1. A method for detecting a user input, comprising: detecting sound with a plurality of acoustic sensors in an electronic device, wherein the electronic device includes openings through which at least some of the sound passes, and wherein detecting sound with the plurality of acoustic sensors comprises, with at least some of the acoustic sensors, detecting sound generated external to the electronic device; and with the electronic device, determining the location of the user input on a surface at least partly using the detected sound that was generated external to the electronic device, wherein determining the location of the user input on the surface at least partly using the detected sound that was generated external to the electronic device comprises: subtracting background noise from sound associated with the user input.