The mass law predicts that transmission loss increases by 6 dB for every doubling of surface mass or frequency. Heavier walls block more sound. Enter the surface mass density of your wall to see the predicted TL across the frequency spectrum.
Common values: 12.5 mm drywall ≈ 10 kg/m², 100 mm concrete ≈ 230 kg/m², double 12.5 mm drywall ≈ 20 kg/m², 200 mm brick ≈ 380 kg/m².
| Frequency | Transmission loss |
|---|
The Mass Law of Sound Insulation
The mass law is the fundamental principle of airborne sound insulation. It states that the transmission loss of a single, limp, non-porous partition depends only on the product of its surface mass density and the frequency of the incident sound:
TL = 20 × log10(f × m) − 47.3 (dB)
where f is frequency in hertz and m is surface mass density in kg/m². The constant −47.3 incorporates the density of air and the speed of sound. This equation predicts two key behaviours: (1) doubling the mass adds 6 dB of TL at every frequency, and (2) every octave higher in frequency also adds 6 dB.
Real walls deviate from the mass law due to coincidence effects (where the bending wave speed in the panel matches the sound wave speed in air), structural resonances, and flanking transmission through connected structures. Nevertheless, the mass law provides a useful first estimate and correctly identifies mass as the primary variable for single-wall isolation.
Double-Wall Construction
A double wall consists of two separate leaves (panels) separated by an air gap. Below a characteristic frequency called the mass-air-mass resonance, the two leaves move in unison and the assembly behaves like a single wall of combined mass. Above this resonance, the air gap acts as a decoupling spring, and TL increases at approximately 18 dB per octave—three times faster than a single wall.
The mass-air-mass resonance frequency is:
f0 = (1 / 2π) × √(ρc² × (m1 + m2) / (m1 × m2 × d))
where ρ is air density (1.21 kg/m³), c is the speed of sound, m1 and m2 are the surface masses of each leaf, and d is the gap width. A deeper gap lowers the resonance, allowing the double-wall benefit to begin at lower frequencies. Filling the cavity with fibrous insulation (mineral wool) adds damping that reduces the resonance peak and further improves isolation.
STC Rating Explained
Sound Transmission Class (STC) is a single-number rating that summarises a partition’s airborne sound insulation performance across the speech frequency range (125–4000 Hz). Higher STC values indicate better isolation:
| STC | Subjective impression | Example construction |
|---|---|---|
| 25 | Normal speech easily understood | Single-pane window |
| 30 | Loud speech understood | Single layer drywall |
| 35 | Loud speech audible but not understood | Standard interior wall |
| 40 | Loud speech heard as murmur | Double drywall, insulated cavity |
| 50 | Loud sounds barely heard | Staggered-stud wall, double drywall |
| 60+ | Excellent isolation | Double-wall with resilient channels |
Practical Soundproofing Strategies
- Mass: add more layers (double drywall, mass-loaded vinyl). Each doubling of surface mass adds 6 dB.
- Air gaps: decouple the two sides of a partition. Staggered-stud or double-stud construction prevents direct structural bridging.
- Absorption in the cavity: fill the air gap with mineral wool to damp the mass-air-mass resonance and add 5–8 STC points.
- Seal all gaps: sound leaks through even small openings. A 1% gap in a wall can reduce effective TL by 10 dB or more. Seal electrical outlets, door frames, HVAC penetrations, and all edges with acoustic caulk.
- Flanking paths: sound can bypass a well-isolated wall via the floor, ceiling, ductwork, or plumbing. The weakest path determines the overall isolation.