What an STC number does and does not tell you, why the lab figure is not what the guest hears, and how to set a realistic target.
Sound Transmission Class is a single-number rating derived from transmission loss measured across standard frequency bands, per ASTM E90 in the laboratory. It compresses a curve into one figure so that products and assemblies can be compared quickly.
That compression is also its weakness. Two assemblies with the same STC can behave very differently — particularly at low frequencies, which the rating weights lightly.
A laboratory figure is measured on a carefully built assembly with no flanking. On site you have service penetrations, door undercuts, continuous ceiling voids and workmanship variation. The field result is routinely several points below the lab number.
Set expectations accordingly. If a client needs a genuine field result, specify to a field-measured target and allow margin against the lab figure.
STC is weighted towards speech frequencies. A guest kept awake by a bass line from the bar downstairs, by a chiller, or by footfall is hearing energy below the range STC treats seriously.
For those complaints, look at the low-frequency behaviour of the assembly rather than the headline number — and consider a barrier formulation tuned for low frequency.
Decide what the space has to achieve in service, not on paper: sleep in a bedroom, confidentiality in a consulting room, no bleed between cinema screens. Then work back to an assembly with margin, and budget for sealing and detailing rather than only for the barrier material.
BlastBlock supplies the full range across Thailand, Malaysia, Indonesia, Vietnam, Philippines, Singapore. Distribution enquiries: info@blast-block.com.
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