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Sound transmission class
View on WikipediaSound 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.[1]
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).[2]
Definition
[edit]The STC or sound transmission class is a single number method of rating how well wall partitions reduce sound transmission.[3] 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.[3]
Sound Isolation and Sound Insulation are used interchangeably, though the term "Insulation" is preferred outside the US.[4] The term "sound proofing" is typically avoided in architectural acoustics as it is a misnomer and connotes inaudibility.
Subjective correlation
[edit]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.[5]
| STC | What can be heard |
|---|---|
| 25 | Normal speech can be understood |
| 30 | Loud speech can be understood |
| 35 | Loud speech audible but not intelligible |
| 40 | Loud speech audible as a murmur |
| 45 | Loud speech heard but not audible |
| 50 | Loud sounds faintly heard |
| 60+ | Good soundproofing; most sounds do not disturb neighboring residents.[6] |
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.[7]
Rating methodology
[edit]Historical
[edit]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.[8][9] 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.[10]
Current
[edit]
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.[11]
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
[edit]Acoustic medium
[edit]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.[12]
Mass
[edit]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.[13] The effect of adding multiple layers of gypsum wallboard to a frame also varies depending on the framing type and configuration.[14][15] 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.[16] 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.[14][15] 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.[14] 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.[17] 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,[when?] 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.
Sound absorption
[edit]Sound absorption entails converting acoustical energy into other forms, usually heat.[18]
Adding absorptive materials to the interior surfaces of rooms, for example fabric-faced fiberglass panels and thick curtains, will result in a decrease of reverberated sound energy within the room. However, absorptive interior surface treatments of this kind do not significantly improve the sound transmission class.[19] Installing absorptive insulation, for example fiberglass batts and blow-in cellulose, into the wall or ceiling cavities does increase the sound transmission class significantly.[14] The presence of insulation in single 2x4 wood stud framing spaced 16 inches (410 mm) on-center results in only a few STC points. This is because a wall with 2x4 wood stud framing spaced 16 inches develops significant resonances which are not mitigated by the cavity insulation. In contrast, adding standard fiberglass insulation to an otherwise empty cavity in light-gauge (25-gauge or lighter) steel stud partitions can result in a nearly 10 STC-point improvement.
Other studies have shown that fibrous insulation materials, such as mineral wool, can increase the STC by 5 to 8 points.[13]
Stiffness
[edit]The effect of stiffness on sound isolation can relate to either the material stiffness of the sound isolating material or the stiffness caused by framing methods.
Framing methods
[edit]Structurally decoupling the gypsum wallboard panels from the partition framing can result in a large increase in sound isolation when installed correctly. Examples of structural decoupling in building construction include resilient channels, sound isolation clips and hat channels, and staggered- or double-stud framing. The STC results of decoupling in wall and ceiling assemblies varies significantly depending on the framing type, air cavity volume, and decoupling material type.[14] Great care must be taken in each type of decoupled partition construction, as any fastener that becomes mechanically (rigidly) coupled to the framing can undermine the decoupling and result in drastically lower sound isolation results.[20]
When two leaves are rigidly tied or coupled by a stud, the sound isolation of the system depends on the stiffness of the stud. Light-gauge (25-gauge or lighter) provides better sound isolation than 16-20-gauge steel, and noticeably better performance than wood studs.[21] When heavy gauge steel or wood studs are spaced 16 inches (410 mm) on center, additional resonances form which further lower the sound isolation performance of a partition. For typical gypsum stud walls, this resonance occurs in the 100–160 Hz region and is thought to be a hybrid of the mass-air-mass resonance and a bending mode resonance caused when a plate is closely supported by stiff members.[22]
Single metal stud partitions are more effective than single wood stud partitions, and have been shown to increase the STC rating by up to 10 points. However, there is little difference between metal and wood studs when used in double stud partitions.[13] Double stud partitions have a higher STC than single stud.[13]
In certain assemblies, increasing the stud spacing from 16 to 24 inches (410–610 mm) increases the STC rating by 2 to 3 points.[13]
Damping
[edit]Though the terms sound absorption and damping are often interchangeable when discussing room acoustics, acousticians define these as two distinct properties of sound-isolating walls.
Several gypsum manufacturers offer specialty products which use constrained-layer damping, which is a form of viscous damping.[23][24] Damping generally increases the sound isolation of partitions, particularly at mid-and-high frequencies.
Damping is also used to improve the sound isolation performance of glazing assemblies. Laminated glazing, which consists of a Polyvinyl butyral (or PVB) inter-layer, performs better acoustically than a non-laminated glass of equivalent thickness.[25]
Sound leakage
[edit]
All holes and gaps should be filled and the enclosure hermetically sealed for sound isolation to be effective. The table below illustrates sound proofing test results from a wall partition that has a theoretical maximum loss of 40 dB from one room to the next and a partition area of 10 meters squared. Even small open gaps and holes in the partition have a disproportionate reduction in sound proofing. A 5% opening in the partition, which offers unrestricted sound transmission from one room to the next, caused the transmission loss to reduce from 40 dB to 13 dB. A 0.1% open area will reduce the transmission loss from 40 dB to 30 dB, which is typical of walls where caulking has not been applied effectively[26] Partitions that are inadequately sealed and contain back-to-back electrical boxes, untreated recessed lighting and unsealed pipes offer flanking paths for sound and significant leakage.[27]
Acoustic joint tapes and caulking have been used to improve sound isolation since the early 1930s.[28] Although the applications of tapes was largely limited to defense and industrial applications such as naval vessels and aircraft in the past, recent research has proven the effectiveness of sealing gaps and thereby improving the sound isolation performance of a partition.[29]
| Transmission loss | % of area open |
|---|---|
| 13 dB | 5% open |
| 17 dB | 2% open |
| 20 dB | 1% open |
| 23 dB | 0.5% open |
| 27 dB | 0.2% open |
| 30 dB | 0.1% open |
| 33 dB | 0.05% open |
| 37 dB | 0.02% open |
| 39.5 dB | Practical maximum loss |
| 40 dB | Theoretical maximum loss |
Flanking
[edit]Building codes typically allow for a 5-point tolerance between the lab-tested and field-measured STC rating; however, studies have shown that even in well-built and sealed installations the difference between the lab and field rating is highly dependent on the type of assembly.[30]
Special variations of STC
[edit]By nature, the STC rating is derived from lab testing under ideal conditions. There are other versions of the STC rating to account for real-world conditions.
Composite STC
[edit]The net sound isolation performance of a partition containing multiple sound isolating elements such as doors, windows, etc.
