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.
