CoachA medium (plural media) is the substance a sound travels through: air, water, wood, steel — any state will serve, for all are crowds of molecules. What no sound can cross is the absence of a crowd: vacuum, the empty stage.
Examples
Example 53.9 (Sonar, in earnest)
The survey ship’s sonar clicks downward; the seabed’s echo returns in . In water, round trip: depth . Dolphins and bats run the same arithmetic by instinct; ships print it on charts. Note the discipline: the medium chooses the speed — feeding an underwater echo the air’s is the chapter’s second classic slip.