sound transmission class
(STC)

See also impact insulation class.

A system devised by the ASTM in 1961 to describe how well various types of interior walls, floors, doors, etc. prevent sound in one room from reaching another.

Sound
Transmission
Class
Approximate Effect
STC 25 Normal speech can be easily understoood.
STC 30 Loud speech can be understood well.
STC 35 Loud speech audible but not intelligible.
STC 42 Loud speech audible as a murmur.
STC 45 Must strain to hear lound speech.
STC 48 Some loud speech barely audible.
STC 50 Loud speech not at all audible.

To assign an STC rating to a barrier separating two rooms, a sound is generated in one of the rooms, the sound power is measured on both sides of the barrier, and the ratio between the two measurements (the transmission loss) is stated in decibels. Sixteen measurements are made in each room, at ¹⁄₃ octave intervals from 125 Hz to 4000 Hz.

The standard specifies a transmission loss curve having 16 points on the same ¹⁄₃ octave intervals. From 125 to 40 Hz, the curve slopes 9 dB per octave; from 400 Hz to 1250 Hz, 3 dB per octave, and it is flat from 1250 Hz to 4000 Hz. The curve is moved up and down until the sum of all 16 differences between the curve's value and the measured values for the barrier is less than 32 dB (providing no single difference is more than 8 dB). The rating is then the curve's loss in decibels at 500 Hz.

Some Typical STC Ratings

STC class Construction
35 2×4's on 16" centers;
½" gypsum board both sides
39 2×4's on 16" centers;
½" gypsum board both sides;
cavities filled with rock wool or fiberglass batting
45 2×4's on 16" centers;
2 layers of ½" type x gypsum board both sides;
one thickness of R-11 fiberglass insulation
40 to 54 as types above, but using plaster instead of wallboard
increases the STC rating
 55 staggered stud 2×4's on 2×6 plate on 24" centers;
2 layers of ½" plasterboard on each side;
 one thickness of R-11 fiberglass insulation
56 2×4's on 16" centers;
2 layers of ½" gypsum board both sides,
but on one side the board is mounted on resilient channels;
one thickness of R-11 fiberglass insulation
56 3⅝" metal studs on 24" centers;
2 layers of ½" type x gypsum board on each side;
one thickness of R-11 fiberglass insulation
63 double wall: 2×4's on separate 2×4 sills, 16" center;
2 layers of ½" gypsum board on each side;
one thickness of R-11 fiberglass insulation

Caveats

The STC system is useful for comparing different ways of building a partition, but it is not a guarantee of a certain level of isolation. It tends to give too much credit to materials which absorb high frequencies, such as sheetrock, and too little to materials and forms of construction which absorb the lower frequencies. Also, workmanship has a large effect, particularly the care taken in sealing (caulking, usually) tiny cracks by which air can pass from one side of the wall to the other.

ASTM E 413.

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