ACOUSTIC TRANSMISSION COEFFICIENTS ACCORDING TO ASTM E413 STANDARD
ASTM is the American Society for Testing and Materials, its technical standards are mainly used in the USA but enjoy international prestige.
ASTM C423 is a standard for the classification of the acoustic transmission of materials and, consequently, of their soundproofing properties.
Sound Transmission Class (STC) coefficients
They are single-digit ratings that indicate how an element that divides two rooms is effective in reducing sound transmission of from one side to the other.
> 60 - Good soundproofing: most noises do not disturb
. 50 - Loud noises are faintly audible
. 45 - A loud speech is heard far away
. 40 - A loud speech is perceived as a murmur
. 35 - A loud speech is audible but not understandable
. 30 - A loud speech is understood
. 25 - Normal speech is understood