8 Ways to Defend Against Acoustical Flanking: Strategies for Improving Field Performance of Operable Partitions

The Operable Partition, whether horizontally acting or vertically acting, delivers flexible options for room use.  Instead of building on more and more wings and expansions to a building, large rooms can be subdivided up into smaller rooms to expand their usage potential.  The expectation of the subdivided rooms is to function as separate rooms for meetings/events/parties without sound washing out the individual function and disrupting the usage of each room.  We are going to focus in on an element of design and coordination that can torpedo the goals of space management in general.  

All operable partitions industry wide are tested in an acoustical laboratory by the criteria in ASTM E90.  Sound Transmission Class (STC) ratings are assigned to the performance of the system, which is well and good in a laboratory environment, but field conditions can quickly undermine the expected performance of the operable partition.   If sound is allowed to leak around the operable partition its space subdivision performance becomes compromised, and the following issues can occur:

  • Revenue generating spaces like hotel ballrooms will be faced with client dissatisfaction and expected refunds due to acoustical bleed over between events.
  • Business conference rooms will not be as private and productive with meetings disrupting each other.
  • Focus in a school classroom will be difficult if the students are distracted by another class.

Sound flanking issues undermine the entire function of operable partition.  If we do not strive to prevent sound leakage through construction surrounding the operable partition, the value of premiums paid for high STC ratings will not be delivered.

ASTM E557, under paragraph 4.3, defines Sound Flanking Paths as “the passage of sound by paths other than directly through the operable partition”.  The following eight 8 principles are intended to prevent acoustical flanking between subdivided spaces.  All eight are not always possible on projects, but minimizing the instances of as many of these as possible will greatly assist in delivering a quality space: 

Flanking Path #1: Open Plenums 

The operable partition is aligned with the ceiling height of the room, providing a sound barrier from ceiling to floor.  Nevertheless, above the ceiling the plenum presents the widest flanking path for sound to transmit trough the ceiling above a deployed operable partition.

Section 5.1 in ASTM E557 describes the solution.  Essentially the plenum barrier is a drywall partition extending from the track/ceiling line up to the roof deck or upper floor deck with acoustical caulking.  Here is an example from the field, as well as Modernfold’s technical details:

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Flanking Path #1: Open Plenums 

Access flooring’s potential for sound leakage is similar to the open plenum path previously discussed; the sound rated operable partition stops its barrier at the floor level; There is a need to secure the gap between the access floor and floor slab to prevent sound from traveling under the operable partition.

Flanking Path #3: Inadequate Intersecting Fixed Wall Construction 

Operable partitions subdivide rooms by sealing against a perpendicular/intersecting fixed wall (or another operable partition).  This intersection point should be designed to be sound resistant like the operable partitions.  For example, standard drywall partitions with 1 layer of 5/8 gypsum installed to both sides of a metal stud at most will provide 36-39STC sound rating.  If the operable partition is rated to 56 STC, the sound will flank through the weaker construction.

ASTM E557 section 5.3 describes multiple construction types and how to ensure sound does not leak through.  5.3.2.2 and 5.3.2.4 describes the proper construction and blocking to be used in a gypsum-board-on-stud fixed partition, 5.3.2.3 indicates properly planing out masonry or brick to prevent grooves in the mortar lines from allowing sound to pass under the operable partition acoustical seals. 

Here is an example in plan view of effective blocking:

Flanking Path #4:  Pocket Wing Walls 

The point where the operable partition seals against fixed walls with its final moving components is typically referred to as where it makes “final closure”.  Similar to flanking path #3, it is recommended to minimize the amount of space in the operable partition’s opening that weaker acoustical construction types fill.  An easy one to forget are the wing walls of the pocket.   

The operable partition can make its final closure to the face of an acoustically rated pair of pocket doors (at the pocket entrance) or at the rear of the storage pocket it stacks into (recommended).  If final closure is made at the front of the pocket, the pocket’s wing walls are now effectively included in the acoustical room separation and would need to be rated to the same acoustical level as the operable partition.  Otherwise, like item #3, sound will travel through the acoustically weaker pocket wing-walls and compromise the field performance of the operable partition.

Instead, we recommend the final closure always be made to the rear of the pocket so that the pocket wing walls do not have to be built to the same acoustical rating of the operable partition.  If the pocket is open without doors, the panels seal to the rear pinning off sound completely across the room.

If pocket doors are desired, a middle flap can be utilized to allow the operable partition panels to pass through and seal to the rear of the pocket, thus non-acoustical pocket doors can be used.

Flanking Path #5:  Plenum Barrier Penetrations 

Adding a plenum barrier to eliminate flanking path number 1 to stop sound from going over the partition. Penetrations through the barrier for other items to pass through opens up additional pathways for sound.  The more pathways, the less effective the plenum barrier solution becomes.

HVAC Ducts are probably the most common: they are like a hollow metal drum that can pick up sound vibrations and transfer them through the plenum barrier.  Sprinkler pipes, electrical conduit, and pneumatic piping all can carry sound vibrations through the barrier.  Simply try your best to avoid this instance to minimize the opportunity for sound flank. Run the HVAC, sprinkler, and other utilities lines through surrounding corridors and dump feeding lines into the separate subdivided rooms.

Flanking Path #6:  Mixed Opening Construction with the Operable Partition 

The sound rating of surrounding construction should always be considered when planning the intended separation.  Plan for the operable partition to extend from one extreme of intended sound barrier to the other.  Make sure whatever alternative construction types (drywall partition, glass, doors) are detailed to match the sound rating of the operable partition, otherwise sound will flank through the weaker sound barrier.

Flanking Path #7:  Non-Rated Swing Doors 

When an operable partition splits a larger room into two, non-acoustically rated egress doors are regularly used to allow emergency egress from each room.  Plan carefully where the egress doors are located relative to the extended operable partition.  If they are close together, sound can travel through each door leaf or pair and enter into the other subdivided room via the non-acoustically rated doors.  The situation gets worse when there is a corridor wall outside of the conference room that helps bounce sound back into the adjacent room. Consider using an acoustically rated egress door instead.

Flanking Path #8:  Undeployed Acoustical Seals 

Operable partitions are equipped with operable bottom seals to account for variations in the flooring in the room.  These seals either deploy manually via cranks in the panels, automatically by spring actuation, or by motor if the operable partition is automated.  With manual crank seals, if a seal is left up and not extended down to the floor as designed, sound will travel right under the operable partition and undermine its acoustics.  This flanking path unfortunately is due to improper setup of the operable partition and can easily be rectified via training or fully automated the operable partition so the seals deploy without relying on human interaction.

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