Without successfully addressing these, many of the other costs incurred to achieve acoustic isolation will be a complete waste of money and effort. Acoustic isolation is achieved with great attention paid to small details.
Also, where air must travel freely, e.g. HVAC supply and return air, the path that the air will take must be planned in advance and acoustically treated.
Both supply and return-air ducts must also be protected from “break-in” sound from the surrounding area. If your main supply-air trunk duct is installed in the ceiling above a common hallway, chances are that sounds made in the hallway will “break-into” the main trunk, and individual rooms.
But that’s not the only way airborne sound travels. It also travels into a structure. Here’s how:
Sound travels spherically out from the source through the air in a room until it hits the wall, floor or ceiling. When the sound strikes these partitions, a portion of the energy will be transferred into them and cause them to vibrate. Another portion of the energy will be reflected off of the partition at and back into the room where it then strikes the next one, and so on...
The remaining portion of the sound energy will continue to reflect off of the partitions until the air molecules in the room absorb this last fraction. How long this process takes and how much energy is transferred into the structure is a function of many factors and ingredients – a few examples are listed below.
How loud was the initial sound? Is the room completely air sealed? What are the partitions made of? Are they hard attached to structure? What are the interior acoustic treatments? How long in time did the initial sound last? What is the frequency content of the initial sound?