What is Sound Transmission Class (STC) Rating?
If you are in the construction industry, especially if you are doing projects in the hospitality business, you may have come across the acronym STC. It is a number that hotel operators like Marriott, Accor, Shangri-La, and a host of other hospitality companies are very particular and stringent about. In fact, they have a unified standard for every hotel brand to maintain the same guest experience anywhere in the world.
Sound Transmission Class Rating or STC Rating is a single-number rating used to define the sound blocking capabilities and sound insulation effectiveness of a soundproofing material. STC Rating is the standard mostly used in calculating how many of the original airborne sound level was reduced after passing through a soundproofing like wall systems, glass, enclosures, etc.
STC Rating was first introduced in 1961 by American Society for Testing and Materials (ASTM) and is currently the standard widely used by professionals in the construction industry.
Sound Transmission Class Rating Calculation
STC Rating of a material can be measured using existing methods presented in ASTM E336-14 and can be classified as per ASTM413-10. The process is actually pretty easy to understand. In a given space separated by a soundproofing material/partition, one part is designated as the noise source and another one is designated as the receiving. The difference of measured sound pressure level on the source and receiving designations are compared across 1/3 Octave Band Centre Frequencies from 125 Hz to 4000 Hz. The transmission loss is plotted on a graph and then compared again on a standard STC contour. The closer a specific transmission-loss graph matches an STC contour, the more that graph resembles that STC Rating.
STC Rating Chart
Acousticians have, over the years, developed tables that give an overall STC Rating and its equivalent real-life experience. General rule of thumb is that the higher the STC Rating the better performance in sound insulation and control.
| STC | What it sounds like |
|---|---|
| 25 | Speech can be heard and understood |
| 30 | Loud speech can be heard and understood |
| 35 | Loud speech can be heard but not intelligible |
| 40 | Loud speech is barely heard |
| 45 | Loud speech is inaudible |
| 50 | Loud sound is faintly heard |
| 60+ | Good soundproofing |
How to improve STC Rating?
There are many ways in order to improve the STC rating of a soundproofing material. Some people confuse this with NRC, but they serve different purposes. STC is how much sound is blocked, while NRC is how much sound is absorbed. STC rating reduces the sound entering or leaving a given space, while NRC rating improves the overall reverberation.
One way of improving the STC Rating of a soundproofing material is increasing its thickness and mass. Increasing the thickness and mass of a soundproofing material prevents the airborne sound from vibrating further and therefore dampening the transfer of transverse wave.
Thickness and mass are directly proportional to density. Using denser materials such as fiber cement boards for drywall and plaster for masonry walls significantly increase the STC rating of soundproofing material.
And lastly, using fibrous insulations such as rockwool and introducing air space to the wall structure increase the STC rating as this acts as another layer/buffer for sound insulation.