How do you evaluate acoustic properties of windows? First, it’s important to understand the rating systems used to gauge performance. Second, you need to be able to put the performance in a context you and your customers can relate to.
» Understanding acoustic rating systems
Acoustics describes the sound control properties of an object. In the building products industry, there are two main rating systems used to communicate how well products block sound: Sound Transmission Class (STC) and Outdoor/Indoor Transmission Class (OITC).
STC: This system has grown to be the most used and recognized method of comparing acoustic performance of building products in the industry. Not because it’s necessarily the most appropriate method, but because it has been in existence far longer than other methods. STC was originally developed to measure the extent of sound control between interior walls (the higher the number, the better the performance). However, asking for or specifying windows with a specific STC rating remains the most effective way to communicate with manufacturers and other professionals involved in the construction process.
OITC: This is a much newer rating system developed specifically to measure the extent to which an exterior wall insulates against outdoor noises in the low to mid frequency ranges: heavy traffic, airplanes, trains, etc. While OITC is often a slightly more appropriate way to evaluate windows, it isn’t nearly as recognized as STC, nor has it been well-embraced by the construction industry. This is because building professionals haven’t fully adopted OITC, and again, STC is better recognized. In the end, both STC and OITC measure the extent of sound transmission loss, and are solid indicators of performance.
» How good is good?
The benefits of windows with a higher STC rating are subjective and sometimes difficult to communicate. There are, however, benchmarks that provide a stable platform for understanding acoustic performance, and more importantly, for managing your customers’ expectations about the extent of sound reduction they can expect.
2x4 exterior wall: A typical 2x4 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.
Single-pane windows: 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.
Dual-pane windows: New dual-pane windows with standard glass fall in the STC 25-27 range. In many situations, this is sufficient, and could dampen sound by as much as 40 percent when replacing single-pane windows with dual-pane. High noise environments may require higher ratings.
» Other benchmarks to consider:
- A 2-point STC improvement is, in many cases, not perceptible. So while STC 34 windows may be sufficient to bring the windows up to the acoustic standard of a typical insulation-filled wall (STC 36), a rating of STC 32 may fall short of that goal.
- A 4-point STC improvement is clearly perceptible.
- A 10-point STC improvement reduces sound by about 50 percent.