Acoustics: Absorb, Block, and Cover

How do you know if a space has poor acoustics? Employees easily overhear conversations and take comments out of context. Rumors spread. Bosses unintentionally leak confidential business agreements. Doctors in an adjacent room are overheard discussing another patient’s treatment. Students’ scores dip. Patrons shout across the table during dinner. While acoustics are invisible, it’s easy to see which spaces lack the sound control needed for privacy, productivity, learning, and enjoyment.

A balanced acoustic design absorbs, blocks, and covers (the ABCs of acoustics) sound. This can be achieved by employing soundmasking technology and installing interior products with acoustic properties. An acoustician can also provide professional advice on layout and design to help optimize acoustics.

To fully appreciate how important the ABCs are, consider how sound travels. The Arlington, VA-based Alliance for Flexible Polyurethane Foam, in its Sound Absorption in the Workplace report, explains: “Much like the expanding ripples that result from tossing a pebble into a pond, sound begins from a point of disturbance. From that point, a series of pressure waves expand out in all directions. When these pressure fluctuations reach our ears, they are processed by the brain and interpreted as sound.”

An acoustically positive environment reduces speech intelligibility (which results in increased speech privacy) and lowers the level of noise distraction. This is accomplished by masking noise or altering and halting the path that sound travels. When sound strikes a surface, a number of reactions occur. The Melville, NY-based Acoustical Society of America (ASA), in the publication Classroom Acoustics: A Resource for Creating Learning Environments with Desired Listening Conditions, explains sound/surface interactions in the following four ways:

• Transmission. The sound passes through the surface into the space beyond it (like light passing through a window).
• Absorption. The surface absorbs the sound (like a sponge absorbs water).
• Reflection. The sound strikes the surface and changes direction (like a ball bouncing off a wall).
• Diffusion. The sound strikes the surface and is scattered in many directions (like pins being hit by a bowling ball).

The ASA reports that multiple reactions can occur simultaneously. “For instance, a sound wave can, at the same time, be both reflected by and partially absorbed by a wall.”

Absorb Sound
“The most direct way to address the control of unwanted noise within a space is to properly treat its shell (floor, ceiling, and walls),” says Mary Ellen Magee, marketing manager, Teknion LLC, Mount Hazel, NJ. A room filled with hard finishes can be an acoustic nightmare - echoes and reverberation distort sound and can make communication difficult. To eliminate these problems, purchase products and systems with sound-absorbing characteristics. “Like a thick paper towel soaking up water, these porous/fibrous sound-absorbing materials soak up sound waves by trapping them in an internal maze of air pockets. As sound waves work their way through that maze, they slowly lose the sound energy they carry as it dissipates into microscopic heat. When they emerge from that maze, sound waves are weaker and, therefore, less loud,” explains Herman Miller Inc. in its 2002 It’s a Matter of Balance: New Understandings of Open Plan Acoustics white paper.

Schools, offices, and healthcare facilities are just a few of the environments using acoustic ceiling systems to improve acoustics. While ceilings absorb sound, they can control sound in other ways as well. “Ceilings can also be used to deflect or distribute sound more evenly throughout a space,” says Kim Graaskamp, director of sales and marketing, Hunter Douglas Contract Ceilings, Thorton, CO.

When comparing products and systems, look for the Noise Reduction Coefficient (NRC), an industry rating that calculates a material’s ability to absorb sound. The rating is expressed as a number ranging from 0 to 1; the higher the number, the greater the material’s ability to absorb sound. According to Graaskamp, “A ceiling is generally judged [as] acoustical (as opposed to reflective) if the NRC is better than 0.50. Obviously, the higher the NRC value, the more absorptive the ceiling. Certain materials are more noise absorbent - for instance, fiber glass has a high NRC.”

Especially in classrooms plagued by long reverberation time, acoustical ceilings are a must. The ASA recommends ceilings with an NRC of 0.75 or higher. In the research paper Making a Cubicle a Better Place to Work, author Guy Newsham, head of the Lighting Research Group at the National Research Council Canada’s Institute for Research in Construction, recommends that offices install ceiling tiles with an NRC of 0.90 or higher to reduce the level of reflected sounds from speech.

