Cork
A natural cellular material
We chose to study cork because it is a natural soundproofing material.
Indeed in the old days, lawyers' and notaries office doors were covered with a thick layer of cork to prevent people from hearing important and confidential information from outside. Therefore, nosy people were not able to eavesdrop!
We bought a pack of 100 cork stopper for 22€ in a supermarket to get this material, as shown on the picture just below.