OITC stands for Outdoor-Indoor Transmission Class and is a measure of the ability of a building material or product’s ability to retard sound transmission. It is similar to STC, but it is not the same. As the name indicates, OITC is a measure of sound transmission from the external environment into the building envelope. Testing to establish an OITC value utilizes ASTM E-1332 Standard Classification for the Determination of Outdoor–Indoor Transmission Class. This standard uses a range of frequencies lower than the STC testing, to more closely replicate the sounds of rail and vehicular traffic.