If you don't know anything about how to improve room acoustics, it can be overwhelming at first to understand all of your options. First of all, you should know what frequency of sound you want to absorb. For most settings, you'll just need to absorb the mid to high frequencies. You can also choose acoustic wall panels from online sources.
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An example would be an office or a daycare center, where most of the sound bouncing around the room is going to come from the human voice. In this case, you'll just need to get acoustical panels. However, if you need to absorb low frequencies, you'll also need to add bass traps. Examples include home theaters and recording studios.
The size of the panel isn't that important. You'll just need to get enough acoustic panels to cover enough square footage of the room. The density of the material does not have much of an impact on the acoustic performance. A 6 lb per cubic foot density core material isn't necessarily going to be better than a 3 lb density.
The density just refers to the rigidity of the material. If the material is rigid enough, you could just wrap the fabric around the acoustic insulation and you would not need a frame to support the material.
There are lots of companies that manufacture acoustic panels and sell them online, so you do have lots of options when shopping for acoustic panels. You can even look in your local yellow pages to find an acoustics engineer to help you treat the acoustics of your space, although this can be an expensive option.