Sound transmission class

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Sound Transmission Class (or 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 Sound Reduction Index (SRI) ISO index is used. The STC rating figure very roughly reflects the decibel reduction in noise that a partition can provide. To improve the sound transmission class and increase soundproofing two techniques are employed; sound isolation and sound absorption. Sound isolation entails blocking the path that sound can travel through by using cavity walls, adding resilient channels or using caulking to seal a room. Sound absorption entails adding extra mass to an already isolated partition, often using extra gypsum layers or by placing rockwool into cavities.


The STC or sound transmission class is a single number method of rating how well wall partitions reduce sound transmission.[1] The STC provides a standardized way to compare products such as doors and windows made by competing manufacturers. A higher number indicates better soundproofing than a lower number. The STC is a standardised theoretical measurement provided by ASTM E413 and E90 with the Field sound transmission class provided by ASTM E336-97 annex a1.[1]

Sound proofing[edit]

STC is used as a means of measuring the sound proofing of partitions between living spaces.

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 neighbouring residents.[2]

Rating methodology[edit]

Sound Transmission Class Report Sample from NTi Audio showing Transmission Loss in the sixteen standard frequencies

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.[3]

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]

The sound transmission class and thereby soundproofing is affected by the two factors; sound isolation and sound absorption.

Sound isolation[edit]

Sound travels from one area to another in two ways. It travels through the air and it travels mechanically through the mass of the structure. 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. [4]

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.[5] Great care must be taken in each type of decoupled partition construction, as any fastener that becomes mechanically (rigidly) coupled to the framing can short-circuit the decoupling and result in drastically lower sound isolation results.[6]

Sound damping tapes and caulking have been used to improve sound isolation since the early 1930s.[7] Although the applications of sound damping 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 damping in interior sound isolation in buildings.[8]

Sound leakage[edit]

For sound isolation to be effective all holes and gaps should be filled and the enclosure hermetically sealed. 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 metres squared. Even small open gaps and holes in the partition have a disproportionate reduction in sound proofing. With 5% or 0.5 metres squared of the partition "open" and offering unrestricted sound transmission from one room to the next transmission loss reduces from 40 dB to 13 dB. A 0.1% open area, or 1 cm squared, where for example caulking has not been applied effectively will reduce the transmission loss from 40 dB to 30 dB.[9] 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.[10]

Transmission loss % of area open
13 dB loss 5% open
17 dB loss 2% open
20 dB loss 1% open
23db loss 0.5% open
27 dB loss 0.2% open
30 dB loss 0.1% open
33 dB loss 0.05% open
37 dB loss 0.02% open
39.5 dB loss Practical maximum loss
40 dB loss Theoretical maximum loss

Sound absorption[edit]

Sound absorption entails turning acoustical energy into some other form of energy, usually heat.[11]


Adding mass to a partition reduces the transmission of sound. This is often achieved by adding additional layers of gypsum. The effect of adding multiple layers of gypsum wallboard to a frame also varies depending on the framing type and configuration.[12] 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.[13] 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.[12] Adding a second additional layer (to the already 3-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.[15] 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.


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.[16] 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" (406 mm) on-center results in only a few STC points. 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. As the stud gauge becomes heavier, the presence and type of insulation matters less.

Legal and practical requirements[edit]

Section 1207 of International Building Code 2006 states that separation between dwelling units and public and service areas must achieve STC 50 (STC 45 if field tested) for both airborne and structure-borne. However, not all jurisdictions use the IBC 2006 for their building or municipal code. In jurisdictions where IBC 2006 is used, this requirement may not apply to all dwelling units.

Common partition STC[edit]

Interior walls with 1 sheet of 1/2″ (13 mm) gypsum wallboard (drywall) on either side of 2x4 (90 mm) wood studs spaced 16" (406 mm) on-center with fiberglass insulation filling each stud cavity have an STC of about 33.[17] When asked to rate their acoustical performance, people often describe these walls as "paper thin." They offer little in the way of privacy. Double stud partition walls are typically constructed with varying gypsum wallboard panel layers attached to both sides of double 2x4 (90 mm) wood studs spaced 16" (406 mm) on-center and separated by a 1" (25 mm) airspace. These walls vary in sound isolation performance from the mid STC-40s into the high STC-60s depending on the presence of insulation and the gypsum wallboard type and quantity.[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.[18]

