Most failed sound insulation tests are not caused by the wall. They are caused by sound travelling around it. Here are the
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September 06, 2026
Why sound insulation tests fail, and the details that cause it
By
September 06, 2026
Most failed sound insulation tests are not caused by the wall. They are caused by sound travelling around it. Here are the details that cause it, and what to check before the tester arrives.
The numbers you have to hit
In England and Wales, Approved Document E asks for airborne insulation of at least 43 dB DnT,w + Ctr for conversions and 45 dB for new build, with impact sound at or below 64 dB and 62 dB L’nT,w respectively. Higher is better for airborne; lower is better for impact.
Flanking transmission: the real culprit
You can build a separating wall that performs beautifully in a laboratory and still fail on site, because sound has found another path:
- A continuous floor screed running under the separating wall from one dwelling into the next. Structurally convenient, acoustically fatal.
- A shared ceiling void above the separating wall, giving sound a clear route over the top.
- Back-to-back sockets and service penetrations — small holes, disproportionate effect.
- Rigid connections between a heavy separating wall and a lightweight external wall.
- Joists built into the separating wall from both sides.
- Unsealed perimeters and gaps at the wall head. Almost always the cheapest thing to fix and the easiest to miss.
Workmanship, not just design
A specification is only as good as the build. The recurring site failures are sequencing errors: resilient layers bridged by fixings, acoustic mastic omitted at perimeters, plasterboard hard-fixed where it should be isolated, insulation left out of a floor void because it was easier to skip.
What to check before the tester arrives
- Is the floor screed genuinely discontinuous at every separating element?
- Is the ceiling void closed above every separating wall?
- Are all perimeters sealed with acoustic mastic, not gap filler?
- Are sockets offset rather than back to back?
- Have any additional fixings been added through resilient layers?
- Is the construction actually the one specified, layer for layer?
If you have already failed
It is recoverable, and usually cheaper than people fear once the actual transmission path is identified rather than guessed at. Send the test report and the as-built construction, and the remedial work can be specified against what the results are telling you.
Note that pre-completion testing itself has to be carried out on site by an accredited testing body. We specify the design and tell you what testing your scheme needs; we do not carry out the tests.