I need the box to block at least 40 dB of sound. The interior size can be as large as a shoebox. I'm thinking about using a plastic box as a frame, and then pad 2 layers of Dynamat or (one layer on the inside, another on outside). And then I want to add 2 layers of styrofoam padding, on top of the Dynamat. And then finally, I plan to add 1 inch thick of acoustic foam to the interior.
Similar thing would be done on the flat surface that the box would be place on, so all 6 sides would have 2 layers of Dynamat, and 2 layers of styrofoam, and 1 layer of acoustic foam. Do you think 40 db is possible?
I'm not sure about using plastic box as the frame. A webpage I looked at recommended particle boards but those are too heavy for the application.
Edit: Thanks. It's valuable knowing that mass is important for insulation. Due to various reasons, I have decided to use 2 layers of acrylic (each being 12mm thick), separated by a layer of acoustic foam for absorption. Is it critical that the absorption foam cover all the areas between the 2 insulation layers?
The box's purpose is to cover up a device from outside noise. It will need to be frequently removed so weight is an issue. For that reason, I have decided to put the device inside a 5 sided box (with the opening facing the top), and just have a flat cover that user can remove easily, rather than removing the whole box. Air tightness has been brought up as a key, so I wonder if it's acceptable to just have a layer of the foam under the cover which would create a seal with the walls of the box due to the weight of the cover itself. Or should I use some sort of rubber seal like they use for doors (that will create a more "airtight" seal).