I need to deaden an echoing drum practice room and am considering acoustic panels, probably DIY ones 3.5" deep filled with rock wool. I'm look at these vs. heavy blankets for aesthetic reasons. I'd like to know they are effective before I spend the labor to assemble them. Also interested in how many (or how much area of wall) I need. My space is small, about 10' x 10'.
You can compare the sound absorption coefficients for the materials you are considering to see how dead different amounts of materials will make your room.
Here is a chart: http://www.acoustic.ua/st/web_absorption_data_eng.pdf
What you could do is approximate the room response by simplifying the math by assuming the non-damped surface area reflects 100% of the sound equally (i.e. treat the walls as if they are a sphere with the drum kit in the center). Then multiply the absorption coefficient by the percent of surface area you intend to cover with the material and estimate that the intensity of the reflections will be that much less. For example, if you cover 50% of the surfaces with materials having absorption coefficients of 0.50, then you could estimate that reflections and reverb will be reduced by about 25%.
Two important reminders: the ceiling and floor count as reflective surfaces for your estimates, and absorbing reflections only affects the reverberant sound level in the room. It does not affect the direct sound level (from the drums to anyone’s ears), nor does it necessarily reduce the amount of sound transmitted to other rooms through the walls of the room. In other words, you can’t really make the drums quieter this way, only deader. And you don’t want it too dead because that would sound weird.
Really a good way to go is to plan to build perhaps four panels and put them up and see if you’re happy. If not, build four more. Keep going until it sounds good. It’s not often effective to do math to predict how something will sound.
It's not easy to tame a 10' x 10' box which you intend to be LOUD in.
You might be well advised to forget the aesthetics and put most of the junk back, leaving JUST room for the drum kit! Then you get free sound treatment AND free storage! I've too often seen people ruin a perfectly useable room with 'design'.