STC Ratings Explained: What They Mean for Soundproof Windows

If you’ve ever shopped for soundproof windows, you’ve almost certainly seen the term STC rating — but what does it actually mean, and how do you know what number you need? This guide breaks it all down in plain language, including a full STC reference chart, how ratings are tested, what the numbers mean in real-world terms, and what to look for when choosing soundproof windows for your home.

What Does STC Stand For?

STC stands for Sound Transmission Class. It’s a standardized rating system developed in the 1960s by the American Society for Testing and Materials (ASTM) to measure how well a building material — a wall, window, door, or floor — blocks airborne sound.

The higher the STC number, the more sound the material blocks. It’s the most widely used acoustic rating in the residential and commercial construction industries, and it’s the primary metric manufacturers use to compare soundproofing products.

STC was originally designed to measure sound reduction in the range of human speech (125 Hz to 4,000 Hz). This makes it very useful for evaluating how well a window or wall blocks voices, TV noise, and mid-range traffic sounds — though it has some limitations with very low-frequency noise, which we’ll cover below.

How Is an STC Rating Measured?

STC testing is done in a controlled lab environment using two adjacent rooms separated by the material being tested. Here’s how it works:

• Sound is generated inside one room across 16 to 18 standard frequencies, ranging from 125 Hz to 4,000 Hz.
• Transmission loss is recorded — the difference in decibels between the sound level on the source side and the sound level on the receiving side.
• The results are plotted on a graph and compared against standard STC reference curves.
• The closest matching curve determines the final STC rating.

For example, if a window generates 70 dB on one side and only 20 dB passes through to the other, that’s a transmission loss of 50 dB — producing an STC rating of approximately 50.

Because STC is measured on a logarithmic scale (similar to the Richter scale for earthquakes), each point increase represents a meaningful improvement. A 10-point jump in STC cuts perceived loudness roughly in half.

STC Rating Chart for Windows

The table below shows how STC ratings translate to real-world performance, with typical examples at each level:

To put this in context: a standard interior wall in most homes rates around STC 33. CitiQuiet soundproof windows achieve STC 48–54, which means they outperform the average wall — making the window no longer the weakest acoustic link in your home.

STC Ratings for Different Window Types

Not all windows are created equal. Here’s how common window types compare on the STC scale:

Single-Pane Windows (STC 26–28)

Standard single-pane windows provide almost no meaningful sound reduction. At STC 26–28, outside noise is fully present — traffic, voices, and city sounds pass through with very little attenuation. Most older NYC apartments have single-pane windows.

Double-Pane Windows (STC 26–33)

Contrary to what many people expect, standard double-pane windows don’t dramatically reduce noise. They’re designed primarily for energy efficiency, not sound reduction. The air gap between the panes helps somewhat, but at STC 26–33, the improvement over single-pane is modest at best. You’ll still clearly hear street noise, traffic, and neighbors.

CitiQuiet Soundproof Windows (STC 48–54)

CitiQuiet interior windows achieve STC ratings of 48 to 54 — whether installed over single-pane or double-pane windows. At this level, the difference is dramatic and immediately noticeable. Most outside noise is reduced to a faint background presence, and many sounds disappear entirely.

This rating is achieved through a combination of laminated glass, precision custom-fit frames, and the additional air space created between the CitiQuiet window and your existing window — all of which work together to absorb and block sound at multiple points.

STC vs. OITC: What’s the Difference?

STC is the most common rating you’ll see, but there’s a second rating worth knowing: OITC (Outdoor/Indoor Transmission Class). Developed more recently, OITC tests a wider frequency range — from 80 Hz to 4,000 Hz — specifically to account for low-frequency outdoor noise like:

• Heavy truck traffic and diesel engines
• Subway and train rumble
• Aircraft flyovers
• Bass-heavy music from bars or venues

Because STC doesn’t measure below 125 Hz, it can sometimes overstate a window’s effectiveness against the deep rumble of a subway or the bass from a nearby nightclub. For urban environments like New York City, OITC can be a more accurate predictor of real-world performance for these specific noise types.

If low-frequency noise is your primary concern — a subway line running nearby, or a busy truck route — ask about OITC ratings in addition to STC.

What STC Rating Do You Actually Need?

The right STC rating depends on your noise environment. Here’s a practical guide:

• STC 35–39: Suitable for relatively quiet streets or suburban areas where you want some noise reduction but the baseline noise level is already low.
• STC 40–44: A reasonable target for moderate urban noise — a busy but not major street, occasional construction, or a mid-floor apartment.
• STC 45–49: Recommended for heavy urban noise environments — major avenues, near highways, or lower floors where street noise is constant.
• STC 50+: The target for serious noise problems — living near an airport, subway, or in a building facing a major intersection. CitiQuiet’s STC 48–54 range puts you squarely here.

An important rule of thumb: soundproofing is only as strong as its weakest point. A high-STC window won’t fully solve a noise problem if sound is also entering through poorly sealed doors, thin walls, or gaps around frames. That’s one reason CitiQuiet includes a complimentary on-site consultation — to assess your entire noise environment, not just the windows.

The Limitations of STC Ratings

STC is the industry standard, but it’s worth understanding what it doesn’t capture:

• Low-frequency sound: STC doesn’t test below 125 Hz, so it may not fully reflect performance against subway rumble, bass music, or heavy truck traffic. Use OITC ratings for these scenarios.
• Real-world vs. lab conditions: STC is tested under ideal conditions. Real-world performance depends heavily on installation quality — even a small gap around a frame can significantly reduce effectiveness.
• The whole-system effect: STC measures a single product in isolation. Your home’s actual acoustic performance depends on the combined STC of all its components — windows, walls, doors, floors, and ceilings.

This is why working with an experienced installer who assesses your specific space matters as much as the product’s rating on paper.

How CitiQuiet Achieves Its STC Ratings

CitiQuiet’s STC 48–54 performance is the result of several design elements working together:

• Laminated glass: Unlike standard glass, CitiQuiet’s laminated glass contains an interlayer that absorbs sound vibration rather than transmitting it through the pane.
• Custom-fit frames: Every CitiQuiet window is built to the exact dimensions of your existing window, eliminating gaps where sound can leak through. No off-the-shelf sizing.
• Air space amplification: The gap created between the CitiQuiet window and your existing window acts as an additional acoustic buffer, enhancing the overall STC of the combined assembly.
• Interior installation: CitiQuiet installs on the interior side of your existing window — no structural changes, no replacement of your existing frame. The result is a two-window system that dramatically outperforms either window alone.

The combined effect: up to 95% of outside noise blocked, an interior that feels genuinely quiet, and installation completed in as little as 7 days from order.

Frequently Asked Questions about STC Ratings

Ready to experience the difference a high-STC window makes? Schedule your complimentary on-site consultation — or call us at (212) 874-5362. We’ll assess your home’s noise environment and recommend the right CitiQuiet solution for your space.