TL was defined as the logarithm
ratio of the acoustic incident energy to acoustic energy transmitted through the wall,
expressed in decibels (dB). Good acoustic insulation is characterized by little energy
being transmitted through the wall. Sound transmission class (STC) was calculated to
determine single-number ratings based upon these measurements according to the
standard ASTM E 413-1987 (1999). The single number rating of sound TL is equal to the
value in decibels of the reference curve at 500 Hz in accordance with the method
specified by the ASTM standard.