Presuming the person has a working ear system: (1) a defined medium in which the sound will travel (water, atmosphere, railroad track, ...); (2) a disturbance to the medium that can be characterized as a change in density versus time such that the frequency of that change is within human hearing capability (20 Hz to 20,000 Hz +/- small delta ) and the amplitude is above human hearing threshold (-10 to about 100 dB Sound Pressure Level depending upon frequency).