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Specification guide

Flanking Paths Explained

Why an expensive wall can still leak sound — because noise takes every route, not just the obvious one. The gaps, ducts, shared floors and sockets that sabotage soundproofing, and why sealing is engineering, not tidying.

You can build a beautiful, heavy, expensive wall and still hear the neighbours perfectly. When that happens, the wall is almost never the culprit — the sound is going around it. These detours are called flanking paths, and they are the single most underestimated factor in soundproofing. A waterproof wall means nothing if the door is left open; a high-STC partition means nothing if sound simply travels over, under and around it. This guide shows you where sound sneaks through and why sealing the leaks is engineering, not housekeeping.

Sound Takes Every Available Route

Sound is energy, and energy is opportunistic: it doesn’t push politely through the one surface you treated, it exploits every path it can find. If you block the direct route through a wall but leave an easier route beside it, the sound simply takes the easier route and your investment in the wall is largely wasted. This is why professionals talk about treating the whole envelope of a room, never a single surface. A chain is only as strong as its weakest link, and sound always finds the weak link.

The maths makes it stark. Because decibels are logarithmic, a tiny leak carries a huge share of the sound: a gap amounting to just 1% of a wall’s area can cost around 10 dB of performance — which, since roughly +10 dB sounds twice as loud, means a single unsealed gap can make the finished wall sound twice as loud as it should. That’s why an unsealed high-mass wall can underperform a cheaper wall that was sealed properly.

Where Flanking Actually Happens

Flanking hides in the details of ordinary construction:

  • Gaps and cracks — the perimeter where a wall meets floor, ceiling or adjoining wall; unfilled joints; skirting and cornice edges.
  • Doors and their undercuts — the gap beneath a door is often the biggest single leak in a room; an untreated door can undo the wall it sits in.
  • Windows — thin glass and leaky frames are a classic weak link, which is why we so often add acoustic windows and secondary glazing.
  • Shared floors and ceilings — sound travels along a continuous concrete slab or floor structure and re-radiates in the next room, bypassing the wall entirely. Common in Ghana’s concrete-frame buildings.
  • Ductwork and HVAC — an air-conditioning or ventilation duct is a clear open pipe for sound from one room to another; see HVAC noise control.
  • Back-to-back electrical sockets and switch boxes — a socket cut into each side of a wall in the same cavity is a direct hole through your carefully built partition.
  • Service penetrations — pipes, cable runs and extractor openings, each a small unsealed tunnel.

Why DIY Soundproofing So Often Fails

This is the usual reason a homeowner’s own efforts disappoint. They add mass to the obvious wall — sometimes a lot of it — but leave the door undercut, the shared slab, the ducts and the sockets untouched. The sound simply reroutes and they conclude, wrongly, that “soundproofing doesn’t work.” It works; the flanking wasn’t addressed. It’s the same trap as buying foam and expecting it to block — the effort went somewhere that couldn’t deliver the result. The broader picture is in How Soundproofing Actually Works, and the airborne-versus-impact question in Airborne vs Impact Noise — note that shared-slab flanking is often impact energy travelling through structure, which mass alone can’t stop.

How Flanking Is Treated

Controlling flanking is methodical, not glamorous, and it’s why a survey matters so much:

  • Seal the perimeter — acoustic sealant and backer rod at every wall-to-floor, wall-to-ceiling and wall-to-wall junction, so the treated surface is genuinely continuous.
  • Treat the door and window — proper seals, drop seals and sweeps to close the undercut, or an engineered door set; secondary glazing for windows. See soundproof doors and seals.
  • Break the structural path — decoupling and floating floors so vibration can’t run along a shared slab, the domain of ceiling, floor and impact soundproofing and vibration isolation.
  • Silence the services — duct silencers and lagging, offset or acoustically boxed sockets, and sealed penetrations.

Every lab STC/IIC figure (see STC, IIC and NRC Explained) assumes these are done; in the real world they are the difference between the rating on paper and the result in your room.

The Honest Frame

Because sound takes every route, no wall alone delivers silence — the honest promise is a measured target for the whole room, verified before and after. Finding and sealing flanking paths is exactly what turns a good-looking wall into a genuinely quiet room, and it’s a core part of every assessment we do: we don’t just measure how loud it is, we trace how the sound is getting through.

Hearing noise you thought you’d blocked? It may be flanking. Book a free noise assessment: +233 20 531 3333.