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Sound Transmission Class (STC)
The Sound Transmission Class is a single number rating of the effectiveness of a material or construction assembly to retard the transmission of airborne sound. The sound transmission loss between the source and receiving rooms are plotted on a graph by frequency and sound level in decibels. The STC curve is a sliding contour that is fitted to the performance data plotted in a manner that will allow no more than 32 deficiencies below the appropriate contour. The maximum deficiency at any given frequency shall not exceed 8 decibels Sound Transmission Class Graph.

Once the appropriate contour has been selected the STC is determined by the decibel value of the vertical scale at 500 Hz. The STC is expressed as a single STC number (Example STC 32)

Note that the STC contour is similar to the inverse of the equal loudness contour, insofar as it discounts the lower frequency sounds to reflect how our ears perceive the lower frequency sounds.

25 Normal speech can be understood quite clearly
Sound Ratings

(IIC)
Impact Insulation Class
Single-number rating that indicates the amount of impact noise isolation provided by a floor/ceiling assembly. The higher the number, the quieter the floor/ceiling assembly.

(STC)
Sound Transmission Class
30 Loud speech can be understood fairly well
35 Loud speech audible but not intelligible
42 Loud speech audible as a murmur
45 Must strain to hear loud speech
48 Some loud speech barely audible
50 Loud speech not audible