Sound Transmission Class (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 ISO Sound Reduction Index (SRI) 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 wythe wall (e.g. two 4-inch-thick [100 mm] block walls separated by a 2-inch [51 mm] airspace) is greater than a single wall of equivalent mass (e.g. homogeneous 8-inch [200 mm] block wall).
Definition The STC or sound transmission class is a single number method of rating how well wall partitions reduce sound transmission. The STC provides a standardized way to compare products such as doors and windows made by competing manufacturers. A higher number indicates more effective sound insulation than a lower number. The STC is a standardized rating provided by ASTM E413 based on laboratory measurements performed in accordance with ASRM E90. ASTM E413 can also be used to determine similar ratings from field measurements performed in accordance with ASTM E336. Sound Isolation and Sound Insulation are used interchangeably, though the term "Insulation" is preferred outside the US. The term "sound proofing" is typically avoided in architectural acoustics as it is a misnomer and connotes inaudibility.
Subjective correlation Through research, acousticians have developed tables that pair a given STC rating with a subjective experience. The table below is used to determine the degree of sound isolation provided by typical multi-family construction. Generally, a difference of one or two STC points between similar constructions is subjectively insignificant.
Tables like the one above are highly dependent on the background noise levels in the receiving room: the louder the background noise, the greater the perceived sound isolation.
Rating methodology
Historical Prior to the STC rating, the sound isolation performance of a partition was measured and reported as the average transmission loss of over the frequency range 128 to 4096 Hz or 256 to 1021 Hz. This method is valuable at comparing homogeneous partitions that follow the mass law, but can be misleading when comparing complex or multi-leaf walls. In 1961, the ASTM International Standards Organization adopted E90-61T, which served as the basis for the STC method used today. The STC standard curve is based on European studies of multi-family residential construction, and closely resembles the sound isolation performance of a 9-inch-thick (230 mm) brick wall.
Current
The STC number is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz. These Transmission Loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour provided by the ASTM. Sound isolation metrics, such as the STC, are measured in specially-isolated and designed laboratory test chambers. There are nearly infinite field conditions that will affect sound isolation on site when designing building partitions and enclosures.
Factors affecting sound transmission class
Acoustic medium Sound travels through both the air and structure, and both paths must be considered when designing sound isolating walls and ceilings. To eliminate air borne sound all air paths between the areas must be eliminated. This is achieved by making seams airtight and closing all sound leaks. To eliminate structure-borne noise one must create isolation systems that reduce mechanical connections between those structures.
Mass Adding mass to a partition reduces the transmission of sound. This is often achieved by adding additional layers of gypsum. It is preferable to have non symmetrical leaves, for example with different thickness gypsum. The effect of adding multiple layers of gypsum wallboard to a frame also varies depending on the framing type and configuration. Doubling the mass of a partition does not double the STC, as the STC is calculated from a non-linear decibel sound transmission loss measurement. So, whereas installing an additional layer of gypsum wallboard to a light-gauge (25-ga. or lighter) steel stud partition will result in about a 5 STC-point increase, doing the same on single wood or single heavy-gauge steel will result in only 2 to 3 additional STC points. Adding a second additional layer (to the already three-layer system) does not result in as drastic an STC change as the first additional layer. The effect of additional gypsum wallboard layers on double- and staggered-stud partitions is similar to that of light-gauge steel partitions. Due to increased mass, poured concrete and concrete blocks typically achieve higher STC values (in the mid STC 40s to the mid STC 50s) than equally thick framed walls. However the additional weight, added complexity of construction, and poor thermal insulation tend to limit masonry wall partitions as a viable sound isolation solution in many building construction projects. In recent years, gypsum board manufacturers have started to offer lightweight drywall board: Normal-weight gypsum has a nominal density of 43 pounds per cubic foot (690 kg/m3), and lightweight drywall has a nominal density of 36 pounds per cubic foot (580 kg/m3). This does not have a large effect on the STC rating, though lightweight gypsum can significantly degrade the low frequency performance of a partition as compared to normal weight gypsum.
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