dwelling units and public and service areas must achieve STC 50 where tested in accordance with ASTM E90, or NNIC 45 if field tested in accordance with ASTM E336. However, not all jurisdictions use the IBC for their building or municipal code. Common partition STC Interior walls with 1 sheet of 1/2" (13 mm) gypsum wallboard (drywall) on either side of 2x4 (90 mm) wood studs spaced 16" (406 mm) on-center with no fiberglass insulation filling each stud cavity have an STC of about 33. When asked to rate their acoustical performance, people often describe these walls as "paper thin." They offer little in the way of privacy. Double stud partition walls are typically constructed with varying gypsum wallboard panel layers attached to both sides of double 2x4 (90 mm) wood studs spaced 16" (406 mm) on-center and separated by a 1" (25 mm) airspace. These walls vary in sound isolation performance from the mid STC-40s into the high STC-60s depending on the presence of insulation and the gypsum wallboard type and quantity. Commercial buildings are typically constructed using steel studs of varying widths, gauges, and on-center spacings. Each of these framing characteristics have an effect on the sound isolation of the partition to varying degrees. STC prediction There are several commercially available software which predict the STC ratings of partitions using a combination of theoretical models and empirically-derived lab data. These programs can predict STC ratings within several points of a tested partition and are an approximation at best. Outdoor-Indoor Transmission Class (OITC) The Outdoor–Indoor Transmission Class (OITC) is a standard used for indicating the rate of sound transmission from outdoor noise sources into a building. It is based on the ASTM E-1332 Standard Classification for Rating Outdoor-Indoor Sound Attenuation. Unlike the STC, which is based on a noise spectrum targeting speech sounds, OITC uses a source noise spectrum that considers frequencies down to 80 Hz (aircraft/rail/truck traffic) and is weighted more to lower frequencies. The OITC value is typically used to rate, evaluate, and select exterior glazing assemblies. See also References Bibliography Construction Noise control