Apparent Sound Transmission Class (ASTC)
[edit]The sound isolation performance of a partition measured in the field according to ASTM E336, normalized to account for different room finishes and the area of the tested partition (i.e. compare the same wall measured in a bare living room and an acoustically dry recording booth).
Normalized Noise Isolation Class (NNIC)
[edit]The sound isolation performance of a partition measured in the field according to ASTM E336, normalized to account for the reverberation time in the room.
Noise Isolation Class (NIC)
[edit]The sound isolation performance of a partition measured in the field according to ASTM E336, not normalized to the room conditions of the test.
Field Sound Transmission Class (FSTC)
[edit]The sound isolation performance of a specific elements in a partition, as measured in the field and achieved by suppressing the effects of sound flanking paths. This can be useful for measuring walls with doors, when you are interested in removing the influence of the door on the measured field STC. The FSTC testing method was historically prescribed by ASTM E336, however the latest version of this standard does not include FSTC.[31]
Door Sound Transmission Class (DTC)
[edit]The sound isolation performance of doors when measured according to ASTM E2964.[32]
Legal and practical requirements
[edit]Section 1206 of International Building Code 2021 states that separation between 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
[edit]Interior walls with 1 sheet of 1⁄2-inch (13 mm) gypsum wallboard (drywall) on either side of 2x4 (1+1⁄2 in × 3+1⁄2 in or 38 mm × 89 mm) wood studs spaced 16 inches (410 mm) on-center with no fiberglass insulation filling each stud cavity have an STC of about 33.[14] 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 wood studs spaced 16 inches on-center and separated by a 1-inch (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.[14] 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.[15]
| STC | Partition type |
|---|---|
| 27 | Single pane glass window (typical value) (dual-pane glass window range is 26–32)"STC Ratings". |
| 33 | Single layer of 1⁄2-inch (13 mm) drywall on each side, wood studs, no insulation (typical interior wall) |
| 39 | Single layer of 1⁄2-inch (13 mm) drywall on each side, wood studs, fiberglass insulation[33] |
| 44 | 4-inch (100 mm) hollow CMU (concrete masonry unit)[34] |
| 45 | Double layer of 1⁄2-inch (13 mm) drywall on each side, wood studs, batt insulation in wall |
| 46 | Single layer of 1⁄2-inch (13 mm) drywall, glued to 6-inch (150 mm) lightweight concrete block wall, painted both sides |
| 46 | 6-inch (150 mm) hollow CMU (concrete masonry unit)[34] |
| 48 | 8-inch (200 mm) hollow CMU (concrete masonry unit)[34] |
| 50 | 10-inch (250 mm) hollow CMU (concrete masonry unit)[34] |
| 52 | 8-inch (200 mm) hollow CMU (concrete masonry unit) with 2-inch (51 mm) Z-bars and 1⁄2-inch (13 mm) drywall on each side[35] |
| 54 | Single layer of 1⁄2-inch (13 mm) drywall, glued to 8-inch (200 mm) dense concrete block wall, painted both sides |
| 54 | 8-inch (200 mm) hollow CMU (concrete masonry unit) with 1+1⁄2-inch (38 mm) wood furring, 1+1⁄2-inch fiberglass insulation and 1⁄2-inch (13 mm) drywall on each side[35] |
| 55 | Double layer of 1⁄2-inch (13 mm) drywall on each side, on staggered wood stud wall, batt insulation in wall |
| 59 | Double layer of 1⁄2-inch (13 mm) drywall on each side, on wood stud wall, resilient channels on one side, batt insulation |
| 63 | Double layer of 1⁄2-inch (13 mm) drywall on each side, on double wood/metal stud walls (spaced 1 inch [25 mm] apart), double batt insulation |
| 64 | 8-inch (200 mm) hollow CMU (concrete masonry unit) with 3-inch (76 mm) steel studs, fiberglass insulation and 1⁄2-inch (13 mm) drywall on each side[35] |
| 72 | 8-inch (200 mm) concrete block wall, painted, with 1⁄2-inch (13 mm) drywall on independent steel stud walls, each side, insulation in cavities |
STC prediction
[edit]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.[36]
Outdoor-Indoor Transmission Class (OITC)
[edit]The Outdoor–Indoor Transmission Class (OITC) is a standard for indicating the rate of sound transmission from outdoor noise sources into a building, based on ASTM E-1332 Standard Classification for Rating Outdoor-Indoor Sound Attenuation.[37] 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. Examples include double- and triple-pane windows and laminated glass units, which reduce sound transmission from outdoor sources. Additional measures, such as window inserts and sealing gaps, can further enhance sound isolation performance.[38]
See also
[edit]References
[edit]- ^ Roller, H. Stanley (November 1985). "Isolating music and mechanical equipment sound sources with gypsum board partition systems". The Journal of the Acoustical Society of America. 78 (S1): S10. Bibcode:1985ASAJ...78...10R. doi:10.1121/1.2022641. ISSN 0001-4966.
- ^ Egan, M. David. (2007). Architectural acoustics. J. Ross Publishing. ISBN 978-1-932159-78-3. OCLC 636858059.
- ^ a b Ballou 2008, p. 72.
- ^ Hopkins, Carl. (2016). Sound insulation. Routledge. ISBN 978-1-138-13770-7. OCLC 933449409.
- ^ Berendt, Raymond D. (1967). A guide to airborne, impact, and structure borne noise-control in multifamily dwellings. U.S. Dept. of Housing and Urban Development. OCLC 5863574.
- ^ Bradley, J.S. (August 2001). Deriving Acceptable Values for Party Wall Sound Insulation from Survey Results. Inter-noise 2001: The 2001 International Congress and Exhibition on Noise Control Engineering. The Hague, The Netherlands. ISBN 9789080655423. OCLC 48937099.
- ^ Cavanaugh, W.J.; Farrell, W.R.; Hirtle, P.W.; Watters, B.G. (April 1962). "Speech Privacy in Buildings". The Journal of the Acoustical Society of America. 34 (4): 475–492. Bibcode:1962ASAJ...34..475C. doi:10.1121/1.1918154. ISSN 0001-4966.
- ^ Knudsen, Vern O. (1988). Acoustical designing in architecture. Acoustical Society of America. ISBN 0-88318-267-X. OCLC 758181173.
- ^ Chrisler, V. L. (1939). Sound insulation of wall and floor constructions. U.S. G.P.O. OCLC 14104628.
- ^ Northwood, T. D. (April 1962). "Sound-Insulation Ratings and the New ASTM Sound-Transmission Class". The Journal of the Acoustical Society of America. 34 (4): 493–501. Bibcode:1962ASAJ...34..493N. doi:10.1121/1.1918155. ISSN 0001-4966.
