Many thanks to you Rod and jwl,
You've been very generous with your advise and it is highly appreciated.
Rod--- I just discovered you're a published author of a book dealing with these issues, so I'm honoured that you've taken the time to help me. The noise level inside my workshop is not a problem. My only concern is the noise level outside while I'm using the grinder.
My workshop is in an apartment building in NYC and my window faces the airshaft which is square shaped and has cement walls. Unfortunately is a perfect sound chamber, so much so if I open the window I can hear my neighbours talking very clearly. Imagine what it must be for them to hear me grinding metal!
After reading your replies I have a much better sense as to how to proceed so thanks again for all your help.
Since adding mass to my plywood door is the way to go, what material should I use? The same question applies to the window plug.
Question 1--- I'm thinking about screwing a sheet of PDB2 on each side of a sheet of plywood so what I will be screwing to my plywood door will be 1 sheet of PDB2 + 1 sheet of plywood + 1 sheet of sheetrock. The plug could also be a PDB2+plywood+sheetrock sandwich, and if it is a good idea I could leave a 1" air gap in between each of the three sheets.
---Would this three-layer sandwich do the job?
As the metal pieces I have to grind are sometimes fairly small in size it occurred to me the idea of building a cabinet similar to a "sandblasting cabinet". sandblasting cabinet - Google Search
so the noise I'm making with the grinder will have an additional barrier to go through before hitting the walls, ceiling, and floor.
Question 2--- What would be the best soundproofing materials to use on the six walls of the cabinet? The shell will have to be plywood but I can make the walls as thick as I wish and I can screw any material on the inner and outer sides.
Question 3--- As the cabinet isn't very large in size it isn't very costly to add thickness to the walls and using different materials such as rubber. The same kind of rubber used on the mat of a weight-lifting room. But what should the formula be? rubber+sheetrock+plywood with a 2" air gap in between???
I found a great variety of soundproofing materials on this link: PDB2 - Google search
so I'm a bit confused with so many sound-blocking products to choose from.
Question 4--- As far as the window frame is concerned... It surprises me knowing the aluminum structure of a double glazed window has nothing but air inside. Doesn't the air allow sound to pass through it thus counteracting the soundproofing effect of the double glass?
Would using a thick coat of rubber paint on the aluminum frame make it less difficult for sound to pass through it?
I look forward to your reply. Hopefully other readers will put in their two cents as well...