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Abbreviation for Sound Transmission Class. This is a number rating that can be used to compare, in a generalized way, the acoustical isolation of different barrier materials or partition constructions. Higher numbers indicate a material will provide more acoustic isolation when used as a barrier.

The tests conducted to determine STC involves two test rooms: a ”source” room and a ”receiver” room. The source room will contain a full-range test loudspeaker. The receiver room will contain a microphone, which is connected to sound-measuring devices. There is a nominal opening between the two rooms – usually about 9′ wide by 8′ high, but can vary in accordance with the standard.

The first step is to measure the sound transmitted from one room into the other through the opening. The sound is measured in decibels (dB) in 1/3-octave bands from 125 Hz to 4000 Hz. Then the opening is plugged with the material or partition construction. This could be a single layer of barrier, such as plywood or drywall, or a complete wall with as many materials, layers, air gaps, etc. that can fit in the opening. The edges are completely sealed and sound transmission between the rooms is measured again. The sound level from the ”after” test is subtracted from the sound level ”before” plugging the opening. The resulting difference is known as the transmission loss or ”TL.”

Next, the TL is plotted on a graph of 1/3-octave band center frequency versus level (in dB). To get the STC, the measured curve is compared to a reference STC curve. Two criteria are used to ”match” the curves:

1. The reference curve shall not exceed the measured TL by more than 8 dB in any 1/3 octave band, and

2. The sum of all the ”negative discrepancies” shall not exceed 32.

(This actually sounds more complicated than it is. A simple spreadsheet can be used to calculate the STC for any range of TL values.)

Once the two above criteria are met, the value of the reference curve at 500 Hz is read as the entire STC of the material or partition type.

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