- ^ Ballou 2008, pp. 72–73.
- ^ Ballou 2008, p. 89.
- ^ a b c d e Ballou, Glen, ed. (2015). Handbook for sound engineers (5 ed.). CRC Press. ISBN 978-1-135-01665-4. OCLC 913880162.
- ^ a b c d e f g Halliwell, R.E.; Nightingale, T.R.T.; Warnock, A.C.C.; Birta, J.A. (March 1998). "Gypsum Board Walls: Transmission Loss Data". National Research Council Canada. doi:10.4224/20331556. IRC-IR-761.
- ^ a b c Bétit, Aaron (March 2010). "Performance Details of Metal Stud Partitions" (PDF). Sound and Vibration Magazine: 14–16.
- ^ "ASTM E413-22 Classification for Rating Sound Insulation". ASTM International. May 2022.
- ^ Warnock, A.C.C. (1985). "Field sound transmission loss measurements". Building Research Note. 1985–06. doi:10.4224/40000485.
- ^ Ballou 2008, p. 97.
- ^ Brown, Steven M.; Niedzielski, Joseph; Spalding, G. Robert (1978). "Effect of sound-absorptive facings on partition airborne-sound transmission loss". The Journal of the Acoustical Society of America. 63 (6): 1851–1856. Bibcode:1978ASAJ...63.1851B. doi:10.1121/1.381924.
- ^ LoVerde, J.; Dong, W. (2010). "Quantitative comparisons of resilient channel design and installation in single wood stud walls" (PDF). Proceedings of 20th International Congress on Acoustics, ICA 2010.
- ^ Halliwell, R. E. (1998). Gypsum board walls: transmission loss data. Institute for Research in Construction. OCLC 155721225.
- ^ Davy, John L.; Fard, Mohammad; Dong, Wayland; Loverde, John (February 2019). "Empirical corrections for predicting the sound insulation of double leaf cavity stud building elements with stiffer studs". The Journal of the Acoustical Society of America. 145 (2): 703–713. Bibcode:2019ASAJ..145..703D. doi:10.1121/1.5089222. ISSN 0001-4966. PMID 30823783. S2CID 73462977.
- ^ Shafer, Benjamin M.; Tinianov, Brandon (October 2011). "Use of damped drywall in architectural acoustics". The Journal of the Acoustical Society of America. 130 (4): 2388. Bibcode:2011ASAJ..130R2388S. doi:10.1121/1.3654567. ISSN 0001-4966.
- ^ Tinianov, Brian D. (September 2005). "Two case studies: QuietRock QR-530 drywall panels in new and remediated multifamily construction". The Journal of the Acoustical Society of America. 118 (3): 1976. doi:10.1121/1.2097073. ISSN 0001-4966.
- ^ Acoustical glazing design guide: laminated glass with Saflex plastic interlayer for superior sound control. Monsanto Company. 1986. OCLC 38400395.
- ^ Ballou 2008, pp. 77–78.
- ^ "Acoustics in Practice - NRC-CNRC". Archived from the original on 2013-09-25. Retrieved 2012-02-07. Acoustics in Practice
- ^ Shafer, Benjamin M. (2013). An overview of constrained-layer damping theory and application. Proceedings of Meetings on Acoustics. Vol. 133. Acoustical Society of America. p. 065023. Bibcode:2013ASAJ..133.3332S. doi:10.1121/1.4800606.
- ^ Shafer, Benjamin M.; Tinianov, Brandon (2011). "Use of damped drywall in architectural acoustics". The Journal of the Acoustical Society of America. 130 (4): 2388. Bibcode:2011ASAJ..130R2388S. doi:10.1121/1.3654567.
- ^ LoVerde, John; Dong, Wayland (2010). "Predictability of field airborne noise isolation from laboratory testing". The Journal of the Acoustical Society of America. 127 (3): 1741. Bibcode:2010ASAJ..127.1741L. doi:10.1121/1.3383509. ISSN 0001-4966.
- ^ ASTM E336-20 (2020). "Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings". Conshohocken, PA: ASTM International.
{{cite web}}: CS1 maint: numeric names: authors list (link) - ^ Standard Test Method for Measurement of the Normalized Insertion Loss of Doors, ASTM International, doi:10.1520/e2964-14
- ^ The Complete Photo Guide to Home Improvement. Creative Publishing international. July 2001. p. 194. ISBN 9780865735804. Retrieved 2011-10-01.
The Complete Photo Guide to Home Improvement
- ^ a b c d "STC Ratings for Masonry Walls". Acoustics.com. Retrieved 2011-10-01.
- ^ a b c "New Data Shows Masonry Wall and Precast Hollow Core Floor Systems Reaching High STC Ratings" (PDF). Masonry Advisory Council. Retrieved 2011-10-01.
- ^ Horan, Daniel (2014). "Computer Modeling of STC- Options and Accuracy" (PDF). Sound & Vibration (December): 8–11.
- ^ "Standard Classification for Rating Outdoor-Indoor Sound Attenuation". Conshohocken, PA: ASTM International. 2016.
- ^ "Noise Cancelling Windows". 2021-01-29. Retrieved 2025-08-13.
Bibliography
[edit]- 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 (4 ed.). Elsevier. ISBN 978-0-240-80969-4.
