Re: "Hollow sound' room fix.

Mic placement is also important. Wrong placement, too distant, or next to a reverberant flat plane, certainly makes things worse.

A lot of inexperienced people throw sonex or sonex look-alike panels up on a wall, and think they've solved the whole room, often, this is not so.

Heavy blankets or carpeting are popular methods to treat a small temporary recording space; their mass and semi-permiable surface help deaden the space. They don't absorb all frequencies equally, though.