The first question in any soundproofing project is: what kind of noise is it? The two fundamental categories — airborne and impact — behave differently and are tackled differently, so misidentifying the problem leads to spending money on the wrong solution. Many real-world complaints involve both at once.
Airborne sound
Airborne sound originates in the air — speech, television, music, a barking dog, traffic — and then vibrates the building structure, which re-radiates it into the neighbouring space. It's the 'I can hear them talking' problem. Airborne sound is tackled primarily by mass, decoupling, cavity absorption and sealing — building a barrier that's heavy, isolated and airtight, since even small air gaps let airborne sound straight through.
Impact sound
Impact (structure-borne) sound is created by direct mechanical contact with the structure — footsteps on the floor above, a dropped object, dragged furniture, a slammed door. The vibration is injected straight into the structure and travels through it efficiently. It's the 'I can hear them walking about' problem, and it's especially common (and intrusive) between flats. Because the energy enters the structure directly, impact sound is tackled at source — resilient layers, floating floors and soft finishes that cushion the impact — and by decoupling the structure, rather than by adding mass alone.
How they're measured
| Metric | Measures | Better performance |
|---|---|---|
| DnT,w (+ Ctr) | Airborne sound insulation between rooms | Higher dB = better |
| L'nT,w | Impact sound transmission between rooms | Lower dB = better |
| Rw | Airborne insulation of an element (lab) | Higher dB = better |
Airborne insulation is a 'how much is kept out' figure, so higher is better; impact transmission is a 'how much gets through' figure, so lower is better. The Ctr term added to airborne ratings is a low-frequency correction — it makes the rating reflect performance against bass-heavy noise (music, traffic), which is the harder, more realistic test.
Building Regulations Part E
In England and Wales, Approved Document E sets minimum sound insulation between dwellings — separating walls and floors must meet airborne and impact performance standards, with somewhat less demanding figures for conversions than for new build, reflecting what's achievable in existing structures. New separating floors must meet both an airborne minimum and an impact maximum; separating walls an airborne minimum. Part E also requires, in many cases, pre-completion testing (or the use of approved 'Robust Details') to prove the built performance — recognising that sound insulation, like airtightness, must be verified, not assumed.