Sound transmission class
View on GrokipediaFundamentals
Definition
The Sound Transmission Class (STC) is a single-number rating that quantifies the airborne sound insulation performance of a building partition, such as a wall, floor, ceiling, door, or window.[1] It provides a standardized measure of how effectively the partition attenuates sound transmission from one space to another, based on laboratory measurements of sound transmission loss (TL).[5] Developed under ASTM International standard E413, the STC rating is derived from TL data obtained via ASTM E90 testing procedures, focusing exclusively on airborne sound and excluding impact or structure-borne noise.[1][6] The TL curve consists of values measured in 1/3-octave frequency bands ranging from 125 Hz to 4 kHz, which encompass the primary range of human speech and common environmental noises.[5] To determine the STC, a standard reference contour is fitted to this TL curve by vertically shifting it upward until the highest possible level is achieved while satisfying specific criteria: no single TL value exceeds the contour by more than 8 dB (i.e., no deficiency greater than 8 dB in any band), and the total sum of all deficiencies (where the contour exceeds the TL) does not surpass 32 dB across the bands.[7] The resulting STC value corresponds to the level of the reference contour at 500 Hz, expressed in decibels (dB), with higher values indicating better sound insulation (e.g., STC 50 reduces normal speech to a low murmur).[1] The reference contour itself has a characteristic shape designed to approximate typical TL behavior of building partitions: it features a 15 dB per decade slope in the lower frequencies to reflect mass-controlled transmission, transitioning to a relatively flat profile at higher frequencies.[8] For the zero-rating contour, specific values include 0 dB at 500 Hz and -8 dB at 125 Hz, with interpolated values across the 16 bands ensuring the fit prioritizes mid-frequency performance relevant to speech intelligibility.[7] This contour-based approach allows STC to condense complex frequency-dependent data into a practical metric for design and specification in building acoustics.[1]Subjective Correlation
The Sound Transmission Class (STC) rating provides a rough guide to human perception of speech privacy, with higher values generally corresponding to greater attenuation of audible speech through partitions. For instance, an STC of 25 allows normal speech to be easily understood, while an STC of 40 renders soft speech unintelligible but may still permit louder voices to be muffled and partially discernible.[9] At an STC of 50, very soft speech is barely audible, and normal conversation is typically inaudible, achieving a level of privacy suitable for many office or residential applications.[10] These correlations stem from empirical assessments linking STC to the Articulation Index (AI), a measure of speech intelligibility, where AI values below 0.05 indicate confidential privacy (often requiring STC >55).[9] However, STC correlations with subjective perception weaken for low-frequency sounds, such as bass from music or machinery, because the rating emphasizes mid-frequencies (125–4000 Hz) relevant to speech but underestimates transmission below 125 Hz.[11] Similarly, for non-speech noises like traffic or appliances, higher STC values do not always predict reduced annoyance, as these sounds often have spectra that bypass the rating's focus on vocal frequencies.[9] These limitations are particularly evident in residential applications involving personal or intimate sounds. An STC of approximately 48 provides good reduction of airborne sounds, making loud speech faint or unintelligible. However, intimate sounds such as moaning at speech-like frequencies may be faintly audible if the source is sufficiently loud. Low-frequency or impact noises, for example bed creaking, can transmit more easily as they lie outside the STC's primary emphasis on mid-frequencies (125–4000 Hz) and may involve structure-borne paths not measured by STC. In practice, field experiences in multi-unit residential buildings show that even STC ratings of 50 or higher do not always prevent the audibility of such neighbor noises, depending on factors including source volume, frequency content, construction quality, flanking transmission, and ambient noise levels.[12][13][10] Building acoustics research, including studies aligned with ASTM E90 and E413 standards, demonstrates that STC can overestimate privacy for certain noise spectra.[14] Empirical field tests in multi-unit buildings show that perceived insulation improves with STC but varies by occupant sensitivity and room layout, with AI-based evaluations confirming marginal privacy at STC 40–50 in quiet environments.[9] STC is not a perfect predictor of subjective experience because perception depends heavily on ambient background noise levels, such as those measured by Noise Criteria (NC); for adequate privacy, STC should exceed the NC by at least 8–10 points to mask transmitted speech effectively.[9] In low-background-noise settings (e.g., NC 25), an STC of 47 is needed for confidential privacy, whereas higher NC levels (e.g., NC 45) reduce this requirement to STC 28, highlighting how environmental factors modulate annoyance beyond the rating alone.[15]Rating Methodology
Historical Development
The foundations of sound transmission rating methods trace back to the 1940s, when wartime acoustics research focused on controlling noise and vibration in military applications, such as aircraft and structures. This era's efforts, including work by Leo Beranek on sound propagation and insulation, established key principles for measuring transmission loss through materials. Beranek's 1949 paper with G.A. Work analyzed sound transmission through multiple-layered structures with flexible blankets, providing early experimental data on frequency-dependent attenuation that influenced later insulation metrics.[16] In the 1950s, building on this research, Beranek's comprehensive text Acoustics (1954) detailed theoretical and practical approaches to sound transmission, emphasizing the need for standardized measurements of transmission loss across frequencies to evaluate building partitions. Early metrics, such as simple averages of transmission loss values or the Noise Isolation Class for field measurements, were employed but suffered from poor correlation with subjective perceptions of speech privacy and annoyance, as they overlooked the uneven frequency response of human hearing. These limitations highlighted the demand for a simplified, perceptually weighted single-number rating. The Sound Transmission Class (STC) emerged in the 1960s as a response to these shortcomings, formalizing a method to derive a single value from laboratory transmission loss data that better approximated reductions in airborne sound, particularly speech. Developed through collaboration among acousticians and engineers, it drew from European single-number systems like the Weighted Sound Reduction Index while adapting to U.S. building practices. The first standardized procedure appeared in ASTM E413-1970, issued by the nascent ASTM efforts in environmental acoustics (later formalized as Committee E-33 in 1972), which defined STC calculation by fitting a reference contour to transmission loss values in 16 one-third-octave bands from 125 Hz to 4,000 Hz. This graphical fitting approach minimized the maximum deviation between measured data and the contour, yielding an integer rating.[1][17] Refinements continued through ASTM Committee E-33, with revisions to E413 addressing inconsistencies in low-frequency weighting to improve correlation with real-world insulation performance. By the 1990s, computational algorithms replaced manual graphical methods, enabling precise, automated contour fitting and reducing subjectivity in rating calculations, as implemented in modern software tools for acoustics testing. These shifts ensured STC's widespread adoption in building codes and design, prioritizing conceptual simplicity over exhaustive frequency details while maintaining ties to empirical transmission loss data.[18]Current Standards
