When a sound wave traveling in a main duct encounters a branch, the sound power contained in the sound wave is divided between the main and branch ducts. This acoustic phenomenon is referred to as branch sound power division.
The portion of sound energy that is transmitted into each branch is associated with the cross sectional area of the main and branch ducts.
If the total areas of the branch and main ducts after the junction are different from the main duct before the junction then there will be a reflected sound wave. The reflected wave travels back up the duct. That sound power is deducted from the overall sound power transmitted into both the branch and main.
The remaining sound energy is divided proportionate the cross sectional area of the branch and main ducts. This sound wave continues down the main and branch ducts.