# Sound Transmission Class

- In acoustics, a single number rating for describing sound transmission loss of a wall or partition.

- The STC is calculated from the 1/3 octave transmission loss (TL) measurement of the partition over the 1/3 octave centre frequencies from 125Hz to 4000Hz. These 1/3 octave values are compared with a set of reference curves. The curve is chosen as that which any value is not more than 8dB below the curve and the total deficiency at all frequencies is not more than 32dB. The STC of the partition is the value of the transmission loss corresponding to the intersection of the chosen reference contour with the 500Hz ordinate.

- The reference contours are split into 3 straight line segments:
- low frequency segment increases by 15dB from 125Hz to 400Hz
- mid frequency segment increases by 5dB from 400Hz to 1250Hz
- high frequency segment that is flat from 1250Hz to 4000Hz

Construction | Mass per unit area kgm^{-2} | STC |

0.5 inch gypsum wallboard | 10 | 28 |

Two 0.5 inch gypsum wallboards bonded together | 22 | 31 |

2 inch by 4 inch studs on 16 inch centres, 0.5in gypsum wallboards on both sides | 21 | 33 |

4 inch hollow block, 0.5in plaster on both sides | 115 | 40 |

4 inch brick, 0.5in plaster on both sides | 210 | 40 |

9 inch brick, 0.5in plaster on both sides | 490 | 52 |

24 inch stone, 0.5in plaster on both sides | 1370 | 56 |

To achieve these values of isolation care must be taken within the construction of the patition. A 5dB difference in STC between the laboratory results and real life can be expected.

### Reference

ASTM E413 - 10 Classification for Rating Sound Insulation

**See also: **Acoustic Barriers, Ceiling Attenuation Class, Field Sound Transmission Class, Field Transmission Loss, Impact Isolation Class, Noise Criteria, Receiving Room, Sound Transmission Loss.

**Subjects: ** Architectural Acoustics Audio Noise & Vibration