The laboratory measurement of airborne sound transmission loss (TL) for determining the Sound Transmission Class (STC) is governed by ASTM E90-23, which outlines procedures in a dedicated transmission suite comprising a source room and a receiving room separated by the test partition or element under evaluation. The source room uses multiple loudspeakers driven by broadband noise (such as filtered pink noise) to establish an approximately diffuse sound field, ensuring uniform sound incidence on the specimen. The receiving room, isolated to limit flanking paths, captures the transmitted sound while maintaining low background noise levels (at least 10 dB below measured levels). Both rooms must have a minimum volume of 80 m³, with larger volumes often used in practice to better achieve diffuse field conditions. Sound pressure levels are measured simultaneously in both rooms using rotating microphone booms or fixed microphone arrays at no fewer than six positions per room to achieve spatial averaging representative of a diffuse field. Measurements occur in one-third octave bands centered from 125 Hz to 4000 Hz, with the TL computed as the difference between normalized source and receiving room levels, corrected for the receiving room's total absorption area (via reverberation time measurements per ASTM C423 or equivalent) and the specimen's exposed area (usually 10 m²).[19][5][20] Instrumentation must meet precision standards, including Class 1 sound level meters compliant with IEC 61672-1 for accurate 1/3-octave band analysis, omnidirectional condenser microphones with random-incidence correction, and power amplifiers paired with loudspeakers capable of at least 90 dB SPL across the frequency range without distortion. Digital signal generators and analyzers are employed for noise production and data processing to ensure stable, repeatable signals. The standard mandates suite qualification through flanking transmission limits (≤3 dB contribution) and reference specimen tests, with precision requirements specifying intralaboratory repeatability of ±2 dB and interlaboratory reproducibility standard deviation of less than 2 dB for TL values from 125 Hz to 4000 Hz.[19][21][22] Following TL measurement, the STC rating is derived per ASTM E413-22 through a curve-fitting process applied to the 16 one-third octave band TL values. A standard reference contour, defined for the 16 one-third-octave bands from 125 Hz to 4000 Hz with values starting at 0 dB at 125 Hz, -1 dB at 400 Hz, 0 dB at 500 Hz, and increasing to 8 dB at 4000 Hz, is shifted vertically in integer steps until it reaches the highest position where no TL value falls more than 8 dB below the contour in any band and the total sum of deficiencies (positive differences between contour and TL where TL is lower) does not exceed 32 dB. The STC is then taken as the contour value at 500 Hz. This method prioritizes balanced performance across speech frequencies while penalizing weak points.[1][23][12] Internationally, the procedures align closely with ISO 10140-2:2021 (Acoustics—Laboratory measurement of sound insulation of building elements—Part 2: Measurement of airborne sound insulation), which specifies similar transmission suite setups, diffuse field conditions via multiple source positions, and 1/3-octave band measurements from 100 Hz to 5000 Hz, often using intensity probes or pressure-based methods interchangeably with ASTM approaches. The corresponding single-number rating, the weighted sound reduction index (Rw), follows ISO 717-1:2020 and employs a reference curve adapted for European speech spectra, yielding values typically 1-3 dB higher than STC for identical data due to contour differences. While STC remains U.S.-centric under ASTM, its methodology is widely adopted globally for product specifications and building codes, often alongside Rw for harmonization.[22] The 2023 edition (ASTM E90-23) incorporates advancements in digital signal processing for enhanced signal generation, filtering, and real-time analysis to reduce measurement time and improve accuracy in non-ideal fields, alongside hybrid testing protocols combining traditional pressure methods with intensity techniques for complex specimens.[19]Factors Affecting STC
Acoustic Medium
The Sound Transmission Class (STC) rating is specifically designed to evaluate the attenuation of airborne sound through building partitions, with air serving as the primary propagating medium in standard laboratory tests conducted per ASTM E90.[20] In these applications, air's relatively low density (approximately 1.2 kg/m³ at standard conditions) and speed of sound (about 343 m/s) facilitate the measurement of transmission loss (TL) across frequencies from 125 to 4000 Hz, focusing on typical indoor noise sources like speech.[24] The density and speed of sound in the acoustic medium directly influence TL by determining the characteristic acoustic impedance (Z = ρc, where ρ is density and c is speed of sound), which governs how sound waves interact with partitions. Higher medium density increases impedance, potentially enhancing reflections at interfaces and thus improving TL, while variations in speed of sound affect wavelength and resonance behaviors in the system. For instance, in air, these properties yield predictable TL curves that align with STC's single-number rating, but deviations in other media alter the frequency-dependent transmission, making direct STC application challenging.[25] STC ratings are less applicable to non-air media, such as underwater structures, because water's much higher density (about 1000 kg/m³) and speed of sound (around 1480 m/s) result in acoustic impedances roughly 3600 times greater than air, leading to severe impedance mismatches and minimal sound transmission across the air-water interface—typically only 0.1% of incident energy transmits.[26] This mismatch causes nearly total reflection of airborne sound into water, rendering standard STC metrics irrelevant for submerged applications like marine vessels or offshore platforms, where specialized underwater TL models are required instead.[27] In rare industrial settings involving gaseous environments other than air, such as high-pressure pipelines or cryogenic facilities with media like helium or carbon dioxide, approximate STC adaptations may be used by scaling TL predictions based on the gas's density and speed of sound to mimic air-like conditions. For example, helium's lower density (0.18 kg/m³) and higher speed (about 1000 m/s) reduce impedance mismatch with partitions compared to air, potentially increasing effective TL at mid-frequencies.[28][29] Fundamentally, the physics of sound transmission involves impedance mismatch between the acoustic medium and the partition, which reduces the transmission coefficient (the fraction of incident sound power that passes through) via partial reflection at the interface. When the medium's impedance differs significantly from the partition's, more energy reflects back into the medium, contributing to higher overall TL, though this effect is most pronounced in standard air-based STC evaluations.[24]Mass
