Choose the absorber stack against the lowest and highest problem frequencies, incidence-angle and polarization range, available depth and the field region. Pyramidal foam, ferrite tile, hybrid treatment and thin loaded sheet use different loss and impedance-transition mechanisms; none can be selected from thickness alone.
Request reflectivity or transmission data for the exact material form, thickness, metal backing, adhesive or air gap and sample size. Reject a normal-incidence best point when the application is oblique, near-field or finite-area, and reject a coupon whose backing differs from the installed enclosure.
Provide the problem band, source/victim geometry, available area and depth, backing metal, angle and polarization range, target reduction and acceptance method. A practical first check is whether the same stack has a full frequency curve from the required setup; if not, plan a representative coupon before committing to the mechanical design.