What is STC and OITC?

STC stands for "Sound Transmission Class," a measure of the extent to which sound is prevented from being transferred from one area to another. The higher the STC value, the less that sound can be transferred from one space to another. The STC scale is a logarithmic progression, meaning that, for instance, a jump of 3 points in an STC rating equates to a doubling in a door's ability to prevent sound transmission. Sound retardant doors are tested as a unit comprised of a door, frame, hinges and a sound sealing system. The chart below illustrates the sound retardant performance associated with a range of STC values.

OITC stands for "Outdoor / Indoor Transmission Class," a rating system developed in ASTM E-1332, was formed in the late 1980s to respond to the perceived need for a more robust classification system that addresses more of the low frequency incident sounds. OITC measures the ability of a product to reduce the overall noise of ground and air transportation. The OITC rating is similar to the STC rating in that it uses ASTM E-90 TL data to derive a single number rating system that increases with increasing sound isolation ability.

STC Ratings
STC Value Rating Description
50-60 Excellent Loud speech heard faintly or not at all
40-50 Very Good Loud spech heard faintly, but not understood
35-40 Good Loud speech heard but hardly intelligible
30-35 Fair Loud speech heard and understood fairly well
25-30 Poor Normal speech heard and understood easily and distinctly
20-25 Very Poor Loud speech audible

Sound Control vs. Standard Hollow Metal Doors

How does an acoustic (sound control) door and frame assembly differ from standard hollow metal? An acoustic unit is comprised of a door, door frame, perimeter and bottom seals. The unit is tested in an independent laboratory. The door must be operable and the test standard (ASTM E90-90, ASTM 413-87) compromises tests at 105 decibles over 18 distinct frequencies.