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Understanding flanking transmission: the number one reason soundproofing fails

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In the past, new homes were seldom built with any consideration for noise, and today most of these properties are not fit for modern living. Today new builds do have to pass a UKAS accredited sound insulation test, but this test doesn’t take into account how people actually live. For example, the indirect noise path of children playing in an upstairs bedroom can affect the next-door neighbour trying to read a book in his living room diagonally below. A barrier on the party walls alone will not reduce this noise transfer. This is why so many new build properties will pass a sound test, yet the new homeowner complains they can hear conversation and TV noise from next door.

To truly make homes soundproof, one must consider how sound actually transmits through the house. With more flexible employment and irregular shift patterns, daily noise is higher than ever, not to mention that land shortages mean we’re building nearer to busy roads.

Soundproofing: only as good as its weakest link

illustration showing the direct and indirect noise transmission paths

The illustration shows how noise in an upstairs room transmits through the property and through the party wall and flanking areas. In many cases the flanking paths (green arrows) are even louder than the direct transmission paths (red arrows). This means that you won’t get the full benefit of soundproofing the upstairs rooms until you’ve also treated the downstairs rooms and addressed the flanking areas.

Indirect noise paths are more complex and numerous. A party wall, for example, could have the floor, ceiling, window wall, and any wall perpendicular to the party wall as an indirect noise path. Also connected to that wall may be an uninsulated Rolled Steel Joist (RSJ), a bay window ceiling, chimney stack, stud wall, cupboard under the stairs, en suite, or wall separating the front room from the back room.

A conflicting ceiling or flat floor will have the walls as indirect noise paths. There are reports now being published showing that “dot and dab” plaster boarded walls actually amplify the noise from neighbours above or below, proving even louder than the direct noise path.

This is why flanking transmission can be understood as the “weakest link” in any soundproofing project, and in many projects, the noise from neighbours is louder from those flanking transmission areas than directly through the party wall.

Flanking transmission – it’s worth getting right the first time round

Lynne loved her semi-detached house in Gloucestershire. She enjoyed the area and she knew all the residents in her street. The neighbours were a lovely, busy family who were there for her if she ever needed anything.

Unfortunately, an unexpected turn of events left her needing some peace and quiet – and the noise from the family next door was not helping. Lynne was woken up every day by slamming doors, TV, the sounds of plug sockets and light switches, and could even make out words from conversations. Lynne had to play music just to give the family their privacy.

She was determined to get her house back and embarked on some research into soundproofing, convinced that the noise was coming from the party wall alcoves and maybe the chimney stacks. She decided to get the whole of the party wall in all the rooms of her property soundproofed. Lynne ended up purchasing a top of the range panel system from a London company with glowing online reviews. They installed it in the alcoves and all the chimney stacks in the four rooms along the party wall.

Sadly, nothing changed. The noise remained. Lynne still felt uncomfortable in her own home.

She didn’t give up, however. She got back online and discovered Quietco, a domestic soundproofing company that specializes in reducing noise in homes. After being educated on how the noise was actually entering her house, she realized the flanking transmission noise paths were louder than the direct noise paths. She was advised that the flanking areas of her house had to be addressed, and the soundproofing re-attempted. This meant uninstalling built-in wardrobes and removing bespoke alcove shelving she’d just painted and decorated.

indirect noise in floorUnderstandably, Lynne was reluctant to go through the upheaval of another house renovation – she’d already spent thousands, how did she know it would work this time? Her concerns disappeared when she was shown the actual mistakes the London company had made, and precisely where the noise was getting into her house. After learning more and speaking with people who’d been through the same process, Lynne decided to invest in her house and make it a home, once and for all.

Quietco installed soundproofing to all the flanking areas of the property and made right the other company’s work. Lynne’s property before the sound insulation test was 39dB dntw+CTR. Afterwards it was 63dB dntw+CTR – a massive 24dB dntw+CTR reduction in noise.

Lynne doesn’t hear her neighbours anymore, even saying, “I think they were away over Christmas as I didn’t hear anything.” We had independent UKAS accredited engineers test the party wall to document the results, which is a valuable asset if Lynne ever comes to sell the property. We managed to speak with the neighbours during the sound testing, and they told us that they had had the whole family round for Christmas – completely unbeknownst to Lynne!

Lynne still lives happily in her semi-detached house and loves it more than ever. Read her full blog here: https://quietco.uk/domestic/soundproofing-semi-detached

Lynne ’s story is not unusual. We commonly find that indirect noses paths are louder than direct ones in semi-detached houses and new builds. It’s usually down to uninsulated RSJs running into the party wall, which can channel noise into every room along their path. Similarly, an uninsulated stud wall fixed incorrectly to the party wall creates an indirect noise path.

In both these cases, you can install the most effective barrier on the party wall possible, but unless you soundproof the flanking areas, you’ll still get unwanted noise.

Common questions

How do I know it will work?

This is a very common concern. But with a little understanding of exactly how soundproofing actually works and why it will simply make sense to have it installed. Most clients wish they had called us years ago.

We give you the opportunity to speak with 50 of these people on our reference list who all decided to have soundproofing installed. They had to go through the same decision process of whether to move or stay in the home they love.

Seems like a lot of disruption…

Depending on which products you decide to have installed, the disruption can range from 2 days to 4 weeks. Which is a lot less disruption than moving!

Whether you live in an older property that wasn’t built for modern noise levels, or you’re in a new build but still experience noise, you might like to consider soundproofing. Since this is an investment, it’s worth taking the time to understand exactly how sound is finding its way into your home, so you can tackle even those sound paths that may otherwise be overlooked.

If this article has been useful, please share or comment.

Have you ever had a soundproofing attempt and it did not work as well as expected, We’d love to hear about your experience.

Simply fill out the tabs on the contact us form or give us a call: 01926 658 638
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