Sound transmission class

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Sound Transmission Class (or STC) is a widely used** integer-number rating of how well a building partition attenuates airborne sound.

STC is used to rate interior walls, ceilings/floors, doors, windows and exterior wall configurations. See ASTM International Classification E413 and E90. The 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. Acoustical engineers fit these values to the appropriate TL Curve (or Transmission Loss) to determine an STC rating. The measurement is accurate for speech sounds but less so for amplified music, mechanical equipment noise, transportation noise or any sound with substantial low-frequency energy below 125 Hz. Sometimes, acoustical labs will measure TL at frequencies below the normal STC boundary of 125Hz, possibly down to 50Hz or lower, thus giving additional valuable data to evaluate transmission loss at very low frequencies, such as a subwoofer-rich home theater system would produce.

STC is roughly the decibel reduction in noise a partition can provide, abbreviated 'dB'. The dB scale is a logarithmic one and the human ear perceives a 10dB reduction in sound as roughly halving the volume - a 40 dB noise subjectively seems half as loud as a 50 dB one. (For more detail on equal-loudness curves see: Fletcher-Munson curves.) If an 80dB sound on one side of a wall/floor/ceiling is reduced to 50dB on the other side, that partition is said to have a STC of 30. This number does not apply across the range of frequencies, since the STC value is A-weighted and is derived from a curve-fit of many datapoints. Any partition will have less TL at lower frequencies. For example, a wall with an STC of 30 may provide over 40dB of attenuation at 3000Hz but only 10dB of attenuation at 125Hz.

Typical interior walls in homes (2 sheets of 1/2" drywall on a wood stud frame) have an STC of about 33. When asked to rate their acoustical performance, people often describe these walls as "paper thin". They do not offer much in the way of privacy. Adding absorbtive insulation (i.e. fiberglass batts) in the wall cavity increases the STC to 39, a much better rating, and doubling up the drywall in addition to the insulation can yield STC 45, provided the wall gaps and pentrations are sealed properly.

Note that doubling the mass of a partition does not double the STC. Doubling the mass (going from two total sheets of drywall to four, for instance) adds 5-6 points to the STC. Breaking the vibration paths by decoupling the panels from each other will increase transmission loss much more effectively than simply adding more and more mass to a monolithic wall/floor/ceiling assembly.

Structurally decoupling the drywall panels from each other (by using resilient channels, steel studs, a staggered-stud wall, or a double stud wall) can yield an STC as high as 63 for a double stud wall (see table below), with good low-frequency transmission loss as well. Compared to the baseline wall of STC 33, an STC 63 wall will transmit only 1/1000 as much sound energy, seem 88 percent quieter and will render most frequencies inaudible.

Due to their high density, concrete and concrete block walls have good TL values (STC's in the 40s and 50s for 4-8" thickness) but their weight, added complexity of construction and poor thermal insulation tend to limit them as viable materials in most residential wall construction, except in temperate climates and hurricane or tornado prone areas.

Another material getting more and more use is mass-loaded vinyl (MLV). Conceived as an acoustical substitute for sheet lead (which, due to its deleterious effect on human health, especially in children, is frowned upon as a modern building material) this material is a polymer, doped with heavy metals, Barium being the most common. When sound travels through MLV its viscoelastic nature converts sound vibrations into minute amounts of heat, which are dissipated through the material. Uses include stapling or gluing between drywall panels or wood panels (for sound reduction in floors or wooden enclosures) or hanging as an acoustic 'curtain' to reduce airborne noise. This material can potentially add 5-10 STC points to a partition, depending on application.

It must be noted that acoustical performance values such as STC are measured in specially constructed acoustical chambers and field conditions such as lack of sealants, outlet boxes, back-to-back electrical boxes, medicine cabinets, flanking paths and structure-borne sound can diminish acoustical performance. The as-built 'field-STC' (FSTC) is usually lower than the laboratory-measured STC.

Another way to reduce the vibration of sound waves is constrained-layer damping (CLD). In this system, a viscoelastic material is adhered between two rigid panels. When the panels vibrate, a shearing force is induced in the viscoelastic layer, converting some of the sound into heat. STC can be improved markedly and companies manufacture both pre-fabricated wall panels [1]and [2] glues[3] that exploit CLD to achieve an increase in STC.

New multifamily dwellings(apartments, condominiums, duplexes, etc.) are required by building code to meet a minimum of STC 50 between units. This is sufficient to block most noise, including normal speech, but may not adequate for the harshest noise; a home theater or loud stereo can reach 110 dB or more. (Legal requirements vary significantly from country to country)

In extreme cases (for instance, a bedroom adjacent to a home theater room, and an inconsiderate nocturnal neighbor, to boot) a partition to reduce sounds from high-powered home theater or stereo should ideally be STC 70 or greater, and show good attenuation at low frequencies. An STC 70 wall would take a great deal of careful design and construction and would probably be easily compromised by 'flanking noise', sound traveling around the partition through the contiguous frame of the structure and reducing the STC significantly. Except for massive structures (solid concrete walls and floor slabs, in addition to resilient mounting of finished wall and floor panels) an overall STC of 70 is terribly difficult to achieve in the field, and some builders and acousticians would say nearly impossible. The cost of such an effort would be enormous as well.

The demanding THX reference standard (a guideline for high-quality audio in movie soundtracks) requires partitions to achieve 50dB of attenuation at 63Hz. Few walls can meet that, as that requires a wall with an STC of 80 or higher. For all practical purposes, no sound will be heard on the other side of the wall with this level of construction. However, as stated before, an STC this high is not achievable in ordinary construction and this level of isolation is only feasible for high-end studios and theaters, where the design and construction can be carefully controlled and the additional cost - tens or even hundreds of thousands of dollars - is justified.

[edit] Sound Transmission Class Examples

STC	What can be heard
25	Normal speech can be understood quite easily and distinctly through wall
30	Loud speech can be understood fairly well, normal speech heard but not understood
35	Loud speech audible but not intelligible
40	Onset of "privacy"
42	Loud speech audible as a murmur
45	Loud speech not audible; 90% of statistical population not annoyed
50	Very loud sounds such as musical instruments or a stereo can be faintly heard; 99% of population not annoyed.
60+	Superior soundproofing; most sounds inaudible

STC	Partition type
33	Single layer of 1/2" drywall on each side, wood studs, no insulation (typical interior wall)
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
54	Single layer of 1/2" drywall, glued to 8" dense concrete block wall, painted both sides
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
72	8" concrete block wall, painted, with 1/2" drywall on independent steel stud walls, each side, insulation in cavities