined to form electrodes of enlarged area to enhance detection range or to form separate electrodes to enhance spatial resolution.