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What is the difference between Flanking Strips and Flanking Noise?

Flanking Strips

Acoustic Flanking Strips are used to prevent hard flooring products and most floor soundproofing products from making contact with the perimeter walls in order to reduce flanking sound transmission. Flanking strips or perimeter strips 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 for the Passage of Sound. Please note that PhoneStar is quite unique because it does NOT require flanking strips to be used around the room perimeter. This is due to its soft structure which contains a compacted but loose sand filling. This also makes PhoneStar quicker to install.


Flanking Noise

Flanking Noise
Sound will move from one room to another through direct and indirect paths. Some amount of energy will travel through or around a wall, floor or ceiling structure. It is important to understand that while sound can travel through the air such as through stud wall and ceiling joist cavities, it can also transfer through rigid surfaces such as along studs, joists, pipes and concrete.

After treating the critical structure e.g. the floor in this case, the sound should be significantly reduced but there still may be a degree of flanking noise that can be heard but this should be tolerable. To gain the highest performance possible, the flanking elements may also need treating with PhoneStar.

The diagram 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, the flanking wall will also need to be treated with PhoneStar, as shown on the wall in the Source Room in this diagram. The wall in the Receiving Room does not need to be treated however.


Flanking Walls

Flanking walls are walls which continue past the treated element into the adjoining room or property. 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 external walls of semi-detached properties.

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


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, and pay particular attention to ensure 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 PhoneStar 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 acoustically treated wall by securing additional studs with a plasterboard lining, thereby leaving the acoustic wall fully sealed. This method of course takes up more of the living space.


Treating Window Reveals in Flanking Walls

If the external flanking walls have a cavity, then there is a considerable risk of flanking noise 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/M³ 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 PhoneStar 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 actually coming from outside. Therefore the external wall and window panes are the only structures preventing this noise from entering your property. The wall may be the weakest structure acoustically but after the external 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 glazing or triple glazing may need to be considered.


Flanking Floors, Joists and Joist Cavities

Flanking floors are floors which continue past the treated wall into the adjoining room or property. 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 even if they are supported on the same wall as the joists in the next room, whether they are touching each other or not. When joists run through the wall the mortar around the joists often shrinks as it dries and so may leave gaps which will allow a lot of noise to pass through, as well as the joists themselves conducting noise. Alternatively 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 and this 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 procedures where practical.


Concrete Floors

1. Lay 15mm PhoneStar 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

Access to the floor joists and cavity is required by removing some of the floorboards adjacent to the wall or some of the plasterboard from the ceiling downstairs beside the wall.

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 PhoneStar cut pieces to the wall at joist level.

4. Fill the first half meter (minimum) of each joist cavity next to the wall with dense mineral wool (45kg/m³), or fill the first joist cavity if joists run parallel to the treated wall.

5. Re-fit 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 PhoneStar as a floating floor treatment with boards butted tightly together.

8. Lay finishing floor surface.


If 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 PhoneStar as a floating floor treatment with boards butted tightly together.

3. Lay finishing floor surface.