What is Flanking Noise?

Sound travels though a building by direct and in-direct means. Direct sound transmission is exactly what it says on the tin. Sound travelling directly through a floor, ceiling or wall construction. For example, a television on in one apartment will be heard by a neighbour through a dividing wall if it is too loud. The noise will be simply travelling directly through the wall construction. This is direct sound transmission. Direct sound transmission is very simple to reduce, you would simply treat the problem area (in this example it would mean treating the poor performing wall construction).

Direct vs. In-Direct Sound Transmission

Direct sound transmission is relatively easy to treat, the main issue with sound travel through buildings is that it also travels in-directly through the wall construction. In-direct sound transmission occurs when sound bypasses the main construction and travels though the floor, the ceiling or the connecting walls at the side. Although sound will seem to be coming though the wall it will also be travelling through these in-direct means and it is these in-direct means that tend to be missed on site installations or at specification generation and can cause acoustic test failures. In-direct sound transmission is more commonly known in the building industry as flanking transmission.

Unfortunately, the UK building industry has a miss-judged perception that if we have a piece of foam around the edge of the room this will be the answer to all the flanking noise problems on site. Although perimeter and flanking strips are important it is the noise that travels through
the structure that is more important to treat.

The Case Study Experiment

Hush got a call from an architects practice on the South Coast of England to help assist with a failed Sound Test. An architect had specified a dividing wall construction to separate two residential units within an existing building. As the existing building was being converted into separate residential dwellings it had to comply with the Material Change of Use Standards of Approved Document E. The architect had specified a twin stud wall construction to divide the two residential units. The wall construction was constructed on the existing ground floor screed. The wall construction that was specified was a twin timber stud construction comprising of two frames of 100mm stud work, a clear cavity between the stu frames, an insulation infill within either frame and two layers of Soundbloc Plasterboard on either side of the wall construction. This type of wall construction is standard and should have easily achieved the requirements of the Building Regulations. Unfortunately, when we tested the separating wall it achieved an airborne performance level of 31 dB DnTw+Ctr. It should of achieved 43 DnTw+Ctr. It was a dramatic failure by 12 dB.

Think about the In-direct Sound path

We found out from the test that the wall construction was actually performing well but noise was bypassing the wall construction and was traveling though the ground floor screed. This was an in-direct sound path (flanking path) causing the wall to fail a test. The architect hadn’t foreseen this at design and specification stage. To solve the problem we simply specified an acoustic matting product called Hush Mat 1 to be laid on the floors either side of the wall. The Hush Mat was laid and we tested the wall construction again. The wall passed the test achieving a figure of 44 DnTw+Ctr. We were able to get it a 13 dB improvement by removing the flanking path from under the wall and achieve the requirements of the Building Regulations.

This one case study highlights the importance of treating all sound paths that will be transmitting sound through a building. On this particular development the architect only thought about the direct sound path, yet it was the in-direct one that caused the wall to fail a test.

The image in this blog also highlights that when there are two adjacent rooms there is one direct sound path through the wall but 12 flanking paths. This is another indication that it is imperative to remove all sound paths when designing systems to reduce sound transmission through buildings.

Why Soundproofing fails?

It does by no means only mean that existing buildings suffer from flanking issues, in fact older buildings are less likely to have flanking problems than most new build developments. We come across so many acoustic test failures with modern day lightweight construction methods. Steel Frame, Timber Frame, Timber Joist and Lightweight Masonry Wall Constructions and Beam and Block Floors are the four most common construction types that experience flanking issues.

These flanking issues occur at the junction details of these systems and if all sound paths are not taken care of you can expect to see many acoustic problems with the development. The main culprit for these failures is continuous block work construction that travels two or more stories high on a building. If the continuous block work is not sufficiently lined to reduce flanking transmission, the floors will fail and acoustic test. It is so critical that these flanking paths are reduced and a suitable specification is being used.

How to reduce flanking issues

It is relatively straightforward to reduce these flanking issues. It is all about correct specification and correct detailing. It is imperative to specify the correct products and systems and to seek technical support so that acoustic specialists can check that all flanking paths are covered within a building project. This is something that was missed by the architects practice on the South Coast and it cost remedial time on site to solve the problem. If construction details are specified correctly at the beginning of a development and the specification is enforced by the design team (so that it is not changed on site), then we will stop seeing acoustic tests fail due to flanking noise.

Please speak to the Hush Technical Team should you have any further questions regarding flanking noise on 01519332026.