How do I select an absorber material for lining the inside of an anechoic chamber?

Selecting absorber material for an anechoic chamber involves matching the absorber's frequency performance, physical size, and cost to the chamber's intended use. The primary absorber types are: pyramidal foam absorber (polyurethane foam loaded with carbon or graphite particles, shaped into pyramidal cones; the gradual taper provides an impedance transition from free space to the lossy material, minimizing reflections; reflectivity: -30 to -50 dB at frequencies where the pyramid height > lambda/4, degrading at lower frequencies; sizes range from 2 inches (effective above approximately 1 GHz) to 72 inches (effective above approximately 30 MHz); used in the main test volume of the chamber where high-quality quiet zone performance is needed), ferrite tile absorber (sintered ferrite tiles, typically 6 mm thick, bonded to the chamber walls; effective from 20 MHz to approximately 1 GHz; reflectivity: -15 to -25 dB; compact (no protruding pyramids) but limited high-frequency performance; used on walls, floors, and ceilings where space is limited or where low-frequency performance is critical), and hybrid absorber (ferrite tiles on the wall with pyramidal foam on top; combines the low-frequency performance of ferrite with the high-frequency performance of foam; effective from 20 MHz to above 18 GHz; the best overall performance but most expensive). The selection criteria include: the lowest frequency of operation (determines the minimum absorber size), the required quiet zone reflectivity (determines the absorber quality at each angle), the available room size (larger absorber requires a larger room for the same test volume), and the budget (ferrite tiles are the most expensive per square meter, followed by large pyramid foam).