Acoustics: reading an STC number properly
Sound Transmission Class compresses a whole frequency curve into one number. It is useful for comparison and misleading if used alone, especially for low-frequency noise.
What STC measures
ASTM E90 measures how much sound an assembly stops at each of sixteen frequency bands between two reverberation rooms. STC is a single number fitted to that curve.
Because the fit is weighted toward speech frequencies, STC describes how well a wall blocks conversation better than it describes how well it blocks traffic, plant noise or a subwoofer.
Rough guide to the numbers
Around STC 35 loud speech is audible but unintelligible. Around STC 45 it is barely audible. Above STC 50 most normal sounds do not carry, and hotel and multifamily demising walls are typically specified in the 50s.
Absorption is a separate property: it describes how much sound a surface soaks up inside a room rather than how much it stops passing through.
Why the assembly, not the panel, is rated
Flanking paths defeat good walls. Sound travels through a shared floor, a continuous ceiling plenum, back-to-back outlets or an unsealed penetration. A wall specified at STC 55 and installed with an unsealed penetration will not perform at 55.
What Guardian publishes
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An STC rating of up to 59 is available in an engineered assembly.
Source · 3-Part Specification 1.5 E.3
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Measured sound absorption: 0.36 at 1,000 cps, 0.54 at 2,000 cps, 0.38 at 4,000 cps.
Source · 3-Part Specification 1.5 E.4
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Written by Data Subsystems, for Guardian Structural Technologies. Last updated .
Drafted from Guardian Structural product literature and published building-science sources. Awaiting Guardian technical review. Confirm any figure against the current product documents or ask us before you rely on it in a design.