Flanking Noise

What is a Flanking Wall, Floor or Ceiling?

Flanking noise, in the field of building acoustics, is the sound that takes indirect routes through walls, floors and ceilings, bypassing the intended sound insulation. When sound encounters an obstacle such as a wall, it can travel through it in several ways. One way is by directly passing through the wall, which is known as the ‘direct transmission’. However, sound can also find alternate paths to reach the other side of this wall creating flanking noise.

These alternate paths can include gaps, cracks, cavities, plumbing or ventilation systems, electrical outlet boxes or other openings in the structure. When sound finds these pathways, it can bypass the sound-insulated structure and become noticeable on the other side of the wall, floor or ceiling.


Reduce Flanking Noise with the PhoneStar Acoustic Insulation Board

The diagram above 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 masonry, with or without a cavity. Flanking walls always transmit sound – the question is whether the level of sound is noticeable and detrimental to the treated floor. To mitigate flanking noise, you must identify and seal any potential pathways for sound transmission. This can involve sealing gaps around doors and windows or closing cavities.

If the noise is still too loud, 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 though. Additionally, employing techniques like decoupling, where the structure is physically separated from the adjacent surfaces, can help minimise the impact of flanking noise.

Flanking Walls

Flanking walls are walls which continue past the soundproofed floor or ceiling into the room or property above or below. This is shown in the diagram above. Noise in party walls between dwellings can travel between the external walls of these properties through cavities etc. If you need to treat a flanking wall, see the Soundproofing Wall options as well as the Window and Window Reveals advice below.

Treating Window Reveals in Flanking Walls

If the external flanking walls have a cavity, 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 needs to be removed. Cavity stops (45kg/m³ high density mineral wool) must then 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 or air vents at either side of a treated wall or floor is important. Often a neighbour’s window could be open and the source of the noise is actually coming from their window and in through your window. If your window is single glazed or draughty it might be better to look at installing secondary glazing or replacing it. This would likely be more beneficial than sound insulating your external wall, as the window is probably the acoustic weak spot. Also, look at inserting cavity stops around the window reveals. Acoustic air vents are available too if sound is travelling through them.

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. Also ensure that they are not positioned in the same cavity bay of a stud wall. We recommend placing the socket in front of the PhoneStar layer so that it will be sitting proud of the wall. Then the acoustic wall remains fully sealed. If a flush finish is required, a PhoneStar box will need to be created and sealed with acoustic sealant so that the socket can be placed in this box.

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. Older floors often have timber joists which run right through the wall into the next room and this creates flanking noise. Even if joists are supported on the same wall as the joists in the next room, this will create flanking noise, regardless if they are touching each other or not. When joists run through the wall, the mortar around the joists often shrinks. When the mortar dries it may leave gaps which will allow a lot of noise to pass through.

Alternatively, when timber joists run parallel to the treated wall, the small wall area within the joist cavity may not be sound insulated, 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 foot. If you need to treat a timber joist flanking floor, then follow the following procedures where practical.

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.

Flanking Strips

Acoustic Flanking Strips are used to prevent hard-flooring products and hard-floor soundproofing products from touching the perimeter walls. This reduces flanking sound transmission. Flanking strips or perimeter strips are usually 5mm or 10mm thick foam strips placed all around the floor and wall junctions. These strips prevent acoustic bridging between the hard and rigid acoustic flooring products and the walls. They are a common cause of problems and failures of the Building Regulations for the Passage of Sound. PhoneStar is quite unique because it does NOT require flanking strips to be used around the room perimeter. This is due to its special structure which contains a compacted, but loose, soft sand filling which does not vibrate off surrounding structures in the same way. This also makes PhoneStar quicker to install. Find out how you can best prevent flanking noise through walls, floors and ceilings with PhoneStar today.