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.

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

2 × 4 on 16" or 24" centers, 3/8-5/8 inch wallboard, rock wool or fiberglass batting	STC 30 to 42
Same as above with plaster instead of wallboard	STC 40 to 54
Staggered stud 2 × 4's on 2 × 6 plate, 2 sheets of 5/8 inch plasterboard on each side, 2" fiberglass inside	STC 51

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.

ASTM E 413.