The ability of a wall assembly to attenuate sound is tested and quantified as Sound Transmission Class (STC). Wikipedia defines STC as:
"Sound Transmission Class (or STC) is an integer rating of how well a building partition attenuates airborne sound. In the US and Canada, it is widely used to rate interior partitions, ceilings, floors, doors, windows and exterior wall configurations."
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STC is tested by the ASTM standard E336 and the STC value is derived from the test results by a procedure defined in ASTM standard E413.
Comparing sound transmission of two walls
STC value can be also determined by a simulation program using a mathematical model of sound transmission through partitions. I have used the INSUL simulation to compare two assemblies, one with a concrete wall (Fig. 1) and one with double steel studs (Fig. 2). Both assemblies have STC 58, but which is better?
Assembly:
- 1/2-inch drywall
- 2.5-inch steel studs 16-inch on centres
- 5-ich concrete wall
- 2.5-inch steel studs 16-inch on centres
- 1/2-inch drywall
Figure 1 - Concrete wall assembly
Assembly:
- 5/8-inch drywall
- 3.5-inch steel studs 16-inch on centres with rockwool insulation
- 1-inch gap
- 3.5-inch steel studs 16-inch on centres
- 5/8-inch drywall
Figure 2 - Double stud wall assembly
Sound transmission of speech sounds
STC is defined in the standard in speech frequency band 125 Hz to 4000 Hz. The STC rating corresponds quite well to people's perception of speech sounds. A higher number represents the better ability of the assembly to block speech.
Sound transmission of music sounds
The problem is that party walls between occupancies must attenuate other sounds, such as music. Two assemblies with the same STC value can have quite different abilities to block low-frequency sounds, the bass beat of amplified music.
Comparing sound transmission
The chart in Fig. 3 displays sound transmission loss values at 1/3 octave frequency bands for both wall assemblies. Both wall assemblies are rated at STC 58.
Figure 3 - Sound attenuation at 1/3-octave frequency bands for both wall assemblies
In the frequency band 80 Hz to 250 Hz, the sound transmission is virtually identical for both walls. At frequencies above 250 Hz, the concrete wall assembly provides about 5 to 10 dB better attenuation.
The biggest difference is at low frequencies. In the frequency band between 50 Hz and 80 Hz, the concrete wall provides as much as 27 dB better sound attenuation than the stud wall. In practical terms, it means that for speech, both walls are about equally effective. For low frequency music sounds, concrete wall blocks such sounds much better. Note that STC rating for both assemblies is the same - STC 58.Please use the button below if you would like to contact the author of the article.
There are other articles in this blog comparing different assemblies, for example Steel Studs vs. Wood Studs.
