Avoid Breaking the Sound Barrier
In October 1947, Chuck Yeager piloted his rocket-powered “Glamorous Glennis” Bell B-1 to a speed of 700 mph to become the first person to break the sound barrier. Nine months, later he was awarded the massive, 525-pound Collier aviation trophy for being “the fastest man alive.” If your acoustician excitedly announces that you just broke through the sound barrier, you probably aren’t going to be expecting the design and construction team to throw you a ticker-tape parade on your way home.
The design and construction of high-performance sound barriers (walls, ceilings, floors, doors, windows, etc.) needed to noise-isolate courtrooms, theaters, and conference rooms is a difficult exercise requiring guided team effort. Unless the owner, architect, designer, engineer, construction manager, manufacturers, distributors and contractors all understand and agree on the project goals and function as a team, sound barriers will be broken and the project will not achieve many of its system-level goals. Unfortunately, the typical design and construction team might be described as a “prisoner’s dilemma,” in which each individual team member is rewarded for not being cooperative.
For the owner, the dilemma might be the chance to purchase a centrally located project site at a below-market-average price but within a high ambient noise environment (e.g. near airports, rail lines and interstates). Having “saved money” on the real estate, the owner sets up the whole team to fail if he also expects to save money on isolating the building from the excessive ambient noise.
Catch 22’s for the architect commonly center upon the placement of noise-producing equipment near rooms that are supposed to be quiet. This practice requires a significantly more complicated and expensive sound barrier to separate the two. For example, trying to hide mechanical units from view tends to place them on the rooftop in the center of the building. As the top floor tends to be executive suites, the architect’s desire to visually hide mechanical units places them directly over boardrooms and executive offices, which then requires a significant increase in the mass of the ceiling, which subsequently requires the structural engineer to redesign the structure to support the increased mass—and so it goes for each member of the team.
For engineers, the need for energy efficiency commonly works against creating good barriers to sound. The most obvious example of this is the need to reduce the energy expended to overcome pressure losses of air distribution throughout the building while maintaining a sound barrier between the occupied spaces and the deafening 110 dB sound pressure levels produced by air handler fans. High energy efficiency is achieved with high-velocity fans and straight, short ducts, which is the perfect antithesis of a sound barrier.
Despite other team members’ dilemma decisions, contractors and manufacturers—who often tend to be blamed when the team fails to accomplish any of its goals—should understand how their work can produce a functional sound barrier.
The best analogy I have for producing a high-performance sound barrier was introduced to me by an elementary school student, the daughter of one of W&C’s readers. For a science project, students in her class were given an alarm clock and a shoebox, with the assignment to modify the shoebox into a sound-proof enclosure for the alarm clock. The best performing enclosures 1) placed the alarm clock on sponges or other vibration-absorbing material, 2) added a layer of Play-Doh or other similarly massive material to all of the box surfaces, 3) eliminated air leaks through the box by using duct tape and plastic wrap, and 4) added sound-absorbing cotton balls within the box cavity.
Although manufacturers and contractors have a few more tricks in their bags, these four principals cover the majority of high-performance sound barrier design:
1. Eliminate Vibration Transfer
Eliminating vibration transfer through a sound barrier is accomplished by adding a break in the structure supporting each side of the barrier. For floor/ceiling structures, this leads to the use of spring-isolated ceiling hangers such as the Model ICC deck-suspended ceiling hanger by Kinetics Noise Control, and floating floor assemblies such as the Eclipse by Robbins Sports Floors. For walls, breaking vibration transfer is accomplished with resilient channel isolators such as Model RSIC-1 by PAC International and the use of staggered and dual-stud construction.
2. Add Mass
For lightweight construction, adding mass to a sound barrier can be an inexpensive option. A common example is the addition of a heavy roof board such as DensDeck by Georgia Pacific to a lightweight roof structure to increase isolation from exterior noise. For sound barrier walls, additional layers of drywall raises the isolation capability of the assembly from STC41 for the typical one layer on each side to STC47 for the addition of one layer on one side to STC55 for two layers on each side to STC57 for three layers on one side and two on the opposite side.
Adding more layers becomes less and less effective. One of the extra tricks in the contractor’s bag is to add a viscoelastic adhesive layer, such as Green Glue by Saint-Gobain/Green Glue Co. between drywall layers. This serves to absorb vibration transfer between layers. To save construction labor and improve consistency, pre-laminated drywall such as QuietRock by PABCO Gypsum may be used.
3. Eliminate Air Leaks
Air leaks through sound barriers range from the small construction gap between floor and wall board, to the wider construction gaps left by plumbing, mechanical and electrical wall and ceiling penetrations, to the painfully obvious gap created by stopping demising wall heights short of the structural deck. In addition to these, functional gaps are created by doors and transfer ducts. Those who aren’t aware of the need to seal these gaps air tight typically will try to stuff fiberglass in the construction gaps or perhaps cover the gaps with baseboard, neither of which makes any improvement in performance. Although there are many acoustical caulks on the market, any compatible, non-hardening sealant will work to seal construction gaps. Larger gaps may first be filled with backer rod before sealing them with caulk. Still larger gaps, such as those around penetrations, either can be built back so they may be treated as small construction gaps, or can be filled using acoustical or fire stop putty. Once the project is built, hidden air leaks may be found using acoustic cameras.
For drywall contractors, probably the hardest part of creating an air-tight assembly is where the top of the demising wall meets a fluted metal deck. For small jobs, skilled contractors may custom contour each piece by hand to fit the deck flutes and then fit them into place like a jigsaw puzzle. Bigger jobs can be set up on a board milling machine such as the PanelMax by Grabber Construction Products. Even bigger jobs can be ordered prefabricated with castle-cut edges from CNC companies, such as BakerTriangle. Firestops designed for this head-of-wall joint may also save this tedious step.
Contractors who spend time and money constructing a sound barrier should be protective of their good work and question anyone who wants to poke holes in it, as sometimes happens with projects that indiscriminately place air transfer ducts through sound barrier walls. If transfer ducts are allowed at all, they should be very long and be internally lined, not short and bare metal. Placing non-sound-rated doors in sound barrier walls also is a waste of effort and money.
4. Add Cavity Absorption
Although it is necessary to include sound-absorbing material within the dead air space of every sound barrier, the type and amount is not critical. Fiberglass batting marketed as thermal insulation can be just as effective as specialized acoustical insulation. Mineral wool and cellulose also are good choices. To prevent transfer of vibration through the insulation, cavities should not be stuffed and the insulation should not be rigid across the cavity.
Understanding how noise is transferred through shoeboxes and other building components puts you in a good position to be the one team member who can make certain the sound barrier is not broken.