Sound Transmission Loss & STC Estimator

How it works

Mass law: TL ≈ 20 log₁₀(m × f) − 47 dB (m in kg/m², f in Hz; at 500 Hz for STC); double leaf: TL of the combined mass + cavity (10 log(d/25 mm)) + 5 dB insulation + decoupling bonus (clips 7, staggered 4, independent 10); mass-air-mass resonance f₀ ≈ 60 √((m₁+m₂)/(m₁ m₂ d))

A single heavy partition blocks sound by inertia — the mass law says every doubling of surface mass gains about 6 dB, and higher frequencies are blocked more than bass, which is why walls are rated with a frequency-weighted single number (STC in North America, Rw elsewhere). Mass alone gets expensive fast; the efficient route is a double leaf: two lighter layers separated by an air gap behave far better than their combined mass would, provided the cavity is deep, filled with absorbent insulation and the two leaves are not rigidly tied together by common studs, which short-circuit the gap. Below the mass-air-mass resonance the double wall is no better than a single one, so bass from music and home theatre needs deep cavities and heavy leaves. In practice flanking paths — doors, ceiling plenums, back-to-back outlets, ducts — usually decide what you actually hear.

Worked example

Two 10 kg/m² leaves (½″ drywall each side) on common studs with a 90 mm insulated cavity: mass law for 20 kg/m² gives about 33 dB, the cavity adds 6 and the insulation 5 — around STC 44, so loud speech next door is heard but not understood, and 75 dB of TV becomes roughly 36 dB. Decoupling one side with clips would lift it to about STC 51.

Frequently asked questions