Codes & Testing :: Sound Transmission Class (STC)

Code: STC rates a partition's or material's ability to block airborne sound.

Enforcement: Appendix Chapter 35 of the ’88 and ’91 UBC, Appendix Chapter 12, Division II of the ’94 and ’97 UBC will be contained in the forthcoming IBC. Although not all municipalities have adopted this appendix chapter, it is still recognized as an industry standard.

General Information: 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.


  • Sound Transmission Class rates a partition’s resistance to airborne sound transfer at the speech frequencies (125-4000 Hz). The higher the number, the better the isolation.

STC Strength: Classifies an assembly’s resistance to airborne sound transmission in a single number.

STC Weakness: This rating only assesses isolation in the speech frequencies and provides no evaluation of the barrier’s ability to block low frequency noise, such as the bass in music or the noise of some mechanical equipment.

Recommended Isolation Level
An assembly rated at STC 50 will satisfy the building code requirement, however, residents could still be subject to awareness, if not understanding, of loud speech. It is typically argued that luxury accommodations require a more stringent design goal (as much as 10dB better – STC 60). Regardless of what STC is selected, all air-gaps and penetrations must be carefully controlled and sealed. Even a small air-gap can degrade the isolation integrity of an assembly.

This code is often used in conjunction with Impact Insulation Class (IIC).

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