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What is STC?

When building your home recording studio one of the important factors you have to deal with is the sound transmission. To determine their soundproofing quality materials are tested for sound transmission loss values, that can be measured, and rated in a single-number rating system – Sound Transmission Class (STC). The STC rating figure very roughly reflects the decibel reduction in noise that a partition can provide.

The Sound Transmission Class (STC) is a single-number rating of a material’s or an assembly’s ability to resist airborne sound transfer at the frequencies 125-4000 Hz. These transmission-loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour. Acoustical engineers fit these values to the appropriate TL Curve (or Transmission Loss) to determine an STC rating. The measurement is accurate for speech sounds, but less so for amplified music, mechanical equipment noise, transportation noise or any sound with substantial low-frequency energy below 125 Hz. Sometimes, acoustical labs will measure TL at frequencies below the normal STC boundary of 125 Hz, possibly down to 50 Hz or lower, thus giving additional valuable data to evaluate transmission loss at very low frequencies, such as a subwoofer-rich home theater system would produce.

In general, a barrier with higher STC rating blocks more noise from transmitting through a partition. In the USA, STC is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (ASTM Standards). Outside the USA, the Sound Reduction Index (SRI) ISO standard is used. The ASTM test methods have changed every few years and over many years have been changed significantly. Thus, STC results posted before 1999 may not produce the same results today, and this difference becomes wider as one goes back in time.

It must be noted that acoustical performance values such as STC are measured in specially constructed acoustical chambers, and that field conditions such as lack of adequate sealing, outlet boxes, back-to-back electrical boxes, medicine cabinets, flanking paths, and structure-borne sound can diminish acoustical performance. The as-built ‘field-STC’ (FSTC) is usually lower than the laboratory-measured STC. Nevertheless STC ratings are the ONLY way to accurately compare various noise reduction products. The STC ratings allow accurate ‘apple to apple’ comparison of materials for soundproofing.

For example loud speech can be understood fairly well through an STC 30 wall but should not be audible through an STC 60 wall regardless of the material the wall is constructed from.

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