The Sound Transmission Class (STC) rating is an integer value that indicates how well a building partition attenuates airborne sound.
In more detail:
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Definition: STC is a single-number rating that measures the ability of a building element (like a wall, floor, door, or window) to reduce the transmission of sound. It's primarily used for speech sounds but can offer a general indication of sound isolation performance.
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How it Works: The STC rating is derived from sound attenuation values measured at different frequencies. These measurements are then compared to a standard contour. The higher the STC rating, the better the sound isolation performance. A higher STC rating indicates that more sound is blocked by the partition.
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Usage: STC ratings are commonly used in the United States to evaluate and specify the sound isolation performance of building components.
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Typical Values:
- 25: Normal speech can be easily understood through the barrier.
- 30: Loud speech can be understood fairly well.
- 35: Loud speech audible but not intelligible.
- 40: Loud speech barely audible.
- 45: Must strain to hear loud speech.
- 50: Loud sounds inaudible.
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Limitations: STC is most accurate for frequencies in the speech range (125 Hz to 4000 Hz). It may not accurately reflect performance at very low frequencies (like bass sounds from music or home theaters) or very high frequencies. It's also important to note that STC only measures airborne sound transmission and doesn't account for impact sound transmission (e.g., footsteps on a floor), which is rated using the Impact Insulation Class (IIC).
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International Equivalent: Outside the US, the Sound Reduction Index (SRI), determined according to ISO standards, is used.
