Expert Insights Into Acoustic Design—the Art & the Science

As both a science and an art, acoustics has the potential to be highly complex. Making that complexity more manageable starts with understanding how sound might behave in a space. In essence, sound has four options:

• It can bounce around—"echoing” off hard surfaces until it dissipates.
• It can pass through a sound-absorbing ceiling or wall panel, which reduces the sound but still allows some to pass through to a space such as the ceiling plenum.
• It can pass directly through an outlet, such as an open door or window.
• Or it can undergo a combination of any or all the above.

Discussions about creating the right acoustics in a space give a good deal of consideration to ceilings and walls. This is because the significant surface area of ceilings and walls can be leveraged to further enhance acoustics through sound absorption and blocking. At the same time, this surface area in ceilings gives sound energy easy and ample access to the plenum, where it can travel to adjacent space. So, it becomes critical that ceiling panels can block sound. Ceilings frequently take even higher priority because acoustical ceiling panels are typically made of a light, porous material, making them convenient to install for new construction or update as a space’s acoustical needs change. 

Consider two adjacent classrooms with all doors and windows shut. When the students in Classroom A start laughing, there are several possible scenarios. The first, highly unlikely, is that Classroom A’s ceilings are made of a solid material—such as metal, gypsum board, or wood—so the sound energy of the laughter stays within the room, bouncing around and creating a distracting reverberation for several seconds until it dissipates. In scenario two, the sound energy is absorbed through the Classroom A ceiling and enters the plenum where, because there is no barrier, it travels through the plenum above the adjacent Classroom B. Classroom B students are disrupted by the laughter of their neighboring classmates.

In scenario three, the sound energy of the laughter is absorbed through the Classroom A ceiling and a barrier exists in the plenum. The sound energy will remain and dissipate within the plenum of Classroom A, never, or very minimally, disturbing Classroom B. In our fourth scenario Classroom A’s ceiling panels absorb and block sound. Because the blocking function prevents much of the sound energy from entering the plenum, Classroom B remains undisturbed by the laughter. While the results of scenarios three and four are virtually the same, achieving them through ceiling panel technology offers several cost, convenience, and functionality advantages compared to creating a barrier in the plenum.

One challenge of acoustic design is determining what the space requires. These requirements can range from a hospital demanding absolute patient privacy to a new night spot counting on sound to energize the space and encourage lively interactions. To that end, the concept of speech privacy has emerged. 

In general, speech privacy references how well an overheard conversation can be understood by an unintended listener. The concept of speech privacy is further broken down into four categories²:

Confidential Speech Privacy

This is the expectation for enclosed rooms.

• SPC of 60 to 70
• A nearby conversation may be partially overheard but there is little-to-no ability for unintended listeners to understand it.
• It does not cause distraction for nearby individuals.
• A minimum CAC of 35 is recommended to achieve confidential speech privacy along with the appropriate background noise level.

Normal/Non-Intrusive Speech Privacy

This is the expectation for well-designed open-plan cubical offices with focused work needs.

• PI rating of 80-95%
• A nearby conversation may be partially overheard, and some words or phrases may be understood.
• The loudness of the speech is not such that it distracts nearby individuals.

Poor/Marginal Speech Privacy

This is the expectation for open-plan workstations with collaboration needs.

• PI rating of 60-80%
• Most nearby conversations can be overheard and are likely understood.
• The loudness and intelligibility of speech can be distracting.

No Speech Privacy

This represents a highly distracting space.

• PI rating 60% and below
• All conversations can be clearly heard and understood.
• The loudness and full intelligibility of speech can be a constant distracting.

Non-intrusive speech privacy is the most common design goal for most open-plan office environments. Although the recommended NRC depends on the size and use of the room, in general, ceiling panels should offer an NRC of 0.70 or higher. While common for open office environments, non-intrusive speech privacy is not an adequate design goal in enclosed environments such as medical facilities, law firms, financial service organizations, or human resource departments, where confidential speech privacy levels are required.

Lastly, when defining acoustical requirements, it’s important to consider the expectations of the occupants. A closed door, for example, typically implies high levels of privacy—so ceilings and walls should support that same degree of acoustical performance. Cubicle occupants, however, typically have more relaxed expectations and look for noise reduction and semi-privacy.

