Just to add: a simple way to view this is to remember...that all disturbances propagate through the air, as a wave, the wave velocity is the sped of sound 'a'
however for slow moving objects that may have created the disturbance the wave moves far faster than the object that creates the disturbance therefore the effects of the wave which is a series of compressions and rarefactions, has no little or nil effect upon the object...however as the object speeds up it begins to 'catch-up' with the forward propagated disturbance and this leads to shock wave which disturbs lift on the airfoil and as the 'bow wave created becomes more and more severe-a loss of lift occurs and the overall center of pressure moves aft and creates a downward pitching moment....as a result of 'compressibility'..i.e all the air molecules scrunched up together...so for low altitudes...one references IAS because...dynamic pressure effects limits...i.e structural limits prevail...however, at higher altitudes stability problems created by shock-wave formation are more of a concern...hence why there's reference to M above a certain altitude...the crossover altitude....because at that speed/altitude combination the effects of compressibility will limit the airplane's performances long before structural issues will ever creep up---providing the ASI with a barber-pole makes the whole worry...obsolete though...
