Sound Transmission Class - STC & Sound Reduction Index - SRI & OITC
Building Noise & Sound Transmission Levels & Sound Isolation or Insulation Measurements
SOUND TRANSMISSION CLASS RATINGS - CONTENTS: What is the definition of STC Sound Transmission Class? How is SRI or Sound Reduction Index defined? Table of Sound Transmission Class Ratings - (STC ratings) for various building wall & floor designs
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STC: Sound Transmission Classes:
This article discusses methods for controlling sound transmission through building walls. Here we define STC - sound transmission class and we provide a table of design details that show STC rating for various building wall framing designs. Sound transmission reduction wall designs are given for both single stud and staggered double stud structural and partition walls.
We include soundproofing suggestions for high noise level areas such as music rooms as well as areas where privacy is a concern such as in counseling and psychotherapy offices.
How effectively a wall or floor reduces
airborne sound is measured by STC ratings (sound transmission
class). Roughly speaking, the STC rating equals
the reduction in decibel levels across the partition. (see Table 5-15).
Definition of STC, Definition of SRI , Definition of OITC
Sound Transmission Class (STC) is defined in the ISO-140 standards series. Sound Reduction Index (SRI) is defined in ISO-140 (2012).
Sound Transmission Class (or STC) is an integer rating of how well a building partition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90).
Outside the USA, the Sound Reduction Index (SRI) ISO index or its related indices are used. These are currently (2012) defined in the ISO - 140 series of standards (under revision).
The STC rating figure very roughly reflects the decibel reduction in noise that a partition can provide - Wikipedia 10/21/2013 
STC or sound transmission class is defined as the level of reduction of sound transmission from outside noise sources to the building interior. Higher STC numbers mean higher resistance to sound transmission to the building interior, or as acousticians would describe it, higher STC means greater sound transmission loss between outdoors and the building interior. Typical STC values for metal buildings are STC=20 to STC=55.
OITC or Outdoor-Indoor Transmission Class describes the sound transmission loss properties of building exterior components like windows and walls against noise from traffic, trains, or low flying aircraft. - General Steel Corporation 
So, for example, a 50 dB noise on the other side of an STC 35 wall
will sound like a 15 dB noise to the average listener (see our
Table of STC ratings and noise reduction left).
Walls and floors in the field often measure
lower than in laboratory ratings due to variations in workmanship
as well as leaks and bypasses.
The higher the STC
rating, the more likely it is to be compromised by site
conditions. For that reason, it is best to select a building
assembly rated at least 5 points above the design goal.
Sound Control Design & Materials for Sound Transmission Through Building Walls
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 Marpac, produces white sound generators, a product that they identify as the Marpac sound conditioner. Marpac can be contacted at http://www.marpac.com/ or contact the Marpac Corporation,
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 Wikipedia provided background information about some topics discussed at this website provided this citation is also found in the same article along with a " retrieved on" date. NOTE: because Wikipedia entries are fluid and can be amended in real time, we cite the retrieval date of Wikipedia citations and we do not assert that the information found there is necessarily authoritative. "STC" retrieved 10/21/2013
 General Steel Corporation, "The Facts About the Acoustical Performance of Metal Building Insulation 2", Sound Transmission Class, General Steel Corporation, 10639 W. Bradford Road, Littleton, CO 80127, web search 4/3/11, original source: http://www.gensteel.com/insulation_facts-5a.htm
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