STC is a single number and represents an average reduction measured at several frequencies between 125Hz and up to about 4kHz (I forget if it is 4 or 8kHz).
The notion of an STC rating of 80 for this type of product is fanciful at best.
For practical reasons, at low frequencies you will probably need to rely on adding mass (double sheets of drywall), staggered stud construction (look it up) or a layer of green glue sandwiched between two layers of drywall (as mentioned above), or limp mass loading (essentially a rubber sheet between the studs and drywall).
As with most things, proper implementation of these techniques is critical. For instance, if you starting cutting into the material for recessed lighting, then you have just wasted a great deal of money.
Let me underscore Art's comment: getting good attenuation (or absorption for that matter) at the lower freqeuncies is especially difficult