Fiberglass has been shown to absorb lower frequencies, down to 250 hertz better than acoustic foam of equal thickness. This is true until about 1,000 hertz. Foam, on the other hand, absorbs some mid and high frequency sounds better than fiberglass, starting at around 1,000 hertz until 4,000 hertz. If you wish to dampen all frequencies and have a sound that is completely dry, than both acoustic foam and fiberglass should be used together.
Fiberglass is a more dense material than acoustic foam, which is the reason it can absorb lower frequencies better. Fiberglass has a density of about 6 lbs. per cubit foot, while acoustic foam has a density of only 2 lbs. per cubit foot. Fiberglass also has more variations in density and can be purchased at a range between 3 and 6 lbs. per cubit foot, while acoustic foam is fairly uniform as far as density is concerned.
Acoustic foam can be easily cut, shaped and mounted onto walls and placed all around a room. Fiberglass must be cut into larger sheets before it is placed on a wall and is not as easy to cut. Because of its larger density, it is also harder to mount onto a wall. For quick soundproofing jobs, acoustic foam may be the best option because it is the easiest and quickest to use.
Fiberglass is the lowest cost noise absorber on the market because it can be easily manufactured. Though fiberglass and foam both vary in price from manufacturer to manufacturer, fiberglass is generally the least expensive choice. Acoustic foam, however, can be purchased in smaller quantities at a time, making it more useful for home studios and quick soundproofing setups.