Sound Transmission Class (STC) represents a numerical classification indicating the efficacy with which a building partition attenuates airborne sound. Within the United States, this metric is extensively employed for rating interior partitions, ceilings, floors, doors, windows, and exterior wall configurations. Conversely, outside the US, the ISO Sound Reduction Index (SRI) is utilized. The STC rating approximately indicates the decibel reduction of noise that a partition can achieve. While effective for assessing discomfort caused by speech sounds, the STC is less suitable for music or machinery noise, as these sources typically contain more low-frequency energy than human speech.
Numerous strategies exist for enhancing a partition's sound transmission class, with the fundamental principles involving the addition of mass and an increase in overall thickness. Typically, 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) surpasses that of a single wall possessing equivalent mass (e.g., a homogeneous 8-inch [200 mm] block wall).
Definition
The STC, or sound transmission class, is a single-number metric for evaluating the effectiveness of wall partitions in reducing sound transmission. This rating provides a standardized method for comparing products, such as doors and windows, from various manufacturers. A higher numerical value signifies enhanced sound insulation performance compared to a lower value. The STC is a standardized rating established by ASTM E413, based on laboratory measurements conducted in accordance with ASTM E90. ASTM E413 can also be applied to derive similar ratings from field measurements performed according to ASTM E336.
The terms "Sound Isolation" and "Sound Insulation" are often employed synonymously, although "Insulation" is generally favored internationally. The term "soundproofing" is typically eschewed in architectural acoustics due to its inaccuracy and the implication of absolute inaudibility.
Subjective Correlation
Empirical investigations have enabled acousticians to develop tables that correlate specific STC values with perceived acoustic performance. Such tables are utilized to assess the degree of sound isolation provided by typical multi-family construction. Generally, a difference of one or two STC points between comparable constructions is considered subjectively insignificant.
These correlations are highly contingent upon the background noise levels present in the receiving room; an increase in background noise levels correlates with an enhanced perception of sound isolation.
Rating Methodology
Historical Context
Before the advent of the STC metric, the sound isolation performance of a partition was quantified and documented as the average transmission loss across the frequency ranges of either 128 to 4096 Hz or 256 to 1021 Hz. While this method proved effective for evaluating homogeneous partitions that adhere to the mass law, it could be potentially inaccurate for assessing complex or multi-leaf walls.
In 1961, the ASTM International Standards Organization adopted E90-61T, which established the foundation for the STC method utilized today. The STC standard curve is derived from European studies of multi-family residential construction and exhibits a strong correlation with the sound isolation performance of a 9-inch-thick (230 mm) brick wall.
Current Practices
The STC number is calculated using sound attenuation values obtained across sixteen standard frequencies, ranging from 125 Hz to 4000 Hz. These Transmission Loss values are then graphically represented on a sound pressure level chart, and the subsequent curve is benchmarked against a standard reference contour provided by the ASTM.
Sound isolation metrics, including the STC, are quantified within meticulously isolated and engineered laboratory test environments. Conversely, a vast array of on-site environmental variables can influence sound isolation when designing building partitions and enclosures in real-world applications.
Factors Affecting Sound Transmission Class
Acoustic Medium
Sound propagates through both airborne and structure-borne paths; consequently, both transmission routes require comprehensive consideration during the design of sound-isolating walls and ceilings. To mitigate airborne sound transmission, all air paths between adjacent areas must be eliminated, which is accomplished through sealing all joints and eliminating acoustic leaks. To mitigate structure-borne noise, it is essential to implement isolation systems that minimize direct mechanical coupling between structural components.
Mass
Increasing the mass of a partition effectively diminishes sound transmission, frequently accomplished by incorporating supplementary gypsum layers. Optimal sound isolation is often achieved with asymmetrical leaves, such as those featuring gypsum of varying thicknesses. The impact of integrating multiple gypsum wallboard layers into a frame is contingent upon the framing type and its specific configuration. It is crucial to note that merely doubling a partition's mass does not equate to a twofold increase in its Sound Transmission Class (STC), given that STC is derived from a non-linear decibel measurement of sound transmission loss. Consequently, while the addition of an extra gypsum wallboard layer to a light-gauge (25-gauge or lighter) steel stud partition can yield an approximate 5-point STC improvement, applying the same modification to single wood or heavy-gauge steel framing typically results in only a 2 to 3-point STC gain. Furthermore, introducing a second additional layer (to an existing three-layer system) does not produce as substantial an STC alteration as the initial added layer. The influence of extra gypsum wallboard layers on double-stud and staggered-stud partitions mirrors the effects observed in light-gauge steel partitions.
Poured concrete and concrete blocks generally attain superior Sound Transmission Class (STC) values, ranging from the mid-40s to mid-50s, compared to framed walls of equivalent thickness, primarily due to their inherent mass. Nevertheless, the considerable weight, increased construction complexity, and inadequate thermal insulation associated with masonry wall partitions often restrict their practicality as a widespread sound isolation solution in numerous building projects.
