You will encounter STC ratings on windows, doors, walls, and acoustic products. The number looks precise and trustworthy, but without context it is easy to misread what it actually tells you about noise reduction performance.
What STC Stands For
STC stands for Sound Transmission Class. It is a single-number rating that summarizes how well a building partition — a wall, floor, door, or window — blocks airborne sound. Higher numbers indicate better sound blocking. The rating is established through standardized laboratory testing defined by ASTM International.
How the Number Is Calculated
During testing, sound is played in one room at a range of frequencies from 125 Hz to 4000 Hz. Microphones in both rooms measure how much sound passes through the partition at each frequency. The resulting data points are compared to a standardized STC contour curve. The STC value is the number at which the test curve best fits the contour — essentially a weighted average that emphasizes the frequencies most important for speech intelligibility.
What Different STC Values Mean in Practice
STC 25: Normal speech is clearly audible through the partition. A typical hollow-core interior door falls in this range.
STC 35: Loud speech is audible but not intelligible. You can hear someone is talking but cannot make out the words.
STC 42: Loud speech is barely audible. Music is faintly heard. A well-constructed standard wall with drywall on both sides reaches approximately this level.
STC 50: Very loud sounds are faintly heard. This requires double drywall, resilient channel, and acoustic insulation.
STC 60+: Very little sound transmission. Requires sophisticated construction: double stud walls, multiple drywall layers, specialty decoupling systems.
The Limitation of STC: It Ignores Bass
STC ratings are weighted toward mid and high frequencies — the frequency range of human speech. They do not measure low-frequency performance well. A wall with an excellent STC of 55 can still transmit significant bass from music, subwoofers, and HVAC equipment because those low frequencies are not captured in the STC calculation.
For noise sources with significant bass content — a home theater, a music studio, a bedroom above a nightclub — look for OITC (Outdoor-Indoor Transmission Class) ratings instead. OITC gives more weight to low frequencies and provides a more realistic picture of performance in real-world urban environments.
Lab Ratings vs. Field Performance
STC ratings are measured in controlled laboratory conditions. Real-world installations almost always underperform lab ratings by 3 to 7 points — a difference called the Field STC or FSTC. Flanking paths (sound traveling around a partition through the floor, ceiling, or adjoining walls), installation gaps, and penetrations for electrical outlets and pipes all reduce field performance.
When planning a project, subtract 5 from any STC rating to estimate realistic field performance. A product rated STC 50 in the lab will likely perform at STC 45 in practice if installed carefully, or lower if installation is imperfect.
Using STC Ratings to Compare Products
STC ratings are most useful for comparing similar products within a category. A window rated STC 40 is meaningfully better than one rated STC 30. Use STC to make relative comparisons, not to predict exact noise reduction in decibels. The decibel reduction you experience will depend on the room’s acoustic conditions, the noise source spectrum, and the quality of installation around the product.