What is flanking noise, and why did my soundproofing not work?
The reason a properly built acoustic wall can make almost no difference: sound going around it through the floor, the ceiling, the chimney and the junctions. How to find the flanking path before spending anything.
- Difficulty
- intermediate
- Time
- 20 min
- Read
- 4 min
Short answer
Flanking is sound travelling around a barrier through connected structure rather than through it. In British houses the common paths are the floor joists into a party wall, a shared chimney, a continuous ceiling void, back-to-back sockets, and the wall junctions. Treating the wall alone leaves every one of them open, which is why the result disappoints.
A wall's performance is only as good as the weakest path around it. Laboratory figures for a wall build-up assume the wall is tested in isolation; in a real house the same construction is connected to floors, ceilings and return walls that carry vibration around it. That gap between the tested figure and the lived result is flanking, and it explains almost every disappointing soundproofing project.
Step by step
- Understand the principle.Sound energy travels through any connected solid. If the floor joists of both houses sit in the same party wall, energy entering the floor on one side arrives in the floor on the other without ever passing through the wall.
- Listen systematically before you build anything.Ear to the wall at several heights, then to the floor, the ceiling, the skirting line, the chimney breast, and beside each socket. Where it is loudest is where the path is. This costs nothing and it is the single most valuable half hour in the whole project.
- Check the floor junction.In terraces and semis the joists commonly bear into the party wall from both sides, sometimes into the same pockets. This is the most common flanking path in British housing and the hardest to treat.
- Check the ceiling void.Where a ceiling void runs continuously over a partition, sound goes over the top of the wall. This is very common where a stud wall stops at the ceiling rather than continuing to the structure above.
- Check the chimney.A shared stack with flues on both sides is a direct air path. It can be the dominant route and it is comparatively cheap to treat, with a properly ventilated closure.
- Check the electrical accessories.Back-to-back sockets in a party wall leave very little material between two openings. Acoustic putty pads around the boxes, or moving one accessory, is cheap and effective.
- Check the wall junctions and the perimeter.Where a new acoustic wall meets the return walls, the ceiling and the floor, any rigid connection or unsealed gap short-circuits it. Details at the edges determine most of the real-world performance.
- Check windows and external walls.In a flat or a terrace, sound can leave a neighbour's window and enter yours. A treated party wall does nothing about that, and secondary glazing is the measure that does.
- Treat the flanking path before or with the wall.The order matters. Sealing, socket treatment and the chimney come first because they are cheap. Then the wall. Treating the perimeter of the floor and ceiling at the junction with the wall is what makes a wall treatment perform closer to its tested figure.
- Set expectations against the weakest path, not the best.The overall result is dominated by whichever route is easiest, which means an untreated flanking path caps what any wall can achieve, however well built.
Tips
- A phone's sound level meter app is not laboratory equipment but it is quite good enough for comparing one spot with another, which is all the diagnosis needs.
- Ask the neighbour to make a consistent noise — a radio at a fixed volume — while you listen around the room. Ten minutes of cooperation is worth more than any amount of guessing.
- If you live in a building converted since sound insulation requirements applied to conversions, the separating construction was required to achieve a tested standard, and a failure to meet it is a matter for the freeholder or the original developer.
- Professional acoustic testing exists and is not enormously expensive relative to the cost of a soundproofing project that does not work.
If it doesn't work
New acoustic wall made almost no difference
Cause: Flanking through the floor or ceiling structure — Fix: Identify the path by listening at the junctions. Treating the perimeter, or isolating the floor, is what remains.
Noise clearly loudest at the skirting line
Cause: Floor structure flanking, or an air gap under the skirting — Fix: Remove the skirting and seal the junction with acoustic sealant, then reassess. Cheap, and sometimes surprisingly effective.
You can hear the neighbour's television in the fireplace
Cause: Shared flue — Fix: Close the disused flue with a ventilated closure. Do not seal it airtight — that causes damp.
Noise arrives at ceiling level in a room with a stud wall
Cause: The partition stops at the ceiling and the void runs over the top — Fix: Take the partition through to the structure above, or fill and seal the void over the wall line.
Sound seems to come from the socket
Cause: Back-to-back accessories — Fix: Acoustic putty pads round the box, or have the accessory relocated.
Questions people ask
Can flanking be eliminated?
Rarely completely in an existing house, because it would mean isolating the floors and ceilings as well as the wall. It can usually be reduced substantially, and identifying the dominant path means the money goes where it will do something.
Why do manufacturers' figures look so good?
They are measured on the construction alone in a laboratory, where no flanking exists. That is the correct way to compare products, and it is not a prediction of what a real room will achieve.
Is it worth getting a professional survey?
If you are considering spending a serious sum on a wall or a ceiling, an acoustic consultant's visit costs a small fraction of the works and can redirect the whole project. It is the sensible step before committing to a large treatment.