@mijostyn J. Gordon Holt wrote about activated carbon in wall filtering in Stereophile. I built a custom listening room, four walls and floor (had to keep the ceiling without them due to thin 2X8 rafters). You can see a description on my profile. The carbon is contained in a four chamber, wooden 12.5" wide X 48" high compartment lining all walls and both doors. The wall width is 16". With 6-12" woofers, I'm not overloading the room with this design and don't require external filtering, including available activated carbon boxes on walls.
From the ad for external boxes:
Carbon technology has the same smooth frequency response starting at 40 – 60 Hz. and going through 6,500 Hz. it is a smooth low-frequency absorbing tool for absorbing unwanted low-frequency energy along the boundary surfaces in small rooms. At a maximum depth of 12″, it won’t take up much wall space and gives you lower frequency absorption down to 40 Hz.
With carbon technology, you achieve a smooth absorption curve that is smooth in rate and level with a special quality of clarity that is not offered with foam or other absorption technologies especially the building insulation types that dominate the industry. If you are looking for a more natural rate and level of absorption that does not over absorb at certain frequencies and under absorbing at others, our carbon and foam technologies will give you that linearity you need.