Soundproofing Materials are acoustical materials that have either absorptive qualities and or sound blocking properties. These Soundproofing Products that are absorptive, are typically porous - soft surfaces that allow sound waves to be received and soaked up rather than being reflective.
These absorbtion products have a NRC number attached that tells the percentage of absorption at various key frequencies. Soundproofing Materials that are sound blockers typically have some mass and are made to stop sound/vibration transfer from one area to another. These products carry an STC number that correlates to the Db reduction when using that particular soundproofing product. Composite panels are Soundproofing products that absorb reverberation and block sound and carry both an NRC rating and STC rating.