Sound Transmission Class is a single-number rating that describes the airborne noise isolation quality of a partition (wall or floor-ceiling assembly) between two adjacent rooms. The standardized field test method for STC determination is ASTM Procedure E-336. The test uses a steady broadband sound source in one room, measures the sound levels in both rooms, and finds the difference or Noise Reduction between the two sides. Ambient noise levels in the receiver room are measured to verify that the source is sufficiently loud.
The STC rating is derived by matching the field test data to a standard spectrum contour using rules specified in ASTM Procedure E-413. The analysis is done in sixteen 1/3-octave bands with center frequencies from 125 Hz to 4000 Hz. The reference contour is adjusted until no deficit is more than 8 dB in any band and the sum of all deficits is no more than 32 dB. The STC rating is the value of the reference contour at 500 Hz.
Results of a typical noise test and data analysis are shown below. The upper chart shows sound levels measured in both rooms, along with the ambient sound level measured in the receiver room. The lower chart shows the noise reduction values and the STC reference contour.