This is achieved by making seams airtight and closing all sound leaks. To eliminate air borne sound all air paths between the areas must be eliminated. Sound travels through both the air and structure, and both paths must be considered when designing sound isolating walls and ceilings. There are nearly infinite field conditions that will affect sound isolation on site when designing building partitions and enclosures.įactors affecting sound transmission class Acoustic medium Sound isolation metrics, such as the STC, are measured in specially-isolated and designed laboratory test chambers. 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. The STC number is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz. Current Sound Transmission Class Report Sample from NTi Audio showing Transmission Loss in the sixteen standard frequencies The STC standard curve is based on European studies of multi-family residential construction, and closely resembles the sound isolation performance of a 9"-thick brick wall. In 1961, the ASTM International Standards Organization adopted E90-61T, which served as the basis for the STC method used today. This method is valuable at comparing homogeneous partitions that follow the mass law, but can be misleading when comparing complex or multi-leaf walls. 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. 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. Good soundproofing most sounds do not disturb neighboring residents. Generally, a difference of one or two STC points between similar constructions is subjectively insignificant. The table below is used to determine the degree of sound isolation provided by typical multi-family construction. Through research, acousticians have developed tables that pair a given STC rating with a subjective experience. The term "sound proofing" is typically avoided in architectural acoustics as it is a misnomer and connotes inaudibility. Sound Isolation and Sound Insulation are used interchangeably, though the term "Insulation" is preferred outside the US. ASTM E413 can also be used to determine similar ratings from field measurements performed in accordance with ASTM E336. The STC is a standardized rating provided by ASTM E413 based on laboratory measurements performed in accordance with ASRM E90. A higher number indicates more effective sound insulation than a lower number. The STC provides a standardized way to compare products such as doors and windows made by competing manufacturers. The STC or sound transmission class is a single number method of rating how well wall partitions reduce sound transmission. two 4"-thick block walls separated by a 2" airspace) is greater than a single wall of equivalent mass (e.g. In general, the sound transmission class of a double wythe wall (e.g. 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. 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. The STC rating very roughly reflects the decibel reduction of noise that a partition can provide. Outside the US, the ISO Sound Reduction Index (SRI) is used. In the US, it is widely used to rate interior partitions, ceilings, floors, doors, windows and exterior wall configurations. Sound Transmission Class (or STC) is an integer rating of how well a building partition attenuates airborne sound.
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