A commonly used measure of soundproofing effectiveness is the sound transmission class, or STC. Sound Transmission Class (or STC) is an integer rating of how well a building partition attenuates airborne sound.
A: The STC rating is calculated in accordance with ASTM E413 (Originally published in 1970). It was created to provide a single number rating for interior building partitions that are subjected to noises from speech, television, radio, office equipment and other mid to high frequency noise sources. The higher the number, the better the noise isolation. The STC rating is calculated over the frequency range of 125 to 4000 hertz. The rating is calculated by comparing the measured sound transmission loss to a reference contour curve. STC stands for Sound Transmission Class. Basically, STC ratings are an established way to average how much sound is stopped by something. STC ratings are used for windows, doors, walls and most building materials. For windows, STC ratings range from 18 to 38. STC ratings are the ONLY way to accurately compare various noise reduction products. An STC rating is an instrument measurement of how much noise is stopped. The STC ratings allow accurate ‘apple to apple’ comparisons. The STC rating is the average amount of noise stopped at 18 different frequencies, measured in decibels. STC ratings are a logarithmic scale similar to the earthquake Richter Scale, which means each number is significantly higher than the one before See additional notes below data for specific details. The STC (Sound Transmission Class) rating is a single-number rating system for interior building partitions and viewing windows used to categorize acoustic performance. Its original intent was to quantify interior building partitions not exterior wall components.  STC Rating. A commonly used measure of soundproofing effectiveness is the sound transmission class, or STC. It is a single number that attempts to characterize the overall ability of a material to reduce the transmission of noise. Any reputable product that is marketed as a good soundproofing material will include the STC in its product literature. STC. STC stands for sound transmission class. This is the most common rating used in North America for determining airborne sound transmission loss between 125 and 4,000 Hz. This range covers the majority of common noises we hear including speech, television, music, dogs barking, and other similar annoyances. This rating increases to STC 42-45 with steel studs. The STC rating for a basic wood structure floor/ceiling is 40-43. The STC rating for a six inch concrete sub-floor is 52-55. Sound Transmission Class (or STC) is an integer rating of how well a building partition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90). Rating methodology [edit]. The ASTM test methods have changed every few years. Thus, STC results posted before 1999 may not produce the same results today, and the differences become wider as one goes further back in time–the differences in the applicable test methods between the 1970s and today being quite significant. STC ratings were introduced in 1961 as a way to compare different types of walls, ceilings, floors, doors and windows. By taking the transmission Loss values and testing them at 18 of the most common frequencies (between 125 Hz-4000Hz), a curve is created, which is then compared to the standard STC curves of reference.  The STC rating for a basic insulated 2×4 wall with a single layer of drywall on each side is 35-38. This rating increases to STC 42-45 with steel studs. The STC rating for a basic wood structure floor/ceiling is 40-43. The STC rating for a six inch concrete sub-floor is 52-55.
http://www.archtest.com/testing/faqs.aspx?id=2 http://www.soundproofwindows.com/stc-ratings/ http://www.viracon.com/acoustic http://www.noisehelp.com/stc-rating.html https://isostore.com/comparing-sound-ratings/ https://en.wikipedia.org/wiki/Sound_transmission_class http://www.audimutesoundproofing.com/how-are-stc-ratings-calculated.aspx https://isostore.com/comparing-sound-ratings/
what is STC rating