Flanking Noise

Flanking Strips
Do not confuse Perimeter Flanking Strips with Flanking Noise.  Flanking Strips are used to protect perimeter walls from the noise vibrating in inferior acoustic flooring products. They are usually 5mm or 10mm thick foam strips placed all around the floor and wall junctions, to prevent acoustic bridges going from the inferior acoustic flooring products into the walls.  They are a common cause of problems and failures of the Building Regulations. Phonewell® does not require flanking strips.

Flanking Noise
All acoustic floors, walls and ceilings are susceptible to flanking noise via other adjoining construction elements whether they are floors, walls or ceilings. To gain the highest performance from the treated structure the flanking elements may need treating also, or at least the largest flanking element must be considered.

The diagram below shows how the noise from the source room can still pose a threat to a well-treated floor via the flanking wall common to both rooms. This flanking wall could be timber, steel or solid block (as shown) and it could also be single leaf, double leaf or twin leaf with a cavity.  Flanking walls always transmit sound - the question is whether the level of sound is noticeable and detrimental to the treated floor - if it is, it needs treating with Phonewell®, as shown on the wall in the source room in this diagram.

Flanking Walls

Flanking walls are easily identifiable - they are walls which continue past the treated element into the adjoining room. A flanking wall is shown in the above diagram continuing past the treated floor.  However some flanking walls continue past the party walls between dwellings, such as to the outside walls of semi-detached properties.

Noise in a flanking wall can usually be heard if it needs treating, especially after the main/direct element has been treated, and often the vibrations can be felt with the bare hand.  If you need to treat a flanking wall see Window Reveals, Windows and the Soundproofing Wall options.

Electrical Wall Sockets

Wherever practical, avoid placing electrical wall sockets in acoustic walls.

When there is no alternative, stagger sockets as much as possible at either side of the wall so sockets are never back to back, pay particular attention that they are not positioned in the same cavity bay of a stud wall. Pack behind and box in the socket with at least one or more layers of Phonewell® and seal with acoustic sealant.

An alternative and more failsafe method to the above procedure is to provide a service void on the surface of the finished treated wall by securing additional studs with a plasterboard lining, thereby leaving the acoustic wall fully sealed. This method of course takes more of the living space.

Flanking Floors, Joists and Joist Cavities

Flanking floors are easily identifiable - they are floors which continue past the treated wall into the adjoining room.  This would be the case in the diagram above if the floor had continued to the left to form a receiving room to the left of the wall. Timber joists also create flanking floors when they run through the wall into the next room, or simply, when they are supported on the same wall as the joists in the next room, whether they are touching each other or not. Additionally, when timber joists run parallel to the treated wall, the small wall area within the joist space and below the floor boards will often be untreated which can also be a source of flanking noise.

Noise in a flanking floor can usually be heard if it needs treating, especially after the wall has been treated, and often the vibrations can be felt with the bare hand. If you need to treat a flanking floor, then follow the following procedure where practical and if access allows.

Concrete Floors
1. Lay 15mm Phonewell® as a floating floor treatment with boards butted tightly together. If the floor is a ground floor or if it has been newly laid and moisture is inherent, first lay a vapour DPM on the concrete, as good practice.
2. Lay finishing floor surface.

Timber Joist Floors
Where there is access to the floor joists and cavity:
1. Fill holes in wall at joist level with mortar.
2. Seal around the joists where they enter the wall (if not parallel to wall).
3. Optionally bond some Phonewell® cut pieces to the wall at joist level.
4. Fill the first half meter (minimum) of each joist cavity with dense mineral wool (45kg/m³), or fill the first joist cavity if joists run parallel to the treated wall.
5. Refit floorboards or fit tongue & groove sub-deck e.g. 18mm chipboard.
6. Make the floor surface sound by sealing any holes or gaps with sealant, or if necessary tack down a 3mm hardboard sheeting.
7. Lay 15mm Phonewell® as a floating floor treatment with boards butted tightly together.
8. Lay finishing floor surface.

Where there is no access to the floor joists and cavity:
1. Make the floor surface sound by sealing any holes or gaps with sealant, or if necessary tack down a 3mm hardboard sheeting.
2. Lay 15mm Phonewell® as a floating floor treatment with boards butted tightly together.
3. Lay finishing floor surface.

Window Reveals in Flanking Walls

If the outside flanking walls have a cavity, then there can often be a flanking noise problem coming through the window reveals.  To reduce this, the plasterboard around the window fitting and the window sill need to be removed, and cavity stops (45kg high density mineral wool) must be inserted around all the window reveals filling the width of the exposed cavity as deeply as possible.  It is advisable to add 15mm of Phonewell® and plasterboard to finish the reveals, if there is room without impeding the window frame.

Windows in Flanking Walls

The proximity of windows at either side of a treated wall or floor is important. Often a neighbour’s window could be open and the source of the full noise is coming from outside. Therefore the outside wall and window panes are the only structures preventing this noise from entering the property. Often the wall is the weakest acoustically but after the outside flanking wall and window reveals have been treated, the window can be the weak spot. Treated walls will reduce noise generally by a minimum of 50 to 60dB, so check with the manufacturer of the double glazing that the noise reduction of the window will be at least this value. Secondary or triple glazing may need to be considered.