Sound Transmission Class (STC): Definition & Meaning

Sound Transmission Class (STC)

A single-number rating that measures how well a window (or wall assembly) blocks airborne sound — higher STC values indicate better sound attenuation.

2 min read · Updated March 2026

Definition

A single-number rating that measures how well a window (or wall assembly) blocks airborne sound — higher STC values indicate better sound attenuation.

The Sound Transmission Class (STC) is a standardized rating that expresses how effectively a window assembly reduces airborne sound transmission across a range of frequencies (125 Hz to 4000 Hz). It is measured by comparing the sound level on the source side to the level on the receiving side in a controlled laboratory test. Higher STC = more sound blocked.

STC Scale for Windows

STC Rating	What You Hear	Window Type
25–27	Normal speech easily understood	Single pane, no seal
28–32	Loud speech audible but not intelligible	Standard double-pane IGU
33–38	Loud speech barely audible	Laminated IGU, triple-pane
39–45	Loud sounds faintly heard	Acoustic laminated IGU
46+	Near-complete sound isolation	Specialty acoustic window systems

How Windows Block Sound

Mass, air gaps, and damping are the three tools for blocking sound:

Mass — heavier glass transmits less sound. Laminated glass (with PVB interlayer) adds effective mass and damping.
Air gap width — wider cavities between panes improve low-frequency performance. The standard 12mm argon gap is optimized for thermal, not acoustic performance. A wider gap (89–150mm) dramatically improves STC.
Asymmetric glass thickness — using different thicknesses for the two panes (e.g., 3mm and 6mm) eliminates the “coincidence dip” frequency where matching-thickness panes transmit sound more efficiently.
Laminated glass — PVB interlayer dampens glass vibration across mid-frequencies where standard glass performs poorly.

Improving Window Sound Performance

If noise is a priority, specify: laminated glass (at minimum the interior pane), asymmetric glass thicknesses, and acoustic-grade PVB interlayer. Secondary glazing (an additional window panel inside the existing frame with a 100mm+ air gap) is the most cost-effective retrofit for maximum sound reduction.