Sound Transmission Class (STC) is a single-number rating defined by ASTM E413 that represents how well a partition blocks airborne sound. Higher STC means better sound isolation. A wall with STC 50 blocks noticeably more sound than one rated STC 35.

How is STC calculated from TL values?

Per ASTM E413, a standard reference contour is shifted vertically until two criteria are met: (1) the sum of all deficiencies (where measured TL falls below the contour) does not exceed 32 dB, and (2) no single deficiency exceeds 8 dB. The highest integer contour value satisfying both rules is the STC rating.

What is a deficiency?

A deficiency at a given frequency band is the amount by which the measured TL falls below the STC reference contour at that band. If TL equals or exceeds the contour, the deficiency is zero. Negative deficiencies are not counted.

What is the difference between STC and OITC?

STC (ASTM E413) emphasizes speech frequencies (mid-range) and is the most common rating for interior partitions. OITC (Outdoor-Indoor Transmission Class, ASTM E1332) weights low frequencies more heavily and is used for facades, windows, and doors exposed to traffic or aircraft noise.

What TL values should I enter if I only know the STC?

If you only have an STC rating and want to reverse-engineer TL values, you can use the tabulated STC reference contour values as approximate TL values. However, two different assemblies can share the same STC while having very different TL curves, so the reverse approach is only an estimate.

What minimum STC is required for apartment walls?

The International Building Code (IBC) requires a minimum STC of 50 (laboratory) or 45 (field, FIIC) for walls and floor-ceiling assemblies separating dwelling units in multifamily residential buildings. Many jurisdictions adopt these minimums or add local amendments.

Sound Transmission Class Calculator

Name: Sound Transmission Class Calculator
Availability: InStock
Author: Nham Vu

Enter octave-band transmission loss values to compute an STC rating, or look up minimum STC requirements by building type and space separation.

Transmission Loss Values

Enter measured TL (dB) at each one-third octave band center frequency. Leave blank or enter 0 for unknown bands.

Freq (Hz)	TL (dB)	Contour

Result

Enter TL values and click Calculate

Minimum STC Requirements

Reference values based on IBC, HUD guidelines, and ASTM E413. Verify with your local jurisdiction before design.

Building Type

Separation Type

Building Type	Separation	Assembly	Min STC	Standard / Source

Summary