Interesting idea majic, building an Anechoic chamber. Iadr used to have one of those floatation tanks in his house about 30 years ago, which he slept in at night for a couple of years. Never did a thing for him though, except help adjust his back, which usually popped while he was in it, as it was more distracting than anything, listening to his heart pound loudly while floating in salt water. We had to drain it when we moved and just never filled it back up again, as it was just too much of a hassle, having to keep the water heated at just the right temperature, which required several water bed heaters.
Along the lines of a Anechoic chamber, we've done a couple of things similar to that, which were fairly effective, although though they weren't sound proof, which ideas might give you some ideas on how you might want to build your Anechoic chamber.
The first thing we tried was a large wooden box that we painted black on the inside, which was large enough for our recliner to fit in, that we meditated in each evening. Because of the darkness inside the box, there were several times that we transported to other places while meditating, although we had no idea what was happening, since we'd never heard of lucid dreaming at the time.
The other thing we tried was a large pyramid that we built in our attic to the specifications we found in this book on pyramids. Although the pyramid was not dark on the inside, and was not soundproof, as we used clear plastic on the outside of it, it did have an eerie silence and peacefulness on the inside of it, which we could definitely feel when entering it. We then got an idea and enclosed it in plywood, which proved to be a mistake, as the eerie silence and peacefulness was no longer there. Seems that certain materials like wood just don't work well for pyramids, as they don't allow that mystical energy in.
We also had a smaller, portable pyramid built over our bed that we slept in at night with the wife, which also had that same eerie feeling in it, as it was covered with sheets. You can just imagine how funny it was when we had some guests visit from out of town, who had a couple of small girls, who just happened to wander into our bedroom, who then came running out of our bedroom excitedly saying "they have a tee pee in their bedroom"
