The air gap changes your absorption more than the material does

This is the single most common reason a customer disputes an absorption result, and it is almost never the laboratory's fault.

A porous absorber works by converting acoustic particle velocity into heat through friction. Particle velocity is at its maximum a quarter wavelength from a rigid surface, and effectively zero at the surface itself. Mount a 50 mm absorber flat against concrete and at 125 Hz — a quarter wavelength of roughly 690 mm — the material is sitting in a region where there is almost no particle velocity to convert. Pull it 200 mm off the wall and you have moved it into a region where there is.

The material has not changed. The measured absorption coefficient at low frequency has, substantially.

The practical consequence is commercial rather than technical. If you publish an αw obtained at Type E-200 and your customer installs the product flat against a slab, the installation will underperform your published figure, and the complaint will arrive at your door rather than ours. Decide how the product is genuinely installed before you decide how to test it.