In general, loud speech can be understood fairly well through an STC 30 wall but should not be audible through an STC 60 wall. An STC of 50 is a common building standard and blocks approximately 50 dB from transmitting through the partition. However, occupants could still be subject to awareness, if not understanding, of loud speech. Constructions with a higher STC (as much as 10dB better - STC 60) should be specified in sensitive areas where sound transmission is a concern.
The Uniform Building Code (UBC) contains requirements for sound isolation for dwelling units in Group-R occupancies (including hotels, motels, apartments, condominiums, monasteries and convents).
UBC requirements for walls: STC rating of 50 (if tested in a laboratory) or 45 (if tested in the field*).
UBC requirements for floor/ceiling assemblies: STC ratings of 50 (if tested in a laboratory) or 45 (if tested in the field*).
* The field test evaluates the dwelling's actual construction and includes all sound paths.
An assembly rated at STC 50 will satisfy the building code requirement. However, as mentioned above, occupants could still be subject to awareness, if not understanding, of loud speech. Therefore, it is typically argued that luxury accommodations require a more stringent design goal.
Rules of Thumb
Weaknesses - What You Should Know
The difference between STC and NRC
STC Ratings for Various Wall Assemblies
STC Ratings for Masonry Walls
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