I need an enclosure to block outside noise by about 30 to 40 DB. The noise I'm most interested in blocking are in the 0 to 2000 Hz range. I understand that mass is important for blocking (insulating) noise. Here's my proposal:
On a table top, place a sheet of acoustic foam or rubber sheet. Place a 1/2 inch thick steel plate on top of the acoustic foam or rubber sheet to serve as my bottom layer. Place a 1mm or 2mm rubber or silicone sheet on top of the steel plate to serve as a seal for my enclosure which is composed of a 5 sided box.
I would like to use acrylic for the 5 sided box because transparency is an advantage for my application and it's easy to buy a custom acrylic 5 sided box rather than making my own (like using MDF). The thickness will be 1/2 inch. To further enhance sound proofing, I would to use TWO 5 sided acrylic boxes each being 1/2 inch thick sized such that there will be a 1/2 thick air gap between the 5 sides between the inner and outer box. Both boxes will be placed on the steel bottom layer with a rubber sheet in between for an airtight seal.
Another option is to use a 1 inch thick Delrin or acrylic plate rather than steel for the bottom layer. I need to drill a hole for placing a wired microphone inside the box and plastic is easier to drill than steel.
Would this cause a lot of echo in the box? I will have a microphone inside the box to record the sound of a object placed inside the box. I wonder if echo will cause issues.