About Acoustic Ratings

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Acoustic Products

The two rating systems used to measure the acoustical performance of windows and doors are OITC (Outdoor Indoor Transmission Class) and the STC (Sound Transmission Class). Below is a brief description of each rating system.

Outdoor Indoor Transmission Class (OITC)
The OITC rating system was developed as a result of an increased demand for measuring the acoustical performance of exterior building partitions to reduce external sounds. Following the guidance of ASTM E1332 (Standard Classification for Determination of Outdoor/Indoor Transmission Class), the OITC rating is calculated within the frequency range of 80 to 5000 hertz. Because the low and mid-frequency sounds most closely exhibit exterior noises (transportation sounds generally have frequencies around 80 hertz), a logarithmic equation which gives a greater weight to sounds of these lower frequencies is used in determining the OITC rating. The OITC rating can be interpreted on a general level as the A-weighted reduction of sound that a partition provides (measured in decibels).

Sound Transmission Class (STC)
The STC rating was the original rating system developed to measure the sound privacy of building
partitions from a variety of common sounds. While originally developed to measure acoustical performance of interior walls, the STC rating system has expanded to be used in rating external partitions as well. Following the guidance of ASTM E413 (Classification for Rating Sound Insulation), this rating is calculated within the frequency range of 125 to 5000 hertz, which is the range for sounds that are typically most frequent indoors such as speech, television, and office equipment. The test that determines the STC rating measures the transmission-loss (TL) values at sixteen standard frequencies over the tested range, and plots these values on a sound pressure level graph which is then compared to a standard reference contour. The TL curve that is the best fit for the graph of the actual TL values measured determines the STC rating that is granted. On a general level, the STC represents the reduction in sound, measured in decibels (dB), that a partition provides. While the decibel scale is non-linear, a 10 dB reduction in sound can generally result in a sound approximately being heard at one-half of its initial level.