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Building noise control - flanking pathways:
This article explains how sound flanking paths, sound leaks around and through building components, defeats incomplete attempts to reduce building sound transmission and noise levels. We include design details for sound reducing details in buildings including soundproof office doorways and doors. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.
Definition of flanking transmission or flanking sound transmission
Definition of flanking transmission or flanking noise transmission: the term flanking transmission in acoustical engineering is used to describe the passage of sound over, under, or around barriers intended to provide sound or noise control or isolation.
For example if sound-transmission-resistant partitions have been constructed between rooms but there are openings at the partition wall top (perhaps through a suspended ceiling) then sound control may be ineffective as sounds can pass over the intended noise barrier.
The page top photograph shows a pair of solid core doors installed at the entry to an office where sound transmission and privacy are a concern. Below we provide more details about soundproofing at doorways. Continuing from from Best Practices Guide to Residential Construction:
Sound takes the path of least resistance
between rooms, through any air leaks or through rigid
connections in the structure itself. These routes that bypass
efforts at sound insulation are called flanking paths.
These flanking transmission pathways can significantly reduce the effectiveness of soundproofing
efforts. Building walls with high STC ratings (SOUND TRANSMISSION CLASS RATINGS) will
do little good if sound can pass easily though electrical
outlets or a thin, loosely fitting door.
For example, an un gasketed
door or the equivalent of a one-inch-square hole
in a wall can reduce an STC 50 wall to STC 30.
Checklist of Common Flanking Transmission Pathways in Buildings
Our photo (left) shows a sneaky flanking noise path - an under-cut door to a room used for massage treatment. Even though the door is solid wood, gaps around the door and the especially wide gap below the door provide a flanking noise pathway.
flanking paths include:
Bathroom medicine cabinets - back-to-back medicine cabinets,
Ceilings: Air leakage around partition walls at ceilings;
Doors: Air and thus sound leaks around doors. Hollow-core doors and single-pane glass, are
good sound transmitters.
Floor noise transmission; for example even with sound isolation between floor levels in a building, if solid floor coverings such as subfloor or finish wood planking extend beneath partitions and into an adjoining area that forms a sound flanking pathway
Framing connections that include solid framing members passing between building areas are noise transmission conduits; Even in a de-coupled framing design intended to address flanking noises, de-Coupled framing omissions: with decoupled framing, a solid path through a band
joist or drywall panel provides a bypass for
Partition edge abutments to adjoining walls, ceilings, floors
Plumbing penetrations: Air leaks & sound flanking through
plumbing penetrations. Also plumbing chaseways that pass between building areas horizontally or vertically
Recessed light fixtures
Resilient channel connector mistakes: with resilient channels, a few drywall screws that
penetrate into the ceiling joists, undermining the
Window noise transmission
How to minimize Flanking Transmission of Noises
Minimizing sound noise transmission flanking paths requires both good planning
and workmanship. Common strategies to control flanking path for noise in buildings include:
Avoid back-to-back holes for electrical and mechanical
Partition wall sealants: Along partition bottoms, leave a 1/4-inch gap between the
drywall and subfloor, and fill with acoustical sealant.
On bathroom partitions, install drywall all the way to the floor before installing the tub and seal all plumbing penetrations through walls with a flexible sealant
Bathroom doors: avoid hollow-core doors (photo at left), use solid wood or sound-resistant doors
For warm air heat or air conditioned bathroom spaces provide return air by ducting; undercutting a door (photo at left) to permit return air flow out of the bathroom provides a flanking sound pathway.
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 General Steel Corporation, "The Facts About the Acoustical Performance of Metal Building Insulation 2", Sound Transmission Class, General Steel Corporation, 10639 W. Bradford Road, Littleton, CO 80127, web search 4/3/11, original source: http://www.gensteel.com/insulation_facts-5a.htm
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