The mass law provides the foundational principle for understanding sound transmission loss (TL) in partitions, stating that the transmission loss is primarily governed by the surface density of the material and the frequency of the incident sound. The approximate equation for TL under normal incidence is given by where is the surface density in kg/m² and is the frequency in Hz.[30] This relationship implies a 6 dB increase in TL for every doubling of frequency or mass, resulting in a slope of approximately 6 dB per octave across the frequency spectrum.[30] In the context of Sound Transmission Class (STC) ratings, which integrate TL over a standard frequency band from 125 to 4000 Hz per ASTM E413, the mass law dominates the overall performance, particularly in the mid-frequency range where speech sounds are prominent.[31] However, the mass law breaks down at low frequencies near the panel's resonance, where the partition's stiffness causes excessive vibration and TL falls below predictions, often by 10-20 dB or more.[30] Doubling the mass of a partition typically increases its STC rating by about 5-6 points, as the added inertia resists airborne sound pressure more effectively, though diminishing returns occur with multilayer assemblies due to the logarithmic nature of the rating.[32] Mass serves as the primary driver of STC for common building partitions above their coincidence frequency, where bending waves no longer limit performance. For instance, a single layer of 12.7 mm gypsum board on wood studs achieves an STC of around 33, reflecting its low surface density of approximately 8 kg/m², whereas a 150 mm poured concrete wall with a surface density of approximately 360 kg/m² yields an STC of about 55, demonstrating how greater mass enhances broadband isolation.[30][3] Empirical data from ASTM E90 laboratory tests confirm that mass accounts for the majority of TL in mid-frequencies (500-2000 Hz), often contributing over 70% to the measured isolation in homogeneous partitions, as the inertial response effectively blocks pressure waves while other factors like stiffness play a lesser role in this range.[30] This dominance is evident in transmission loss curves, where deviations from mass law predictions are minimal away from resonances or coincidence dips.[3]Sound Absorption
Sound absorption within partition materials plays a key role in enhancing the overall Sound Transmission Class (STC) performance by dissipating acoustic energy inside cavities and multi-layer assemblies, thereby reducing internal reverberation and minimizing sound energy that could otherwise transmit to the receiving space.[4] In laboratory measurements of transmission loss (TL), which forms the basis for STC ratings, the absorption in the receiving room is standardized to at least 1.9 times the test specimen area across frequencies to ensure a diffuse sound field; variations in this absorption can lead to small systematic changes in apparent TL, with high-absorption setups indirectly boosting measured values by up to 3-6 dB compared to lower-absorption conditions before normalization.[33] Common materials for achieving this absorption include porous types such as fiberglass batts, which provide broadband dissipation by converting sound energy to heat through viscous and thermal losses in their open-cell structure, effectively reducing cavity flanking paths in double walls.[34] In contrast, resonant absorbers, such as membrane or Helmholtz resonators integrated into partition cavities, target specific low-frequency resonances to suppress transmission at critical bands, offering narrower but more pronounced improvements in STC for tuned applications like thin double-leaf panels.[35] These materials' impact on STC occurs via reduced energy buildup within the partition, distinct from direct transmission loss mechanisms. While sound absorption primarily influences the Noise Reduction Coefficient (NRC) under ASTM C423 by quantifying a material's ability to reduce room reverberation, it contributes to STC in multi-layer systems by attenuating transmitted sound within the assembly itself, leading to measurable enhancements in overall isolation. Studies on double walls demonstrate that incorporating cavity absorption, such as fiberglass or cellulose fill, can improve STC ratings by 5-10 points, equivalent to roughly 10-20% gains relative to baseline empty-cavity configurations (e.g., from STC 45 to 50-54).[36][23] This improvement is most evident at mid-to-high frequencies where flanking via cavity modes is prominent, though cavity effects interact with framing stiffness to optimize results.[37]Stiffness and Framing
Stiffness in building partitions plays a critical role in sound transmission, particularly at low frequencies, where it can lead to panel resonance that reduces the sound transmission loss (TL) and thereby lowers the overall STC rating. At the resonant frequency , where represents the stiffness and the mass, the partition vibrates efficiently, causing a significant dip in TL, often in the 50-200 Hz range for typical walls, which disproportionately affects the STC due to the logarithmic averaging in the rating method.[38] This resonance is especially pronounced in single-panel or framed constructions where structural rigidity couples the surfaces, amplifying vibration transfer across the partition.[39] Framing configurations directly influence stiffness and vibration paths, with choices like stud material, spacing, and decoupling methods altering low-frequency performance. Wood studs, being more rigid, transmit vibrations more readily than steel studs, which can flex slightly and dissipate energy, often yielding 3-5 STC points higher for equivalent assemblies; for instance, a wood-stud wall with double-layer gypsum board achieves STC 56, while a comparable 20-gauge steel-stud version reaches STC 60.[40] Resilient channels, installed between studs and gypsum board, decouple the finish layers from the frame, reducing stiffness-controlled flanking by 5-10 STC points, particularly beneficial at 125 Hz where many ratings are limited.[40] Staggered stud walls, where studs alternate between two parallel tracks, further minimize direct structural paths, improving isolation by isolating the cavity and lowering effective stiffness. The coincidence effect exacerbates stiffness issues at mid-to-high frequencies, occurring at the critical frequency where bending wave speed in the panel matches the airborne sound speed, leading to another TL dip and potential STC reductions of 5-15 dB if unmitigated.[41] This effect is more evident in lightweight, stiff panels like gypsum board, where the dip shifts based on material properties and thickness. Damping materials can help control these resonances and coincidence dips, as detailed in subsequent sections. In practice, double-stud walls, which inherently reduce framing stiffness coupling, achieve STC ratings of 50 or higher, compared to single-frame wood-stud walls with similar mass that typically rate around 35-40, as verified in standardized building code tests.[40]Damping
Damping in sound transmission involves the dissipation of vibrational energy in building partitions through viscoelastic mechanisms, where mechanical vibrations are converted into low-grade heat via internal friction within the material. This energy loss is quantified by the loss factor η, a dimensionless parameter representing the ratio of dissipated energy to stored elastic energy per cycle of vibration; higher values of η indicate greater damping efficiency.[42] By elevating the effective loss factor of a partition, damping substantially boosts transmission loss (TL) in the vicinity of resonant frequencies, where structural vibrations would otherwise cause pronounced dips in performance, with improvements reaching up to 15 dB at low frequencies in constrained layer configurations.[43] Damping mechanisms are particularly valuable for addressing resonances stemming from stiffness and framing in partitions, as they broaden and attenuate these vibrational modes without altering the underlying structure.[44] Key damping approaches include constrained layer damping (CLD), where a thin viscoelastic layer—such as polymer sheets or compounds—is inserted between rigid panels like gypsum board layers to induce shear deformation and maximize energy dissipation, and free-layer damping, involving the direct application of viscoelastic coatings to a single surface to absorb extensional vibrations.[45][43] In multi-layer wall, floor, and ceiling assemblies, damping integration typically enhances the Sound Transmission Class (STC) rating by 3 to 8 points, with greater benefits observed in floors and ceilings due to their larger resonant contributions at low frequencies.[46] This improvement arises from smoothing TL curves and elevating performance across the STC evaluation band (125 Hz to 4 kHz), often without requiring additional mass.[47] A prominent example is the application of Green Glue Noiseproofing Compound, a viscoelastic damping material applied between gypsum layers in walls; laboratory transmission loss curves for assemblies treated with Green Glue reveal filled resonance dips compared to untreated equivalents of similar mass, yielding STC gains of approximately 9 points and enhanced low-frequency isolation.[47]Sound Leakage and Flanking
