There are products such as Fibraboard Barrier Backer that can be used as a soundproofing tile backer in conventional grid ceiling applications. To use the FibraBoard Barrier Backer you would simply lay these panels of 1/2″ thick dense cotton based backerboard on top of your existing acoustical ceiling panels. This may require using a step ladder to access above the grid in order to set the FibraBoard Panels on top of the existing tiles. These panels can reduce the amount of sound conducted through the area above the grid by up to 60% depending on the noises that are needing to be blocked.

You may need additional suspension wires, but unless the ceiling is over 500 sq ft the additional wires are probably not necessary.
There you have it, an easy and effective way to soundproof your T-Bar grid ceiling.
For more information about these products or any of our soundproofing or acoustical treatment products, talk to the professionals at Soundproofing America US, Inc, your #1 Soundproofing Resource. This Soundproof Scott Signing Out.
Call For Free Consultation (800) 823-6817