Flanking sound

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.

Flanking can result from both impact sounds and airborne sounds.

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.
• Sockets.
• 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.

Typical measures to limit flanking include avoiding continuous screeds, ceilings and raised floors running beneath or above separating walls, taking separating walls through to the underside of the floor or roof structure, sealing gaps and service penetrations, and using resilient layers or isolation at junctions. For new dwellings and rooms for residential purposes, compliance with Approved Document E can be demonstrated by pre-completion sound testing, or by the use of Robust Details for the relevant separating walls and floors.

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. The current edition, which applies in England, was published in 2015.

ISO 10848 provides standards for the laboratory and field measurement of the flanking transmission of airborne and impact sound between adjoining rooms. Part 4 of the series, which covers junctions with at least one Type A element, was originally published as BS EN ISO 10848-4:2010 but has since been revised, and the 2010 edition has been withdrawn and replaced by ISO 10848-4:2017.

[edit] Related articles on Designing Buildings

• Airborne sound.
• Ash deafening.
• British standards.
• Building acoustics.
• Building Bulletin 93: acoustic design of schools.
• Decibel.
• Flanking element.
• Flanking transmission.
• Noise nuisance.
• Raised floor.
• Reverberation time.
• Sound absorption.
• Sound insulation.
• Sound v noise.
• Structure-borne sound.
• Suspended ceiling.