Sound leakage occurs through gaps, cracks, or unsealed joints in building partitions, allowing airborne sound to bypass the primary transmission barrier and significantly degrade the effective sound transmission class (STC). Even small openings dominate the overall transmission loss because the transmission loss through an open area is effectively 0 dB, following the principle that the effective transmission loss is limited by the ratio of total partition area to leak area, approximately 10 log₁₀(A_total / A_leak). For instance, a 1% unsealed area (1/100th of the total area) can result in an effective transmission loss of about 20 dB, potentially dropping the STC rating by 10 or more points compared to a sealed assembly.[48][49] Flanking transmission refers to indirect sound paths through structural elements, such as beams, floors, ceilings, HVAC ducts, or shared corridors, rather than direct passage through the partition itself. These paths are quantified using the flanking level difference, which measures the sound level attenuation via the indirect route and often reveals reductions in isolation performance. In field conditions, flanking can lower the apparent STC by 5-15 dB relative to laboratory measurements, as demonstrated in studies of wood-frame constructions where structural interactions reduced field STC ratings by 4-7 points on average.[48][49] Mitigation strategies for both leakage and flanking emphasize sealing and isolation to restore intended performance. Acoustic caulks, gaskets, and perimeter seals effectively close gaps, improving STC by 6-10 dB in affected assemblies, while isolation joints or resilient mounts decouple structural paths, reducing flanking by up to 10 dB in resonant frequency ranges. Field measurements incorporating these techniques, such as ASTM E336 for apparent sound transmission loss, confirm that addressing flanking paths in situ can align field results more closely with lab STC values by ascribing all transmission—including indirect routes—to the overall partition performance.[48][50]Variations of STC
Composite STC
The composite sound transmission class (STC) applies to building assemblies comprising multiple elements or paths for sound transmission, such as walls incorporating windows, doors, or vents, where sound can propagate through parallel or series configurations. In such systems, the overall performance is determined by a weighted average of the transmission losses (TL) from each component, as weaker elements often dominate the rating. This approach accounts for the relative areas of each path, ensuring the composite STC reflects the net sound isolation of the entire partition. The calculation begins with the transmission coefficient for each component , where is the transmission loss in decibels. The area-weighted average transmission coefficient is then , with as the area of component and as the total area. The composite transmission loss follows as , from which the composite STC is derived by applying the standard STC contour to the resulting TL spectrum per ASTM E413.[51] This method is commonly applied to multi-layer walls or room enclosures with dissimilar elements, where elements like vents or glazing act as weak paths that significantly reduce the overall rating despite robust surrounding construction.[52] For instance, an office partition consisting of 80% solid wall (STC 45) and 20% glazing (STC 30) typically yields a composite STC of approximately 35, illustrating how the lower-rated component controls the assembly's performance.[51] The composite STC assumes independent, uncorrelated transmission paths and is best suited for laboratory-based evaluations of direct transmission. It may overestimate isolation in real installations where flanking transmission—sound traveling via structural or indirect routes—dominates, as this is not captured in the area-weighted model.[53]Apparent and Field Measures
In building acoustics, apparent and field measures of sound transmission class (STC) adapt laboratory-based ratings to real-world, in-situ conditions, accounting for factors like room reverberation, background noise, and flanking paths that are absent in controlled tests. These metrics provide more accurate assessments of actual noise isolation in occupied spaces, often revealing lower performance than lab results due to installation imperfections and environmental variables. The Apparent Sound Transmission Class (ASTC) is determined using ASTM E336, a standard method for measuring the apparent sound transmission loss in buildings via airborne sound transmission in the field. This approach involves generating noise in one room and measuring the sound pressure levels in both the source and receiving rooms, then adjusting for field-specific reverberation times and background noise levels to derive an apparent transmission loss (TL) curve. Unlike laboratory STC, ASTC typically yields ratings 5-10 dB lower, reflecting real-world degradation from non-ideal conditions such as uneven surfaces or air gaps. Field Sound Transmission Class (FSTC) extends this by directly evaluating the noise reduction between two rooms without isolating the partition, thereby incorporating flanking transmission through floors, ceilings, or structural elements. Measured similarly to ASTC but emphasizing overall field performance, FSTC often results in ratings significantly below lab STC; for instance, a wall assembly rated at STC 50 in the lab might achieve only FSTC 40 in situ due to sound leaks around edges or junctions. This metric is particularly useful for diagnosing installation flaws in completed buildings. Normalized Noise Isolation Class (NNIC) and Noise Isolation Class (NIC) offer alternative field metrics that parallel STC but are tailored to site-specific acoustics. NIC represents the uncorrected noise reduction between rooms, derived from measured sound pressure levels without normalization, making it sensitive to room volumes and absorption. NNIC refines this by normalizing for differences in room sizes and reverberation, providing a value akin to STC that better compares diverse field scenarios; both are outlined in ASTM E336 for field measurements of sound isolation. These classes highlight how actual isolation can vary, with NNIC often aligning closely with ASTC but emphasizing volume-adjusted fairness across installations.Door and Outdoor Measures
The Door Transmission Class (DTC) is a single-number rating specifically developed to evaluate the sound isolation performance of door assemblies in field conditions, as defined in ASTM E2964.[54] This standard measures the normalized insertion loss (NIL) of a door by comparing sound pressure levels in the receiving room with the door open and closed, adjusted for source room variations, across third-octave bands from 125 Hz to 4000 Hz.[55] Unlike laboratory-based Sound Transmission Class (STC) ratings under ASTM E90, DTC emphasizes practical installation factors such as edge sealing and perimeter gasketing to account for real-world air leaks and flanking paths that degrade performance.[55] The DTC value is calculated analogously to STC using ASTM E413, fitting the NIL data to a standard reference curve, but it provides a more reliable assessment for doors where diffuse sound fields are difficult to achieve in typical rooms.[54] Standard interior doors, such as hollow-core wood models without enhanced sealing, typically achieve sound transmission class (STC) ratings of 20 to 25, allowing normal speech to be audible through the door.[56] Solid-core wood or basic steel doors with basic gasketing can reach STC ratings of 28 to 30, where loud speech is faintly audible but words are indistinct.[57] Configurations specifically certified for STC 38 include the HMF Express STC 38 Package, featuring a 1-3/4" flush steel door with proprietary core, mortar-filled frame, Zero #770 perimeter seal, #367 automatic door bottom, #564 threshold, and #119W rabbet seal.[58] NGP/Zero International Systems provide various door assemblies with static STC ratings of 37–39, combined with specific gasketing sets such as perimeter seals (e.g., 5020, 5050) and automatic door bottoms (e.g., 780S, 335N) to achieve operable STC 38–40.