I assumed it must be somewhere between 1.5 and 2 atmospheres for a sinusoidal piston motion that creates alternating compression and rarefaction. I also would guess that plenums, vents and other resonant chambers would have certain frequencies where they were more sensitive due to resonant conditions. At frequencies where a vent or plenum is resonanting, we'll get a ram-charge effect where critical pressures are reached at lower piston excursions.

But putting resonance aside, I wonder where air rarefaction/compression starts becoming noticably asymmetrical, causing non-linear distortion. Kinda thought you might know a reference or two.