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 USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90). Outside the USA, the Sound Reduction Index (SRI) ISO standard is used.

The ASTM test methods have changed every few years and over many years have been changed significantly. Thus, STC results posted before 1999 may not produce the same results today, and this difference becomes wider as one goes back in time (that is the differences in test method from the 1970's to today are vast).

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. 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 125 Hz, possibly down to 50 Hz 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. Alternatively, Outdoor-Indoor Transmission Class (OITC) is a standard used for indicating the rate of transmission of sound between outdoor and indoor spaces in a structure that considers frequencies down to 80 Hz (Aircraft/Rail/Truck traffic) and is weighted more to lower frequencies.

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 an STC of 30. This number does not apply across the range of frequencies, since the STC value 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 3000 Hz but only 10dB of attenuation at 125 Hz.

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 offer little in the way of privacy. Adding absorptive insulation (i.e. fiberglass batts) in the wall cavity increases the STC to 36-39, depending on stud and screw spacing. Doubling up the drywall in addition to insulation can yield STC 41-45, 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) typically 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 or more 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. Various Cellulose insulation installation options can result in an STC of 50 or greater. [1]

Materials which can improve STC's in walls include mass-loaded vinyl (MLV) and soundproof drywall, such as QuietRock.

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 adequate sealing, 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.

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 normal speech, but may not adequate for a home theater or loud stereo which can reach 110 dB or more.

In serious 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 can require detailed design and construction and can be easily compromised by 'flanking noise', sound traveling around the partition through the contiguous frame of the structure, thus reducing the STC significantly. STC 65 to 70 walls are often designed into luxury multifamily units, dedicated home theaters, and high end hotels.

The demanding THX reference standard (a guideline for high-quality audio in movie soundtracks) requires partitions to achieve 50dB of attenuation at 63 Hz. 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, an STC this high is not achievable in simple 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 is justified.

Contents

[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

STC partition ratings taken from: "Noise Control in Buildings: A Practical Guide for Architects and Engineers"; Cyril M. Harris, 1994

[edit] See also

[edit] External links

[edit] References

  1. ^ ICC Legacy Report ER-2833 - Cocoon Thermal and Sound Insulation Products, ICC Evaluation Services, Inc., http://www.icc-es.org
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