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).