By: Taylor Blaine, MCH Senior Acoustical Consultant
Sound Transmission Class (STC) is a single-number rating used to characterize the airborne sound isolation properties of a construction assembly, such as a wall, window, ceiling, etc. In general, higher STC ratings indicate greater levels of sound isolation performance.
STC, along with speech effort, expectations of privacy, and background sound levels, are important parameters for estimating satisfaction with speech privacy.
How is STC calculated?
The STC rating procedure involves fitting a standard STC contour to laboratory measured transmission loss (TL) data. The graphic below shows an example of the STC contour fitted to measured data.
Note that this procedure collapses all of the detailed information, which varies with frequency, into a single number.
Limited frequency range
We can hear a wide range of frequencies, approximately 20Hz to 20,000Hz. STC was designed to address sound isolation of human speech which fits in the middle of this range, and STC is specifically limited to the 125Hz through 4,000Hz bands.
Importantly, STC does not address low-frequency sounds of the sort generated by many problematic noise sources such as mechanical equipment, transportation, and music. For example, the “thump” of your neighbor’s subwoofer is not addressed by the STC descriptor.
Same STC rating, two different results
To help illustrate the challenges of single-number STC ratings, the chart below shows levels of the same sound transmitted through two walls that are each rated STC-50. The spectra of transmitted sound are markedly different, even though they share the same STC rating. The blue line (concrete wall) exhibits significantly improved low-frequency sound isolation and results in substantially less transmission of low-frequency sound, compared to the red line (stud wall).
Please show us the acoustical laboratory test report
Single-number ratings are helpful for quick general reference, but can over-simplify noise reduction performance.
An optical analogy for the STC acoustical descriptor would be to characterize the overall color and brightness of a multi-colored stained-glass window using nothing more than a single number.
Fortunately, STC ratings are derived from laboratory test reports, which describe the specific construction assembly tested, the testing methodology and, of course, detailed sound isolation performance by frequency. Especially whenever sounds other than speech are of concern (music, equipment, traffic, loudspeakers, etc.), acousticians base recommendations on the frequency-specific performance found in such laboratory test reports. An STC rating means that a laboratory test was performed, but sometimes these lab reports can be difficult to obtain, in which case we have to approximate a frequency performance.
Commonly available reference resources, such as fire rating handbooks that also include STC ratings, may provide summary STC ratings, but the lack of important component descriptions and details, such as stud gauge or type/thickness of internal insulation, too often lead to overly-optimistic expectations, by 6 or 8 STC points or more!
Summary
While of good initial value, STC ratings should be used with caution, especially if the sounds of concern include frequencies lower than the typical human voice produces. Proper acoustical design often requires frequency-specific analysis from information that should be found within the laboratory test reports that yielded the STC rating.
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