There is a persistent myth in recording that you can get good studio acoustics by lining a room with foam. This is just so untrue – as you will realize to your cost if you do it!
Firstly, for 'foam' read 'mineral wool'. The sort of foam you find in soft furnishings is expensive and not as effective as the mineral wool that is commonly used as insulation in buildings. Also, don't confuse fiberglass with mineral wool. Fiberglass is also more expensive and generally not as effective for acoustic treatment.
OK, now we've got that sorted out, what's wrong with lining the entire room with a couple of inches of mineral wool?
The answer is that it will make the room very dull and lifeless. It will be unpleasant even to be in that room, let alone record or mix in it. Mineral wool is a porous absorber and it can only absorb sound with wavelengths up to four times the thickness of the layer you apply. So a typical 50 mm layer would absorb frequencies above 1700 Hz. Below this, sound will reflect from the surface of the wall underneath as though the mineral wool wasn't there.
This overemphasis of low-mid and low frequencies reflecting back and forth in the room will give it the dull sound I mentioned. Also, if you mix in a room like this, the excessive bass produced acoustically in the room will lead you to put less bass in your mix. So your mixes will sound bass-light to anyone else!
The answer is to use a combination of porous absorption, together with diffused reflections and specific low frequency absorption. More on this in another Audio Masterclass resource…