I passed ACX's QC...but the end result sounds horrible.

it’s literally an empty room

Our preference is a Victorian overstuffed parlor with lots of padded divans and reclining sofas and thick carpeting with heavy drapes. Ferns optional.


So an empty room is a good start.

I have a “portable studio” I made from sticks and furniture moving pads.

One finished wall is double layer.

Note the doubled-over moving pad on the floor on top of the industrial carpeting. This was a quiet room in one corner of our factory whose only shortcoming was bare-wall live echoes. I shot several animation voice tracks in that room. Note the contraption on the right is home-made vibration isolation for the microphone. Those are US Postal Service rubber bands.

http://www.kozco.com/tech/pvcShockMount/shockmount.html
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You don’t have to soundproof the whole room (although not a dreadful idea). There are a number of “announcing tunnel” concepts out there.
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Both of these are wrong because neither one pads the floor under the microphone and only the second one has a “spider” to isolate the box from the microphone. Metrobus rumble through the floor right up to the microphone in the case of the first one.

Also I would somehow deaden the wall behind you. The microphone is picking up your face and anything behind you. Alternately, put a corner of the room behind you, not a flat wall.

Fill the room with storage. Cardboard boxes full of old vinyl records and tax forms from 1982 are a terrific soundproofing. I did a sound shoot in a storage closet once—with the storage. I had to stop when the company ran out of room and turned it into somebody’s office. I actually time-shared with a Producer for a week or so.

Koz