Where speech privacy, at any level, is required the goal is to achieve a balanced acoustical design. The approach to this is as easy as A, B, C:

Absorb sound with the use of high-performance acoustical ceiling and wall systems. These panels will lower RT—eliminating the distraction of “echoes”—and prevent sound from traveling to adjacent spaces

Block sound transmission between spaces with a combination of high-performance ceilings and partition wall(s).

Cover the remaining sound with a sound-masking system. Recent studies indicate that biophilic sound—such as birdsong, water, or wind—can provide highly effective masking³.

The key to speech privacy is making sure all three elements play a part in the acoustics of a space and that the design achieves the right balance for that space’s intended function and the well-being of its occupants.

An extreme example of non-balanced acoustical design is an anechoic room or chamber. An anechoic space is completely silent—there are no sound reflections or echoes. While, at first thought this may seem incredibly peaceful and conducive to work, too much quiet can be distressing and counterproductive to creativity and other important workplace dynamics. In fact, according to a recent article by Smithsonian Magazine, in an anechoic chamber, “without the hum of everyday life, people have trouble orienting and even standing.”⁴

In addition to the A, B, Cs, the good-better-best approach can help zero in on the level of acoustical performance needed from a space’s ceiling and wall panels. Choosing the ceiling panels to get the acoustics right for a given space can be aided by following these good, better, or best guidelines:

Good (NRC 0.60-0.65 and CAC 35+)—Spaces where less sound absorption is required but sound blocking is needed to maintain privacy between adjacent areas, such as enclosed rooms.

Better (NRC 0.70-0.75 and CAC 35+)—Spaces where strong sound absorption and sound blocking are needed for all-around acoustical performance, such as high-traffic spaces or multipurpose spaces. An example is a medical waiting area.

Best (NRC 0.80+ and CAC 35+)—Where maximum sound absorption and sound blocking are needed to for maximum sound quality within the room and privacy between rooms. A conference room is a good example.

Today’s architects, designers, and acousticians have a tremendous number of acoustical options for continuous ceilings, exposed structure applications, and walls—meaning that bridge between sight and sound can stretch as far as the imagination will go. Whether a space needs to be clinical and private, high-energy and interactive, or anywhere in between, solutions for achieving high-performance acoustics are available in a growing number of design choices to support highly creative visions.

In addition to traditional white mineral fiber ceiling panels being used exclusively in a space, wood, wood-look finish, and metal ceilings can be paired with an acoustical infill to reach a space’s “good, better, or best” performance needs. Acoustical performance of these latter ceiling panels will be determined by the product, perforations—which allow sound energy to pass through the panel and be absorbed by the infill—type of infill, and the installation method.

Many teams have been gravitating toward metal ceiling products which, in addition to traditional square and rectangular panels, now include stand-out options such as blades, mesh, and open cells that offer a more opened look. 3D options and cloud configurations add dimension. Unique shapes can be used to create visual interest, as well as a wealth of color and finish options (including natural wood looks) and more.

There are also many options for adding attractive, functional acoustical improvements on a wall. Acoustical wall panels complement acoustical ceilings by dampening the portion of sound that travels horizontally. These panels usually consist of a 3/4" to 1" thick mineral fiber or fiberglass substrate. Depending upon the substrate and installation method, they can absorb 50% to 90% of the sound striking them, reducing noise not only within a space but also between spaces.⁵ 

Lastly, furniture, floor treatments, permanent or temporary room partitions, doors, window treatments, and other elements in a space may help enhance the aesthetics and acoustical performance in a space.

The interactive nature of acoustics can make designing for this particular type of performance challenging.  Tools like the Custom Acoustical Report from Armstrong offer product recommendations and comparisons based on the acoustical needs of a project. 

With the best practices, tools, and range of product innovations now available, it's easier than ever before for architects and acousticians to create spaces that look and sound beautiful. 

¹ The Journal of the Acoustical Society of America, Relationship Between the Privacy Index and Privacy Class.

² Armstrong World Industries, Attaining Speech Privacy with Acoustical Ceiling Panels.

³ National Library of Medicine, National Center for Biotechnology Information, A Review of Benefits of Nature Experiences: More than Meets the Eye.

⁴ Smithsonian Magazine, In the Earth’s Quietest Room, You Can Hear Yourself Blink.

⁵ Armstrong World