Recently, gypsum board manufacturers have introduced lightweight drywall products. Standard gypsum typically exhibits a nominal density of 43 pounds per cubic foot (690 kg/m3), whereas lightweight drywall possesses a nominal density of 36 pounds per cubic foot (580 kg/m3). While this density difference does not substantially impact the overall Sound Transmission Class (STC) rating, lightweight gypsum can considerably diminish a partition's low-frequency acoustic performance when contrasted with conventional normal-weight gypsum.
Sound Absorption
Sound absorption involves the transformation of acoustic energy into alternative forms, most commonly thermal energy.
Incorporating absorptive materials onto interior room surfaces, such as fabric-faced fiberglass panels and heavy curtains, effectively reduces reverberated sound energy within that space. Nevertheless, these types of absorptive interior surface treatments do not substantially enhance the Sound Transmission Class (STC). Conversely, the installation of absorptive insulation, including fiberglass batts or blow-in cellulose, within wall or ceiling cavities demonstrably improves the STC. In single 2x4 wood stud framing spaced 16 inches (410 mm) on-center, the inclusion of insulation yields only a marginal STC increase, typically a few points. This limited effect occurs because such wall configurations tend to develop significant resonances that cavity insulation alone cannot adequately mitigate. In stark contrast, introducing standard fiberglass insulation into an otherwise empty cavity within light-gauge (25-gauge or lighter) steel stud partitions can lead to an improvement of nearly 10 STC points.
Research indicates that fibrous insulation materials, including mineral wool, are capable of augmenting the Sound Transmission Class (STC) by 5 to 8 points.
Stiffness
The influence of stiffness on sound isolation can be attributed to either the inherent material stiffness of the sound-isolating component or the structural rigidity imparted by specific framing methodologies.
Framing Methodologies
When properly implemented, the structural decoupling of gypsum wallboard panels from partition framing can substantially enhance sound isolation. Common examples of structural decoupling techniques in building construction encompass resilient channels, sound isolation clips with hat channels, and staggered- or double-stud framing. The Sound Transmission Class (STC) improvements achieved through decoupling in wall and ceiling assemblies exhibit considerable variation, influenced by factors such as framing type, air cavity volume, and the specific decoupling material employed. Meticulous attention is paramount in every form of decoupled partition construction, as any fastener that establishes a rigid mechanical connection to the framing can compromise the decoupling mechanism, leading to significantly diminished sound isolation performance.
The acoustic insulation of a system comprising two panels rigidly connected by a stud is contingent upon the stud's rigidity. Light-gauge studs (25-gauge or thinner) offer superior sound isolation compared to 16-20-gauge steel and significantly outperform wood studs. When heavy-gauge steel or wood studs are installed at 16-inch (410 mm) intervals, additional resonances emerge, which diminish the partition's sound isolation capabilities. In standard gypsum stud wall constructions, this resonance typically manifests within the 100–160 Hz frequency range, hypothesized to be a combination of mass-air-mass resonance and a bending mode resonance induced by the close support of a plate by rigid elements.
Single metal stud partitions demonstrate greater efficacy than their wooden counterparts, potentially elevating the Sound Transmission Class (STC) rating by as much as 10 points. Nevertheless, the distinction in performance between metal and wood studs becomes negligible when incorporated into double stud partition designs. Double stud partitions inherently achieve a higher STC rating than single stud configurations.
For specific structural assemblies, expanding the stud spacing from 16 to 24 inches (410–610 mm) can enhance the STC rating by 2 to 3 points.
Damping
While the terms 'sound absorption' and 'damping' are frequently used interchangeably in discussions of room acoustics, acousticians differentiate them as distinct characteristics pertinent to sound-isolating wall systems.
Numerous gypsum manufacturers provide specialized products employing constrained-layer damping, a type of viscous damping. Damping typically augments the sound isolation performance of partitions, especially across mid-to-high frequency ranges.
Damping techniques are also applied to enhance the acoustic insulation of glazing assemblies. Laminated glazing, featuring a Polyvinyl butyral (PVB) interlayer, exhibits superior acoustic performance compared to non-laminated glass of identical thickness.
Sound Leakage
To achieve effective sound isolation, all apertures and gaps must be sealed, and the enclosure rendered hermetic. An illustrative table presents soundproofing test outcomes for a wall partition possessing a theoretical maximum sound transmission loss of 40 dB between adjacent rooms and a partition area of 10 square meters. Even minor open gaps and perforations within the partition lead to a disproportionate decrease in soundproofing efficacy. For instance, a 5% opening in the partition, facilitating unimpeded sound transmission between rooms, resulted in a reduction of transmission loss from 40 dB to 13 dB. A mere 0.1% open area can diminish transmission loss from 40 dB to 30 dB, a scenario common in walls where caulking has been applied ineffectively. Partitions that are insufficiently sealed and incorporate elements such as back-to-back electrical boxes, untreated recessed lighting, and unsealed pipes create flanking paths for sound, leading to substantial leakage.
