STC and OITC are not the same number.
Which one matters next to a rail line, and why the glass decides both.
Vistaza Engineering · Jul 8, 2020 · Updated Sep 18, 2026 · 3 min read
Both numbers say how much sound a window stops. Both come from the same laboratory measurement. They are not interchangeable. For a window that faces a street or a rail line, read the OITC first.
Two ratings from one test.
In the lab, the window is built into a wall between two rooms. Sound is played on one side and measured on the other. The drop in level, in decibels, is recorded band by band from low frequencies to high. That set of values is the transmission loss.
A simple picture of it: a commuter train measures 105 dB on the platform and 80 dB inside the building next to it. The facade took out 25 dB.
STC and OITC are two ways of turning that curve into a single number.
| Rating | STC | OITC |
|---|---|---|
| Full name | Sound Transmission Class | Outdoor-Indoor Transmission Class |
| Frequency range | 125 to 4,000 Hz | 80 to 4,000 Hz |
| Written for | Interior walls, floors, ceilings, doors | Exterior walls, windows, doors |
| Leans toward | Speech and everyday indoor noise | Traffic, construction, rail, aircraft |
Why OITC is the lower number.
STC was written for partitions inside a building. It starts at 125 Hz and follows the range of speech. OITC starts lower, at 80 Hz, and gives more weight to the low end. That is where trucks, buses, trains and aircraft put most of their energy. Low-frequency sound travels farther and is harder to block, and glass is at its weakest there. So the same window scores lower on OITC than on STC.
On a Vi6 casement the gap runs from five to eight points, depending on the glass.
| Glass on a Vi6 casement | STC | OITC |
|---|---|---|
| Standard insulated double-pane | 33 | 27 |
| Double-pane with lites of unequal thickness | 39 | 34 |
| Laminated acoustic double-pane, wider cavity | 46 | 38 |
| Laminated acoustic triple-pane | 47 | 40 |
Look at the second and third rows. STC jumps seven points. OITC moves four. Now the last two rows: STC moves one point, OITC moves two. A specification that names only STC cannot tell you which of these windows is quieter beside a highway.
Next to a rail line, read the OITC. It was written for that noise.
The glass decides both.
The frame and the seals have to be tight. Sound takes any path that air can take, so a window that leaks air leaks noise. Once the frame is tight, the glass sets the rating. Four things in the glass move the number:
- Thickness. Heavier glass stops more sound.
- Unequal panes. Two panes of different thickness cover a wider range of frequencies than two panes that match.
- Laminated glass. Two layers of glass bonded to a plastic interlayer. The interlayer damps the vibration that carries sound through the pane.
- The number of panes and the space between them. A wider cavity and a third pane both help, as the table shows.
Acoustic glass does not have to cost you the energy rating. One Vi6 casement glass is both NFRC-certified at U 0.18 and tested at OITC 40.
How to specify it.
- Get the required rating from the acoustical consultant, and check which rating it is. If the noise source is outside the building, the requirement should be in OITC, or in both.
- Ask for the lab report for the whole window, in the type you are buying. A fixed window and a casement with the same glass do not test the same. A glass-only number is not a window rating.
- Put the rating in the construction documents by elevation. The side facing the tracks may need the acoustic triple. The courtyard side may not.
- Check the perimeter on site. Gaps and voids around the frame let sound past the best glass. The joint between window and wall has to be filled and sealed.
What not to rely on.
Some product pages promise to remove a percentage of outside noise. A percentage is not a rating, and it cannot be checked. Ask for STC and OITC, the report number, and the glass makeup that was tested.
Be careful with STC ranges quoted for secondary windows fitted behind an existing window. Those are two windows and a deep air space, tested together. They do not describe a single new window in a new opening.
The tested sound ratings for every Vistaza type are on the sound page. To see which glass carries which rating, and what it does to the U-factor, use the glass finder.