Articulation Class (AC) is another measure of a ceiling’s acoustical performance. In the recently updated Sound Solutions: Increasing Office Productivity Through Integrated Acoustic Planning and Noise Reduction Strategies report, the American Society of Interior Designers (ASID) explains: “The AC rating of the ceiling is a measure of its performance in limiting the sound reflection off the ceiling plane over an angle of approximately 46 to 60 degrees, which represents most situations for adjacent cubicles separated by furniture panels.” In open-plan offices, invest in ceiling products within the range of 170 to 210 AC.

To reap the greatest acoustic benefits from these high-performance ceilings, Norcross, GA-based soundmasking provider Dynasound Inc. recommends adding closure/seal components to prevent sound leaks through return-air grills, lay-in light fixtures, sprinkler heads, and other utility penetrations.

Installation of carpet - especially when it includes a layer of cushion - is also beneficial. The Herman Miller report identifies numerous advantages, including the absorption of airborne sound and reduced footfall noise: “In addition, in many settings, carpet helps to create an aesthetic ambiance conducive to lowered voices, a heightened sense of privacy, and reduced distractions.” Like acoustic ceiling tiles, it’s important to review the NRC rating of the floorcovering under consideration. Lab results in ASID’s Sound Solutions reveal that polyurethane cushion-backed carpet has a higher NRC rating when compared to bare concrete and conventional jute-backed carpet. Additional tests conducted by the Dalton, GA-based Carpet and Rug Institute (CRI) show that cut-pile (vs. loop pile) carpet with a permeable carpet backing laid over cushion exhibits higher NRC ratings.

A carpet’s Impact Insulation Class (IIC) rating reflects the carpet’s insulation capabilities in floor-ceiling assemblies. Results from independent tests were consistent with NRC test results; polyurethane cushion-backed carpet has higher IIC ratings.

Block Sound
Sound absorption is just one way to improve acoustics; blocking sound from spreading into adjacent areas or workstations is equally important. Both walls and carpet block sound transmission; however, in open-plan offices, the most widely used sound-blocking product is furniture panels. Herman Miller’s It’s a Matter of Balance report explains: “Whereas the best sound absorption relies on lightweight, porous materials, the best sound-blocking materials (solid masonite, metal, or hardboard) are dense and heavy. With no air spaces for sound waves to slip into and through, panels containing these tightly packed interiors essentially cut off the direct path into adjacent workstations.” These divider panels can be extremely effective at blocking and absorbing sound simultaneously. To be effective at both and provide non-intrusive levels of speech privacy, the panel should meet the following criteria (as recommended by ASID):

• Panel heights should be a minimum of 60 inches; 72 inches is preferred.
• The recommended NRC is 0.60.
• A Sound Transmission Class (STC) rating between 18 and 20 is ideal.

STC is another helpful industry rating, and can be used to distinguish which products are more equipped to block sound than others. “STC [is] a single-number rating of the acoustical-blocking capability of a single panel or system. It rates the amount of sound that is transmitted through a wall, panel, ceiling, etc. A higher STC means that the panel will ‘block’ more sound and result in better privacy,” explains Kenric Van Wyk, president, Acoustics By Design Inc., Grand Rapids, MI. Unfortunately, office accessories hung on furniture panels can reduce the acoustic properties of systems-furniture panels.

Remember, partition height and STC are both important: While a tall panel with a poor STC rating might increase visual privacy, it will do little to improve speech privacy and workspace acoustics. Panels should be high enough to block conversational noise from traveling over the top of partitions into adjacent spaces. “The lower the panel, the less acoustics it provides. This has more to do with the fact that noise travels in all directions. If the panel is low, the likelihood is that sound will just travel over it. The higher the panel, the more sound blocking that occurs,” says Mark Murdoch, senior product manager, Herman Miller Inc., Zeeland, MI. While panels taller than 60 inches are best, studies have found that sound-blocking benefits level off in panels that are 75 inches or higher.

To successfully block sound in open-plan workspaces, workstation design should limit line-of-sight layouts between workers and, when possible, ensure that occupants are not facing into the panel (find alternatives to locating computers in a workstation corner).