Acoustic joint tapes and caulking have been employed to enhance sound isolation since the early 1930s. Historically, the application of such tapes was predominantly restricted to defense and industrial sectors, including naval vessels and aircraft. However, contemporary research has substantiated the effectiveness of sealing gaps, consequently improving the sound isolation performance of partitions.
Flanking
Building codes generally permit a 5-point discrepancy between laboratory-derived and field-measured STC ratings. Nevertheless, research indicates that even in meticulously constructed and sealed installations, the variance between laboratory and field ratings is significantly influenced by the specific assembly type.
Specialized Variations of STC
Inherently, the STC rating is established through laboratory testing conducted under optimal conditions. Consequently, alternative versions of the STC rating exist to address real-world environmental factors.
Composite STC
This metric represents the overall sound isolation performance of a partition that incorporates multiple sound-isolating components, including doors and windows.
Apparent Sound Transmission Class (ASTC)
This refers to the sound isolation performance of a partition, as measured in situ in accordance with ASTM E336. The measurement is normalized to compensate for variations in room finishes and the area of the tested partition, enabling a comparative analysis of the same wall in diverse acoustic environments, such as a bare living room versus an acoustically dry recording booth.
Normalized Noise Isolation Class (NNIC)
This metric quantifies the sound isolation performance of a partition, measured in the field per ASTM E336, and normalized to adjust for the room's reverberation time.
Noise Isolation Class (NIC)
This represents the sound isolation performance of a partition, measured in the field according to ASTM E336, without normalization for the specific room conditions prevalent during the test.
Field Sound Transmission Class (FSTC)
This metric quantifies the sound isolation performance of specific elements within a partition, as measured in situ and achieved by mitigating the effects of sound flanking paths. This proves valuable for assessing walls incorporating doors, particularly when the objective is to isolate and exclude the door's influence from the overall measured field STC. Historically, ASTM E336 prescribed the FSTC testing methodology; however, the most recent iteration of this standard no longer incorporates FSTC.
Door Sound Transmission Class (DTC)
This metric quantifies the sound isolation performance of doors, as determined by measurements conducted in accordance with ASTM E2964.
Legal and practical requirements
According to Section 1206 of the 2021 International Building Code, partitions separating dwelling units from public and service areas are mandated to achieve an STC of 50 when tested per ASTM E90, or an NNIC of 45 if evaluated in the field according to ASTM E336. Nevertheless, it is important to note that not all jurisdictions adopt the IBC for their local building or municipal codes.
Common partition STC
Standard interior walls, featuring a single layer of 1⁄§34§-inch (13 mm) gypsum wallboard (drywall) on each side of 2x4 (§67§+§910§⁄§1112§ in × §1415§+§1718§⁄§1920§ in or 38 mm × 89 mm) wood studs spaced 16 inches (410 mm) on-center and lacking fiberglass insulation within the stud cavities, typically exhibit an STC rating of approximately 33. When individuals are surveyed regarding the acoustical performance of such walls, they frequently characterize them as "paper thin," indicating minimal provision for privacy. In contrast, double-stud partition walls are commonly fabricated with multiple layers of gypsum wallboard panels affixed to both faces of parallel 2x4 wood studs, spaced 16 inches on-center and separated by a 1-inch (25 mm) airspace. The sound isolation performance of these configurations ranges from the mid-STC 40s to the high-STC 60s, contingent upon the inclusion of insulation and the specific type and thickness of the gypsum wallboard utilized. Commercial structures typically employ steel studs, which vary in width, gauge, and on-center spacing. Each of these framing attributes influences the sound isolation capabilities of the partition to differing extents.
STC prediction
Numerous commercial software applications are available that forecast the STC ratings of partitions by integrating theoretical models with empirically derived laboratory data. While these programs can estimate STC ratings within a few points of a physically tested partition, their outputs represent approximations.
Outdoor-Indoor Transmission Class (OITC)
The Outdoor–Indoor Transmission Class (OITC) serves as a standardized metric for quantifying the transmission rate of external noise sources into a building, established upon ASTM E-1332 Standard Classification for Rating Outdoor-Indoor Sound Attenuation. In contrast to the STC, which relies on a noise spectrum primarily focused on speech frequencies, OITC employs a source noise spectrum that encompasses frequencies as low as 80 Hz (relevant to aircraft, rail, and truck traffic) and exhibits a greater weighting towards lower frequencies. The OITC value is commonly applied for the assessment, evaluation, and selection of exterior glazing assemblies. Illustrative examples include double- and triple-pane windows, as well as laminated glass units, all designed to mitigate sound transmission from external origins. Furthermore, supplementary interventions, such as the installation of window inserts and the sealing of gaps, can augment sound isolation performance.
References
References
Bibliography
- Harris, Cyril M. (1994). Noise Control in Buildings: A Practical Guide for Architects and Engineers. McGraw-Hill. ISBN 978-0-07-026887-6. OCLC 869588871.Ballou, Glenn M. (2008). Handbook for Sound Engineers (4th ed.). Elsevier. ISBN 978-0-240-80969-4.Source: TORIma Academy Archive