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Flanking sound (or flanking noise) is sound that transmits between spaces indirectly, going over or around, rather than directly through the main separating element. This can allow sound to transmit between spaces even though the main separating element itself provides good acoustic insulation.
Approved document E: Resistance to the passage of sound, defines ‘flanking transmission’ as, ‘Sound transmitted between rooms via flanking elements instead of directly through separating elements or along any path other than the direct path’. It defines a ‘flanking element’ as, ‘Any building element that contributes to sound transmission between rooms in a building that is not a separating floor or separating wall’.
A common example of flanking is sound transmitted between two spaces through a floor void (or even a floating screed) that runs under the separating partition, even though the partition provides good acoustic insulation preventing the direct transmission of sound.
Any building element that penetrates or circumnavigates a separating element can result in flanking. This might include:
- Windows and doors.
- Flanking ceilings, floors and walls which continue past the separating element into the adjoining space.
- Voids such as wall cavities, suspended ceilings and raised floors.
- Penetrating joists.
- Corridors and other circulation spaces.
- Ductwork and pipework.
- Poor workmanship.
Flanking should be considered early in the design stage of new developments and detailing should eliminate or minimise the inadvertent downgrading of sound insulation. Junctions between elements in particular can offer a potential flanking route if they are not carefully detailed and constructed. Good briefing, supervision and inspection on site can help to ensure that the quality of workmanship remains high so that details are constructed as designed.
Flanking can be a particular issue where adjoining spaces have different uses, such as; a lecture theatre next to an office, a private room adjacent to a circulation space, or between neighbouring houses with different patterns of occupancy and behaviour.
Flanking can be difficult to treat in older buildings, where the addition of sound insulation to one element simply reveals a flanking path through another.
Approved document E of the building regulations: Resistance to the passage of sound, sets out requirements for sound insulation between spaces and provides guidance on how to detail separating elements to avoid flanking.
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