Note that in both cases the bulk modulus varies directly with pressure. For air under standard atmospheric
conditions with 1abs2 and the isentropic bulk modulus is 20.6 psi.
A comparison of this figure with that for water under the same conditions shows
that air is approximately 15,000 times as compressible as water. It is thus clear that in dealing with
gases, greater attention will need to be given to the effect of compressibility on fluid behavior. However,
as will be discussed further in later sections, gases can often be treated as incompressible fluids
if the changes in pressure are small.