FWIW - Once again, I agree with KCT. Particularly here, about the best construction method to reduce sound transmittance.
We have been involved with multiple projects that have been designed by engineers to reduce sound transmittance through walls. Our job was to supply the materials and build these actual sound reducing walls according to the design plans. Several of these projects have been in medical buildings and hospitals.
One in particaular, was an entire wing of Doctor's executive offices at a major regional-hospital remodeling and expansion project. The wing also contained multiple conference and meeting rooms. Confidential discussions in privacy was more than a preference, it was about legality. The engineer-designed partition walls were not built with Mass as the primary sound reducing material. They were as KCT described: Multiple layers of sound reducing sheetrock, with staggered stud arrangements, inter-woven "sound attenuation" blankets (mineral wool insulation), channels placed over these, with additional layers of sound reducing sheetrock. All facets of the areas had sound dampening caulk and pads at floor, ceiling, and outlet boxes (as these, and other connected metal components like venting) will transmit sound.
I am not going to get into the technical details, like decibal counts, or STC levels with this (no offense to anyone). The point being, that in the construction community, the best methods & designs, are not speculated, they are being put into plans, built, and tested...and they work.
With standard buildings and structures (as opposed to buildings with specific purpose-design), when you create air chambers/pockets and variable layers of sound reducing materials, these greatly reduce sound transmittance, over the use of mass, alone, in wall construction.
This is speaking on first hand involvment with such, and not based on, reading about things on the internet (no offense DIY Chatroom)...