OITC meaning in General ?

Sound Transmission Class (or STC) is an integer rating of how well a building partition attenuates airborne sound. In the US, it is widely used to rate interior partitions, ceilings, floors, doors, windows and exterior wall configurations. Outside the US, the Sound Reduction Index (SRI) ISO index is used. The STC rating very roughly reflects the decibel reduction of noise that a partition can provide. The STC is useful for evaluating annoyance due to speech sounds, but not music or machinery noise as these sources contain more low frequency energy than speech.

There are many ways to improve the sound transmission class of a partition, though the two most basic principles are adding mass and increasing the overall thickness. In general, the sound transmission class of a double leaf wall (e.g. two 4"-thick brick walls separated by a 2" airspace) is greater than a single wall of equivalent mass (e.g. homogeneous 8" brick wall).