I've had modest success using the mattress foam from Walmart and painting it with latex spray paint. It even has a diffuser type of surface.

Foam, especially of the sort that is not designed for acoustical treatments (open vs. closed cells), is not particularly good for this purpose. I suggest cloth-covered fiberglass panels of 2" thickness if you have no need for low frequency treatments.

Since they are cloth covered, you can have them in any color or pattern of your choice, including black. Many, many providers to be found on the internet. An audio specialist is GIK Acoustics but there are cheaper sources.

sleestack wrote:

Do you have to spray the fiber glass with something? Im afraid of breathing that in. Also, Working with plaster walls, No studs

Not usually. The fiberglass is compressed and bonded before being enclosed in the cloth.

I think he is talking about fiberglass duct board. You can buy it in 4x8 sheets and varying thicknesses. If Home Depot doesn't have it, an HVAC supply house should carry a wide variety. I've seen it up to 2" thick and it does have great sound deadening characteristics. Part of its design characteristics does involve sound absorption and I'm sure there are white papers on how effective it is based on frequency range.

Actually, I was talking about commercially prepared panels such as this: https://www.acoustimac.com/acoustic-panels/acoustic-panels-dmd