When discussing sound transmission ratings for windows and doors, there are two different scales. In this post, we’ll look at Sound Transmission Class (STC) and Outdoor-Indoor Transmission Class (OITC) ratings and what they mean.
Sound Transmission Loss (TL)
A door’s ability to reduce noise is called its TL effectiveness. TL is a decibel value, determined by measuring sound pressure levels and certain frequencies. It also accounts for reverberation time (the length of time required for sound to decay 60 decibels from its initial level). The higher the TL, the better the door or window is at reducing noise transmission.
Because TL ratings are based on a range of frequencies, it’s hard to know how accurate they are. STC ratings provide a single value of the acoustic performance of a door. STC is a weighted average of TL values over 16 frequencies. The higher the STC value, the better the performance.
We can divide STCs for interior doors into groupings:
- STC 40-60: Best (loud sounds or speech heard faintly)
- STC 25-40: Better (loud speech heard fairly well)
- STC 20-25: Good (low speech audible)
Sound transmission ratings: The difference between STC and OITC
OITC is much newer than STC, originating in 1990. It also emphasizes the transmission of street sounds (horns, sirens, airplanes) through exterior walls, windows and façade elements. STC doesn’t focus on specific kinds of sounds, only how they transmitted through walls, doors and windows. OITC is not universally adopted, so STC ratings are still more prevalent.
Professionals have been using the STC rating system to measure sound transmission for decades. Originally evaluating transmission through interior walls only, you can use STC to assess almost any type of barriers: exterior walls, interior walls, windows and doors.
Exterior noise tends to be a lower frequency than interior noise (such as voices), so the OITC rating system emphasizes low-frequency sounds in its calculations. Professionals use OITC less frequently than the STC.
How OITC is measured
Like the STC rating System, OITC measures sound intensity loss in decibels. If a 105-decibel above-ground subway only registers as 80 decibels after traveling through a window, the sound experiences a 25-decibel deficit. The ability of a barrier to create a specific decibel deficit varies according to the frequency of the sound passing through it. In general, very high and very low frequency sounds are more difficult to block.
A barrier’s OITC rating is measured using data gathered over an 80 to 4000 hertz frequency range. After data collection, testers calculate the barrier’s OITC rating in accordance with standards laid out by the American Society for Testing and Materials (ASTM). Typically, a barrier’s OITC rating is lower than its STC rating.
What do STC and OITC ratings really mean
Ratings of sound transmission can be subjective. There are benchmarks that provide a standard platform for deciphering acoustic performance. Here are some examples:
2×4 exterior wall
A typical 2×4 wall with insulation in a stick-built home has an STC rating of around 36. Unless the entire structure is engineered for sound control, there is little value to purchasing windows with a higher rating than the wall. If the windows rate higher than the wall, sound enters the structure through the walls. Additionally, if the structure is old and not well-designed, sound-control windows may do little to nothing to improve the situation.
Existing single-pane windows often have an STC rating between 18 and 20. Replacing old single-pane units with new acoustic windows will likely have a noticeable effect.
New dual-pane windows with standard glass fall in the STC 25-27 range. In many situations, this can reduce sound by as much as 40 percent when replacing single-pane windows with dual-pane. High-noise environments may require higher ratings.
When building or remodeling a space, it’s important to consider how sound will travel and where the weak spots might be. By looking at STC and OITC ratings, you can choose the windows and doors that best fit your needs.