In a nutshell, the air gap extends low frequency absorption by placing the absorptive material at a distance from a reflective boundary so that it is closer to the one quarter (and / or three quarter) wavelength for a given frequency whose total wavelength is greater than the thickness of the absorptive material. Think of a classic sinusoidal wave ââ'¬â€œ maximum amplitude occurs at the one quarter and three quarter wavelengths for any given frequency. Minimum amplitude occurs at the point of wave reflection increasing sinusoidally after this.

There's an animated giff out there somewhere that explains this far better!