When researching soundproofing, you'll encounter "STC" ratings. Understanding what they mean helps you evaluate materials, compare products, and understand what to expect from your apartment's construction.
What Is STC?
STC stands for Sound Transmission Class. It's a standardized rating that measures how well a building element (wall, floor, door, window) blocks airborne sound. Higher numbers mean better sound blocking.
The rating is determined by laboratory testing across a range of frequencies. The result is a single number that represents overall performance.
STC Ratings in Practice
| STC | What You'll Hear | Typical Construction |
|---|---|---|
| 25 | Normal speech easily understood | Single-pane window |
| 30 | Loud speech understood fairly well | Hollow-core door |
| 35 | Loud speech heard but not understood | Basic interior wall |
| 40 | Loud speech heard as murmur | Standard apartment wall |
| 45 | Must strain to hear loud speech | Good apartment wall |
| 50 | Loud sounds barely heard | Enhanced construction |
| 55+ | Most sounds blocked | Luxury/specialized |
Typical STC Values
Walls
- Single drywall on wood studs: STC 33-35
- Double drywall on wood studs: STC 38-40
- With insulation added: Add 4-6 points
- With resilient channels: Add 5-8 points
- Concrete block: STC 40-50 depending on thickness
Floors/Ceilings
- Basic wood frame: STC 35-40
- With carpet above: STC 45-50
- Concrete slab: STC 45-55
Windows
- Single pane: STC 26-28
- Standard double pane: STC 28-32
- Laminated glass: STC 34-38
- Specialized acoustic windows: STC 40+
Doors
- Hollow-core interior: STC 20-25
- Solid-core interior: STC 30-35
- Solid exterior door: STC 35-40
- Acoustic door: STC 45+
Limitations of STC
STC has some important limitations:
Doesn't Measure Low Frequencies Well
STC focuses on speech frequencies (125-4000 Hz). Bass frequencies are not well represented. A wall with high STC might still let bass through.
Lab vs Reality
STC is measured in controlled lab conditions. Real-world performance is often lower due to flanking paths, gaps, and imperfect installation.
Doesn't Measure Impact Noise
STC only rates airborne sound transmission. For impact noise (footsteps), look for IIC (Impact Insulation Class) ratings.
IIC: Impact Insulation Class
IIC measures how well a floor/ceiling assembly blocks impact noise. Like STC, higher is better.
- IIC 30-40: Footsteps clearly audible
- IIC 45-50: Footsteps audible but muffled
- IIC 55-60: Footsteps faint
- IIC 65+: Footsteps barely perceptible
Building codes often require IIC 50 minimum for floors between units, but this still allows considerable noise transfer.
What Does This Mean for You?
If you're apartment hunting, ask about STC/IIC ratings for walls and floors between units. Values below 50 for either mean you'll likely hear neighbors to some degree.
If you're trying to improve existing construction, understand that going from STC 35 to STC 50 is a substantial improvement requiring significant intervention—not something achieved with curtains and rugs.
The 10-Point Rule
Each 10-point increase in STC roughly halves the perceived loudness. Going from STC 30 to STC 40 makes a noticeable difference; from 40 to 50 is another significant improvement.