A portable electronic device comprising: an enclosure having an enclosure wall that forms an interior chamber and a sound output port to an ambient environment; a speaker positioned within the interior chamber, the speaker having a front volume chamber coupling a sound output surface of the speaker to the sound output port and a back volume chamber acoustically coupled to the interior chamber by an aperiodic vent; and a valve operable to open and close the aperiodic vent in response to a pressure change.