Flanking noise or flanking sound – as it’s commonly known, is defined as sound from a source room that is not transmitted via the separating building element e.g. the wall or floor partition. The sound is transmitted indirectly via paths such as external walls, windows, doors and internal corridors. The term ‘Sound insulation’ describes the reduction of sound that passes between two spaces separated by a dividing element, such as a wall or floor partition. The sound energy passes through the dividing element (direct transmission) and through the surrounding structure (indirect or flanking transmission).
One of the most popular reasons for excessive flanking noise and potential sound insulation test failure is when the inner leaf of the perimeter wall is built with light weight blocks. This can act like a large snare drum allowing the sound to travel straight up and across the walls from one flat to the other. Even if you have used an acoustically robust wall and/or floor partition the sound insulation testing may still fail. If you have used lightweight blocks in your onsite construction and the building fails the sound test, you may need to construction independent internal plasterboard lining throughout the inner perimeter wall, this should isolate the lightweight blocks and ensure the flanking path is minimized between dwellings.
Invariably, sound will usually find the weakest link through construction materials and this is usually via material flanking. For instance, construction workers sometimes run the inner envelope walls straight through the whole building elevation. Unfortunately, Even though acoustically robust walls may be pulled of the inner envelope, it doesn’t stop sound travelling from unit to unit via the continuous (lightweight) wall behind. In his instance the envelope wall should stop and return along the dividing wall partition providing a cavity or sound insulation break in the envelope wall and between flats.
Typical Sound Flanking Pathways
Typical Flanking Sound Transmission Pathways are as follows:
- Dividing Ceiling Partitions – Above and Through the Ceiling Space (where an adequate acoustic break has not been carried on through the ceiling void)
- Dividing Floor Partitions – Through Floor and Floor Joist Space (if insulation has not been installed or direct fixing to joists without a drop ceiling below the partition under test)
- Shared Structural Building Components – Floor Boards, Floor Joists, Continuous Drywall Partitions, Continuous Concrete Floors, and Cement Block Walls.
- Through Structural Steel (structural steel beams are often a major cause of noise transmission as plasterboard is often fixed directly to the steel without sound breaks)
- Plumbing Chases – Junctures Between the Walls & Floor Slab Above or at the Exterior Wall Juncture (this should be filed with mortar etc. to add mass to this weakened area)
- Through Windows (if they are no double glazed or have secondary glazing as a minimum)
- Fixtures & Outlets – Light Switches, Telephone Outlets, and Recessed Lighting Fixtures (if penetrations have been cut back to back with the opposite dwelling under test)
- Structural Joints – Perimeter Joints at Wall & Floor, Through Wall & Ceiling Junctures (these should be filled with acoustic mastic)
- Around the End of the Partition Through the Adjacent Wall (acoustic mastic should be used to seal this junction)
To reduce the chance of sound insulation test failure on your project, it is imperative that flanking transmission is considered at the design stage and any construction detailing is specified to minimise any potential of noise flanking which will downgrade the acoustic performance. Good detailing at the design stage will minimise this effect and optimise the overall levels of acoustic privacy achieved. If designing for residential units, design advice on flanking details must be followed to maximise the possibility of achieving the specified acoustic performance. It is very important that the design advice is followed during the onsite construction; otherwise the site sound insulation values may not meet the performance criteria required and subsequent expensive remedial treatment may be required.
One of the easiest ways of dealing with sound flanking issues is to use isolation strips around the perimeter of the partitions at the edges of floors and walls – as per the details below). Acoustic sealant should also be incorporated wherever possible.
The 5 Main Design Considerations
To improve the chance of a successful sound test and reduce the chance of noise flanking you will need to ensure that you allow for the following 5 acoustic design considerations, they are:
We can advise on all types of acoustic design, whether it’s accomplished during initial construction or during a refurbishment/renovation project. We also undertake UKAS accredited sound testing providing a ‘one stop’ solution for all your acoustic requirements.
Alternately, If you would like more information on how to prepare for your sound testing please download our sound test checklist.