Advice and questions on keeping your instruments in top form
Hi everyone, this is my first post so I wish to say thanks for all the infos found here, this site made me the player I am. Not so good to be fair but I have my personal satisfaction climbing the learning curve :)

I decided to register because I wish to share my solution to the ringing problem.
It is incredibly simple, tunable and effective. I just love it even if I tried it only once and with a problematic traditional djembe, I guess this will work with the infamous remo ringing problem.

put some spray foam inside the djembe body to damp vibrations.
Poliuretanic foam is the perfect material for this application, her only real enemy is continous uv radiation (no chance in a djembè).

Disassemble and mask the parts where the foam should not stick, my advice is to wrap the outside with cellophane, protecting also the rim and other visible parts.
IF you interior is smooth please make it raw with sanding paper, use the lower number you find comforable to use, the rawest the scratches the best.
Make a sticky test with a single application, leave it there for 12hrs before you try to remove it with your hands, also make some trial to find your preferred application method (remember to provide humidity, see below).

Now that all is prepared and tested lets go, remember that if all is masked you can make any kind of error. If it goes in a wrong place leave it there, when is dry is way easy to remove.
I made a torus of foam but we can even damp all the surface with a thin layer, use single points, stripes or you can fill it and reshape completely your interior and/or duct!

Important: clean with a humid rag and nebulize little water before and after the foam application, the foam needs humidity to stick and inflate herself and build an external crust faster (depends on climate anyway, if you are not comfortable with water and is not a dry day well, don't use it).
Don't overdo that, djembe surface should be NOT DRY but not dripping.
The foam inflate a lot, it is easy to work with when you are used so try it elsewhere on a scrap part.
You may need to make the work in steps so the foam does not pulled away from gravity while is soft.

If the foam sticks to your hands, clothes or anything not silky smooth you will be in real troubles, use latex gloves.
When the foam is dry (12hrs) reassemble your djembè and tune it with same tension as before.

The foam is extremely easy to cut so you best use more than necessary and remove it reaching the inside from the duct.
You can use anything that can cut or rip the foam in a controlled way, even a hot wire on a solder works, you can cut it like butter with a blade or rip it with fingers.
You can finely tune your ringing changin volume, shape and roughness of the foam and with ropes/keys. You can even try to put back foam using drops of new foam as glue but I didnt do that.

The foam will modify ringing but if you drastically change the shape of the interior part also the main bass resonance could change.
I use a calibrated microphone with an audio analyzing software to help tuning, if you have some knowledge in that field you can calculate and modify precisely frequencies, resonances and ringing time ;)
Also you can help with masking tape on the skin (2-5 inches long, multiple layers needed) until you have the sound you wish (that in my engineer mind is just the right combination of numbers from the audio software, once you understand how the instrument spectrum works it is very easy to recognize what you don't like and what to modify to avoid it).

What a long post :)
Thanks everybody for this great forum!
Interesting, if rather elaborate, method :)

Personally, I just tune up until the ringing disappears or the skin breaks (whichever happens first). Different skins ring to different degrees; if I have a skin whose sound I don't like, I don't mind it breaking. Life's too short to waste it playing a drum with bad sound ;)