There are two ways to 'cheat' a little with LF porous absorption using relatively thin materials.

The first is to fix a foam tile (100mm as example) at a distance from a room boundary that equates to either the one-quarter or three-quarter wavelength of a given frequency. These are the points where wave velocities are highest and consequently where most absorption (or more properly energy conversion) occur. Hanging pairs of 100mm foam tiles from the ceiling at distances of about 88cm and 264cm from a room boundary will be useful at frequencies around 100Hz.

The second method is to increase density - essentially taking a thick material and compressing it.

Both very low WAF