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″ drywall on each side, wood studs, no insulation (typical interior wall)
39 Single layer of 1/2″ drywall on each side, wood studs, fiberglass insulation [19]
44 4″ Hollow CMU (Concrete Masonry Unit) [20]
45 Double layer of 1/2″ drywall on each side, wood studs, batt insulation in wall
46 Single layer of 1/2″ drywall, glued to 6″ lightweight concrete block wall, painted both sides
46 6″ Hollow CMU (Concrete Masonry Unit) [20]
48 8″ Hollow CMU (Concrete Masonry Unit) [20]
50 10″ Hollow CMU (Concrete Masonry Unit) [20]
52 8″ Hollow CMU (Concrete Masonry Unit) with 2″ Z-Bars and 1/2″ Drywall on each side [21]
54 Single layer of 1/2″ drywall, glued to 8″ dense concrete block wall, painted both sides
54 8″ Hollow CMU (Concrete Masonry Unit) with 1 1/2″ Wood Furring, 1 1/2″ Fiberglass Insulation and 1/2″ Drywall on each side [21]
55 Double layer of 1/2″ drywall on each side, on staggered wood stud wall, batt insulation in wall
59 Double layer of 1/2″ drywall on each side, on wood stud wall, resilient channels on one side, batt insulation
63 Double layer of 1/2″ drywall on each side, on double wood/metal stud walls (spaced 1″ apart), double batt insulation
64 8″ Hollow CMU (Concrete Masonry Unit) with 3″ Steel Studs, Fiberglass Insulation and 1/2″ Drywall on each side [21]
72 8″ concrete block wall, painted, with 1/2″ drywall on independent steel stud walls, each side, insulation in cavities

Sound proofed partitions[edit]

  • Single metal stud partitions are more effective than single wood stud partitions, achieving a gain of between 2 and 10 STC. However for double stud partitions there is little difference between the two.[22]
  • A fibre or rock wool cavity fill will increase STC by between 5 and 8.[22]
  • Resilient channels are more effective on wood than metal studs.[22]
  • Increasing partition stud offset from 16 to 24 inches increases STC by 2 to 3 points.[22]
  • It is preferable to have non symmetrical leaves, for example with different thickness gypsum.[22]
  • Double stud partitions have a higher STC than single stud.[22]

See also[edit]


  1. ^ a b Ballou 2008, p. 72.
  2. ^ Bradley, J. S. (August 2001). Deriving Acceptable Values for Party Wall Sound Insulation from Survey Results. The 2001 International Congress and Exhibition on Noise Control Engineering. The Hague, The Netherlands. CiteSeerX
  3. ^ Ballou 2008, pp. 72-73.
  4. ^ Ballou 2008, p. 89.
  5. ^ NRC IRC-IR 761,
  6. ^ LoVerde and Dong, Proceedings of 20th ICA 2010,
  7. ^ Shafer, Proceedings of Meetings on Acoustics Vol. 19,
  8. ^ Shafer and Tinianov, The Journal of the Acoustical Society of America Vol. 130,
  9. ^ Ballou 2008, pp. 77-78.
  10. ^ "Archived copy". Archived from the original on 2010-03-15. Retrieved 2012-02-07.CS1 maint: archived copy as title (link) Acoustics in Practice
  11. ^ Ballou 2008, p. 97.
  12. ^ a b NRC IRC IR-761, and Sound and Vibration Magazine March 2010,
  13. ^ ASTM E413 Classification for Rating Sound Insulation,
  14. ^ a b c NRC IRC IR-761,
  15. ^ NRC IRC BRN-217,
  16. ^ The Journal of the Acoustical Society of America, Vol. 63 No. 6 pp 1851-1856,
  17. ^ NRC IRC IR-761,
  18. ^ Sound and Vibration Magazine, March 2010,
  19. ^ The Complete Photo Guide to Home Improvement. Creative Publishing international. July 2001. ISBN 9780865735804. Retrieved 2011-10-01.
  20. ^ a b c d "STC RATINGS FOR MASONRY WALLS". Retrieved 2011-10-01.
  21. ^ 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.
  22. ^ a b c d e f Balou 2008, pp. 76-77.


  • Cyril M. Harris. 1994. Noise Control in Buildings: A Practical Guide for Architects and Engineers.
  • Glenn M Ballou. 2008. Handbook for Sound Engineers 4ed. Elseveir. USA.