[59] Republic Doors offers proprietary lighter-weight sound-core doors with perimeter seals, gaskets, and thresholds that reach STC 38 or higher, tested per ASTM E90 and E413.[60][61] For project-specific needs, consult manufacturers for certified test reports, as performance varies by exact assembly, and requirements like fire-rating may limit maximum STC. Acoustic doors, incorporating dense cores, automatic door bottoms, and perimeter seals, often exceed 40, reducing loud sounds to barely perceptible levels and suitable for high-privacy environments like recording studios or hospitals.[62] The Outdoor-Indoor Transmission Class (OITC) addresses sound attenuation from exterior sources into buildings, particularly low-frequency transportation noise, as standardized in ASTM E1332.[63] It evaluates transmission loss or noise reduction data over one-third octave bands from 80 Hz to 4000 Hz, using an A-weighted reference spectrum derived from averaged spectra of aircraft takeoff, road traffic, and rail operations.[63] This spectrum features a steeper low-frequency contour compared to STC, with greater weighting below 400 Hz to reflect dominant energy in traffic and aircraft sounds—for instance, emphasizing reductions at 80 Hz where outdoor noise spectra peak.[63] The OITC is computed as the difference between the overall A-weighted outdoor reference sound level and the corresponding indoor level after subtracting the measured transmission loss values across the frequency bands, resulting in a rating typically 5 to 10 points lower than STC for assemblies exposed to highway or aviation noise due to poorer low-frequency performance.[64] For example, a wall assembly rated STC 35, common for basic residential partitions, might achieve an OITC of around 30 when evaluated against road traffic spectra, as the OITC penalizes inadequate bass attenuation from tires and engines. Similarly, window systems with STC 45 can yield OITC 35, highlighting the metric's sensitivity to exterior low-frequency sources. OITC finds primary application in designing building envelopes near transportation corridors, where it informs requirements for facades, windows, and doors to mitigate environmental noise. In regions with high traffic exposure, such as highways, facades often target OITC 30 or higher to limit indoor levels to acceptable thresholds.[65] For airports, U.S. Federal Aviation Administration guidelines under Advisory Circular 150/5000-9B recommend sound insulation programs achieving at least 30 to 35 dB of noise reduction (equivalent to OITC 35 or better for facades) in residences within the 65 dB DNL contour to ensure interior levels below 45 dB.[66] In contrast, general interior partitions prioritize STC 50 or higher for speech privacy, underscoring OITC's specialized role in outdoor-indoor scenarios.[67]Applications and Prediction
Legal and Practical Requirements
In the United States, the International Building Code (IBC), as adopted by many jurisdictions, mandates a minimum Sound Transmission Class (STC) rating of 50 for airborne sound insulation in multifamily dwelling unit separations, including walls and floors/ceilings, effective since the 2009 IBC updates under Section 1206 (unchanged as of 2025). Field verification requires an apparent STC (ASTC) or Field STC (FSTC) of at least 45 to account for installation variations and flanking paths, with testing conducted per ASTM E413 and E90 standards by accredited laboratories.[68] Internationally, national acoustic standards in European Union member states vary but typically require airborne sound insulation equivalents to STC using the weighted sound reduction index (Rw) of 50-55 dB for dwelling separations, calculated per ISO 12354. In Canada, the National Building Code (NBC) 2020 (editorially revised January 2025) requires an apparent STC (ASTC) of 47 or greater for multifamily units, building on the 2015 edition's shift from lab STC 50 to field-inclusive measures that limit flanking transmission. Australia's Building Code of Australia (BCA) 2022, under Part F7, specifies Rw + Ctr ≥ 50 (airborne equivalent to STC 50) for Class 2 and 3 buildings, with impact sound limits to ensure privacy between sole-occupancy units (unchanged as of 2025).[69][70][71] Practical applications enforce these ratings through sector-specific mandates and third-party verification. In hospitals, patient room partitions must achieve STC 50 or higher to minimize noise disruption, often verified via on-site ASTM E336 testing by certified acousticians per FGI Guidelines (2022). Hotels adhere to IBC's STC 50 minimum for guest room walls, with many chains targeting 55+ for premium privacy, enforced during occupancy inspections. Schools require STC 45-50 for classroom dividers under current guidelines like ANSI/ASA S12.60-2010 (R2020), prioritizing speech intelligibility while allowing for budget constraints, with compliance confirmed through post-construction audits and integrated into IBC Section 1207 as of 2021.[72][73][68] As of 2025, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook—HVAC Applications (2023 edition, building on 2022 updates) emphasizes adaptive noise control in urban environments, recommending STC enhancements for exterior walls to counter traffic and climate-exacerbated ambient noise, integrated into sustainable design per ASHRAE Standard 90.1. Post-pandemic building trends have amplified telework privacy needs, with increasing emphasis on higher STC ratings for dedicated office spaces in multifamily retrofits to support hybrid work, though not yet codified federally.Common Partition Ratings
Common partition ratings provide benchmark Sound Transmission Class (STC) values for standard building assemblies, helping architects and engineers select configurations that meet acoustic performance needs. These ratings are derived from laboratory tests under ASTM E90 standards and can vary based on specific materials, thicknesses, and installation details. For instance, increasing the mass of a partition by doubling it typically raises the STC by about 6 points, following the mass law principle of sound transmission loss.[74] Typical wall assemblies illustrate how basic constructions perform. A single wood stud wall (2x4 studs at 16 inches on center) with one layer of 1/2-inch gypsum board on each side achieves an STC of 33 without insulation, rising to 39 with fiberglass batt insulation. Adding a second layer of gypsum board on both sides boosts this to around 45. Concrete masonry units (CMU), such as an 8-inch ungrouted block wall, commonly rate at STC 45-50, with grouting or added finishes increasing performance further.[23][75] Floor-ceiling assemblies also have established ratings. A carpeted 6-inch concrete slab without suspended ceiling typically yields an STC of 50 for airborne sound transmission. For wood-framed floors, a joist system (2x10 joists at 16 inches on center) with 3/4-inch plywood subfloor, resilient channels, and gypsum board ceiling achieves STC 55, enhanced by underlayment materials that decouple layers.[76][77] Windows and doors often represent weak points in partitions. Single-pane glass windows have low ratings of STC 25, allowing significant sound leakage. In contrast, laminated acoustic glazing in double-pane configurations reaches STC 40, providing better isolation through added damping in the interlayer.[78][79] The following table summarizes representative STC ratings from authoritative sources like the Gypsum Association's GA-600 Fire Resistance and Sound Control Design Manual and National Research Council Canada publications, noting that actual values depend on exact configurations.[80][81]| Assembly Type | Description | Typical STC |
|---|---|---|
| Walls: Single Wood Stud | 2x4 studs, one 1/2" gypsum layer per side, insulated | 35-40 |
| Walls: Double-Layer Gypsum | 2x4 studs, two 1/2" gypsum layers per side, insulated | 45 |
| Walls: Concrete Masonry | 8" CMU block, ungrouted | 50 |
| Floors: Carpeted Concrete Slab | 6" slab with carpet and pad, no ceiling | 50 |
| Floors: Wood Joist | 2x10 joists, plywood subfloor, resilient underlay, gypsum ceiling | 55 |
| Windows: Single-Pane Glass | 1/8" glass in standard frame | 25 |
| Windows: Laminated Acoustic | Double-pane with laminated interlayer | 40 |