Sound isolation between homes depends first on the mass of the wall and second, and more often decisively, on whether it is sealed: by the mass law a 30-40 cm wall of dense earth blocks roughly 60-65 dB of mid-frequency sound, but an open gap of just 1% of the wall’s area limits the whole wall to 20 dB however thick it is. Build thick dense party walls, add an air gap or independent double leaf where sleep must be protected, and seal every crack, door edge and pipe hole.
Part of Multi-Family Housing Design
Sound isolation between units is the reduction of airborne noise, such as speech, crying and music, passing through the walls and floors shared by neighbouring homes.
Mass Helps, Gaps Defeat It
The multi-family guide names noise as the biggest complaint in shared housing and sets four strategies: mass (target shared walls at least 30 cm thick with dense materials), decoupling (an air gap between two separate wall layers; even 2 cm helps, and loose insulation in the gap helps more), double-wall construction (two independent walls with a 5-10 cm gap, each on its own foundation or base plate with no rigid link between them) and sealing gaps under doors, around pipes and where walls meet the ceiling.
The mass law gives numbers to the first and last of these. The transmission loss TL of a single heavy wall, for sound arriving from all directions, is approximately
TL = 20 × log₁₀(f × m) − 47 dB
with f the frequency in hertz and m the wall’s surface mass in kg/m². Doubling the mass or the frequency adds 20 × log₁₀(2) = 6 dB. This is the usual field form of the mass law and is an ideal for a limp, airtight panel; real walls do worse because of flanking paths, gaps and resonances.
Worked Example: Wall Mass, Then Gap Leakage
Inputs: earth density 1,500-2,000 kg/m³ (Earth Load Calculations); timber about 500 kg/m³ (assumed); frequency 500 Hz, in the middle of speech.
- A 40 cm earth wall: m = 0.4 × 1,500 = 600 kg/m² to 0.4 × 2,000 = 800 kg/m². f × m = 500 × 600 = 300,000 to 500 × 800 = 400,000. log₁₀ = 5.48 to 5.60. TL = 20 × 5.48 − 47 = 62.5 dB to 20 × 5.60 − 47 = 65 dB.
- A 30 cm earth wall: m = 450-600 kg/m², TL = 60 to 62.5 dB.
- A 15 cm solid timber wall: m = 0.15 × 500 = 75 kg/m². TL = 20 × log₁₀(37,500) − 47 = 91.5 − 47 = 44.5 dB.
The earth wall beats the timber wall by 16-20 dB at 500 Hz, the multi-family guide’s claim that a 40 cm earthbag wall blocks more than a 15 cm timber wall. Low frequencies are weaker: the 40 cm earth wall gives 50.5-53 dB at 125 Hz.
Now the gap. If a fraction a of the wall area is an open path (transmission 1) and the rest transmits negligibly, the composite loss is TL = −10 × log₁₀(a).
- a = 1% = 0.01: TL = 20 dB. a = 0.1% = 0.001: TL = 30 dB. a = 0.01% = 0.0001: TL = 40 dB.
So a 15 m² party wall with a 15 cm × 10 cm hole (150 cm², 0.1%) is limited to 30 dB, far below the 62.5 dB of the wall itself. Sealing matters more than adding thickness once the wall is already massive.
What You Need
- Dense, solid material, earth, earthbag, brick or stone, free of voids
- Mortar, plaster or clay to close every joint
- Loose insulation (straw, wool) for any air gap
- An independent second wall where double construction is used
Method
Step 1: Build the party wall solid and continuous from foundation to roof, with plaster on both faces.
Step 2: For sleeping rooms of different families, build a second independent leaf with a 5-10 cm gap and no rigid connection; fill the gap loosely.
Step 3: Seal under doors, around pipes and flues, and at the ceiling junction.
Step 4: Do not back-to-back noisy rooms (kitchens, workshops) against another family’s bedroom.
Step 5: Test by ear with a loud sound on one side and a quiet listener on the other; leaks show as bright patches of sound.
Common Mistakes
- Thickening a wall that leaks — the gap sets the limit.
- Rigid ties across the air gap that carry vibration straight through.
- Hollow back-to-back cupboards or flues in the party wall.
Safety
Sealing a wall must not seal in smoke
Sealing for sound also stops accidental ventilation, so a stove in a tightly sealed unit needs a proper vent and flue to the outside, and a smoke or carbon monoxide path through a hole in the party wall is a hazard in itself. Never use a party wall as part of a flue.
Sources And Limits
The mass law, its constant and the gap formula are standard acoustics results. The formula is not stated elsewhere in this corpus, and the wall masses use the corpus’s own earth-density range; verify the constant (−47) against an acoustics text before using it for a binding design. Model building codes set minimum sound ratings for walls between dwellings, but no figure is given here because none is sourced.
What’s Next
- Learn next: Thermal Efficiency (Party Walls) — the same thick wall also holds heat
- Combine with: Sound Absorption Materials — controlling echo inside a room