Sound Isolation Estimator & STC Calculator

The single biggest factor in how much sound a simple wall blocks is its mass: the heavier the panel, the harder sound is to push through it. The mass law captures this — for a single layer of surface mass m (kg/m²), the transmission loss rises with both mass and frequency as TL ≈ 20·log10(m·f) − 47 dB, so it gains roughly 6 dB every time you double the mass (or the frequency). This tool computes that TL at the sixteen one-third-octave bands from 125 Hz to 4 kHz and then runs the real ASTM E413 procedure — sliding the standard STC reference contour up as far as it can go without the total shortfall exceeding 32 dB or any single band falling more than 8 dB below it — to read off the single-number STC. Note that STC is a laboratory rating; the corresponding field sound transmission class (FSTC) measured in a finished building typically runs 3–5 points lower due to workmanship and flanking, which is why building codes often set a field limit 5 points below the lab requirement. For practical acoustic isolation planning, the assemblies table provides more realistic targets than the mass-law estimate alone.

Where mass law stops being the whole story is exactly where good soundproofing lives. It assumes one limp leaf, so it misses the coincidence dip — a stiffness-driven trough in transmission loss that occurs near a panel’s critical frequency, where bending waves in the panel couple efficiently to airborne sound — and, far more importantly, decoupling: putting mass on two separated leaves with an air gap (a cavity, resilient channel, or a double/staggered stud wall) creates a far more effective soundproof wall than the same mass in one slab. That’s why the assemblies table — built from typical published STC ratings — shows a double-stud wall reaching the mid-50s while its mass alone would predict much less. Two more honest cautions: STC is a mid-frequency rating that ignores everything below 125 Hz, so it tells you little about bass or home-theatre isolation; and in real buildings the limiting factor is usually not the wall but the flanking transmission paths around it — gaps, back-to-back outlets, ductwork, and structure-borne sound travelling through the shared floor and ceiling. If your goal is to measure the actual noise reaching a room rather than estimate what a wall should block, a decibel meter can reveal how much sound is already getting through. Seal first, decouple second, add mass third, and get a real test before you rely on a number for code.