Air’s springiness needs an equivalent measure since it lacks Young’s modulus

Evidence

So far the speed of sound in solids has been considered. However, sound is more usually considered as something that propagates through air, and for music this is the normal medium for sound propagation. Unfortunately air does not have a Young’s modulus so Equation 1.1 cannot be applied directly, even though the same mechanisms for sound propagation are involved. Air is springy, as anyone who has held their finger over a bicycle pump and pushed the plunger will tell you, so a means of obtaining something equivalent to Young’s modulus is a measure of the “springiness” of a material. — David Howard, Jamie Angus, coustics and Psychoacoustics, 43

Categorization

• DEFINE: Establishes that air lacks a Young’s modulus yet compresses, setting up the bulk modulus concept to come.
• corroborated: 2 independent passages support this claim — David Howard, Jamie Angus, coustics and Psychoacoustics, 43