Party walls decide whether your neighbours become a problem

The options around sound insulation are set out side by side below, with the conditions that genuinely favour one over the other.

Two different sounds, two different fixes

Airborne sound — speech, television, music — travels through the air and is resisted by mass and by isolation between layers. Impact sound — footsteps, dropped objects, furniture moving — is generated in the structure itself and travels through it directly. Adding mass helps airborne transmission and does relatively little for impact, which is why heavy concrete floors still transmit footfall.

Resilient layers, floating floors and soft finishes address impact by breaking the structural path.

Flanking is the usual culprit

Sound frequently reaches the neighbour not through the separating wall but around it, through the floor, ceiling, facade or a continuous structural element. A well-specified party wall connected rigidly to a continuous floor slab performs far worse than its laboratory rating suggests. Flanking detailing is invisible, unglamorous and the difference between a quiet flat and a miserable one.

This is why acoustic performance is a construction quality issue as much as a specification one.

Small defects cause large failures

Sound behaves like water in that it will exploit any continuous path, and a small unsealed gap around a service penetration can dominate the performance of an entire wall. Back-to-back electrical sockets in a separating wall are a classic example, as are unsealed pipe penetrations and gaps at the wall head. The measured difference between a sealed and unsealed detail can be larger than the difference between two entirely different wall constructions.

Site supervision therefore matters more than the specification once a reasonable specification is chosen.

Regulation sets a minimum, not a standard

Most countries set minimum sound insulation values for separating walls and floors between dwellings, and the values and test methods differ. Some regimes require pre-completion testing of a sample of units; others accept approved construction details without testing. A building can therefore comply fully and still perform poorly in the specific unit somebody lives in.

Where testing is by sample, the incentive to get every unit right is weaker than it should be.

Layout does much of the work

Placing a bedroom against a neighbour's living room guarantees conflict regardless of the wall construction. Stacking bedrooms above bedrooms, wet rooms above wet rooms and circulation against circulation resolves much of the problem for free.

At street level, lift shafts, refuse chutes, plant rooms and communal stairs adjacent to habitable rooms are avoidable design failures. These decisions are made at plan stage and cost nothing, which makes them the highest-value acoustic intervention available.

What works in a dense grid may not transfer to a low-density suburb.

Retrofit is possible and lossy

Independent linings, floating floors and suspended ceilings can improve an existing separation considerably. Each consumes room dimension and ceiling height, which is why it is more painful in a building already built to minimum dimensions. Soft floor finishes reduce impact noise at source and are the cheapest single measure available to an occupant.

In many buildings the rules governing floor finishes are set by the lease or the management company rather than by building regulations.

Side by side

The takeaway

Look at what is on the other side of your bedroom wall before you look at what the wall is made of.

Measure the walk, not the map.