What is Flanking Sound/Indirect Sound?

Posted in Soundproofing & Sound Testing on Oct 17, 2018

Flanking and indirect sounds move from room to room and travel indirectly over or around a soundproof element. A common occurrence of flanking is when sound is transmitted through a floor, beyond a separate partition despite the fact that the partition may provide good sound insulation to prevent sounds that are directly transmitted.

It's imperative that the issue of flanking is addressed during the design and construction phase in order to ensure that the building is soundproofed for both direct and indirect sound transmission.

What are the common ways that flanking sounds travel?

There are some typical paths for flanking sounds to travel, and one of them is through the framing of a floor under a wall. Floor systems are connected on either side of the wall, so the floor frame itself allows flanking to occur.

The joint system of a ceiling is another area where indirect sounds can travel. An attic room may run above two separate rooms, and the sound is transmitted through either the joists or an air cavity.

Another pathway is through sidewalls. A building will usually have walls that are directly connected, so sound can be transmitted around the elements used in the construction of this wall.

Air ducts can transmit airborne sound, and allow flanking sound to travel into a building, so it's important to take steps to reduce the risk of flanking.

Tips to prevent flanking noise

In order to reduce or prevent flanking and indirect sound in your buildings, there are some key things you should consider.

Doors - When designing the building, think about the type of doors you will use. Doors that are heavy, weather-stripped, and made with effective seals work best to reduce flanking.

Sockets - Flanking can also occur in electrical sockets. In order to prevent this, install your outlets in separate cavities, instead of back to back. Insulation can be added for mass, and ensure they are sealed correctly.

Floating Floors - Making use of floating floors is another way that you can reduce flanking sound. Effectively, this is a floor with a floating surface on top of a layer made from a material like rubber or fibreglass.

When designing and constructing a building, flanking sound needs to be considered at the earliest possible stage to prevent or minimise the risk of it occurring.

What should be considered when designing a building?

When it comes to designing a building, it's important to consider some of the aspects that can have an impact on the acoustics. For example, partitions should be designed in such a way that they close against permanent walls.

When using base seals, they should be installed against a hard and smooth surface instead of a carpet because the fibres from the carpet will allow sound to travel through.

Gaps, cracks and holes allow airborne sounds to travel, and have an impact on the quality of the sound insulation of a building, so a building should be airtight to provide the best possible protection against flanking sound.

Design and construction are essential to acoustic performance

When it comes to insulating against both direct and indirect sounds, the most important phase of any building development is the design and construction. During this stage of development, it's essential to consider the insulation for external, internal and separating walls.

External walls should limit the amount of sound that can be transmitted into a building, internal walls should be designed in such a way that flanking is prevented between rooms, and separating walls need to be considered in order to ensure two different areas within the same building do not suffer from flanking.

It's important to remember that junctions between elements open up the possibility of flanking to occur, so the need to design and construct this aspect of a building carefully is essential.

When there are spaces that are adjoined, but have different uses, it's imperative that the necessary steps have been taken in order to minimise or prevent flanking sound from occurring, so that the different areas within a building can be used for their intended purposes.

There are specific building regulations for noise and sound insulation called Approved document E which sets out the necessary requirements for sound insulation between different spaces, and gives guidance on how to separate elements in order to avoid the issue of flanking sound.

Just like direct sounds, indirect sounds can have a huge impact on the performance of a building, so ensuring buildings are designed within the constraints of the published regulations is an important part of the design and construction of a building.