The Science of Sound PART III: Let’s Talk About STC



Currently, the most widely accepted standard for ranking the acoustical performance of accordion and operable partitions is STC or Sound Transmission Class. Virtually every manufacturer conducts laboratory tests for STC.


Sound Transmission Class is a two-digit number describing the laboratory performance of a single building element in stopping the transmission of sound through it. It is found from individual STL figures of 16 frequencies between 125 and 4000 Hertz.  The STLs are found through the formula described earlier, completely defined by ASTM Standard E90.

The procedure for finding the STC is defined in ASTM E413. Refer to the graph while following the steps below.

  1. Plot the STLs on the graph.
  2. Plot the standard STC curve.
  3. Move this standard curve as high as possible so that:
    1. no frequency’s STL falls below the standard curve by more than 8 dB (Each dB below the curve is called a “deficiency”).
    2. total deficiencies do not exceed 32.
  1. Locate the level on the standard curve corresponding to the 500 Hz frequency. This is the STC.

How Good is an STC Rating? Find out next week! Until then, this may help you.

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