10/30/2020 • 6 min read

Acoustical Design: Room vs. Building

Identifying solutions to manage each

by Jim Goodchild

Universally, people say they want “great acoustics” in their work environment. And that makes perfect sense. Let’s be honest: Who wants bad acoustics?

But, when people are asked to define what makes an acoustic environment good, the response is often blank stares. It is sometimes easier for people to say what they don’t want:

“The constant noise in the open office area drives me nuts!”

“I can’t understand the people on the other end of this Teams call unless they’re sitting right in front of the phone.”

“I can overhear conversations from the next room.”

Each of these statements describes an expectation that is not being met. They reflect what’s at stake: Get the acoustic conditions right, and people will go about their business without giving acoustic conditions much thought—because their expectations are being met. But get acoustics wrong, and people will notice immediately. Sadly, fixing acoustics done wrong often means much more effort—and money—must be spent than would have been necessary to do things right in the first place.

Too often, the problem is simply that good materials were used in the wrong places.  While every acoustic material can address a specific problem, not all problems can be fixed by every material. Using acoustic materials effectively requires an understanding of the problem to be addressed.

In tackling an acoustic design challenge, one of the first questions to be asked is whether the talkers and listeners are in the same room, or in different rooms. If they are in the same room (in person or via a remote technology setup), the issue falls into the category of “room acoustics.” If they are in different rooms, it’s an issue of “building acoustics.” This simple categorization can help dictate what materials—and what material properties—should be considered.

Room Acoustics: It’s in the Fuzz

In the field of room acoustics, the primary concerns are noise levels and a fundamental property called “reverberation time,” or “reverb.” To a great degree, these issues are managed by adding or subtracting materials that act as acoustic absorbers.

Reverberation time (reverb) is the time it takes a very loud sound to decay to the point where it is inaudible. The standard measurement for reverb time is called “RT60,” the time it takes for a sound signal to drop by 60 decibels. In rooms with a lot of hard surfaces, such as a bathroom or gymnasium, the sound is reflected over and over, leading to higher sound levels. If the reverb is too long, the multiple echoes will overlap and make speech unintelligible. On the other hand, a short reverb time will make a space seem quiet and calm, but if it gets too short, the lack of reflections can make people feel uncomfortable. In most office environments, a desirable reverb time is between 0.3 and 0.8 seconds.

Reverb is controlled using materials known as acoustic absorbers, which acousticians might refer to as “fuzz.” Common absorbers are woven or fibrous materials that work by breaking up the sound waves, turning the acoustic energy into heat. The opposite of an absorber is a reflector; smooth surfaces—such as glass, steel, or wood—reflect sound, making them poor absorbers.

The first two examples at the beginning of this article each relate to room acoustics challenges. In the first, a room with a lot of hard surfaces becomes very noisy with relatively little human activity in the space. Such spaces can be dramatically improved by adding absorptive materials. Traditionally, this might be done by adding a suspended ceiling. But while ceilings contribute substantial quantities of absorption, more and more spaces are being fitted without full ceilings. These spaces instead use baffles or wall treatments, light fixtures, or other feature elements that carry absorptive properties and reduce sound levels.

In the second example, rooms used for teleconferencing require careful attention to the specific placement of the absorptive materials on the ceiling, walls, and floors to eliminate troublesome echoes, while adding some diffusive elements to spread the speech sounds evenly throughout the space. These spaces also benefit from careful placement of microphones close to the talkers.


Building Acoustics: Size—or Rather, Mass—Matters

Where room acoustics is concerned with whether sound is reflected or absorbed by a surface, building acoustics is concerned with how well a wall (or floor or ceiling) blocks sound transmission, and how well those sounds are heard on the other side.

The fundamental measurement for building acoustics is “noise isolation,” which is a measurement of the space itself, not just the walls. People’s experience of noise isolation between two spaces is the result of the combined effects of the sound blocking properties of the construction and the absorption of the rooms. The common measurement for noise isolation is NIC, or Noise Isolation Class.

What makes a wall a good sound blocker is mostly dictated by its mass and thickness, and how well the layers of the wall are isolated. Insulation between layers further improves performance. The ability of the wall to block sound is commonly measured by the STC, or Sound Transmission Class.

Whether or not a space is experienced as having adequate noise isolation is also dependent on people’s ability to hear the sounds that are transmitted. This is dictated by the background sound levels in the space. Background sound exists in the space when the human activity has been removed, and in most buildings is primarily composed of HVAC sounds along with some environmental sounds, such as traffic noise.

When people are in the space, however, the relationship of speech sounds (signal) to these background sounds (noise) must be considered. To maintain a beneficial signal-to-noise ratio requires that both the noise isolation offered by the construction and the background sound levels are adequate for the task. To a limited degree, increasing one allows a reduction in the other—but both are necessary. Controlling background sound generally requires investment in a well-designed, tunable background sound-masking system.

Back to the beginning of the article, the last statement relates to building acoustics, and to the signal-to-noise ratio. In both instances, an adequate combination of noise isolation via construction, combined with a controlled background sound level, will ensure expectations are met.

The Short of It

Understanding how to address the acoustic expectations of the users of an office environment can be simplified by first understanding if the concern relates to room acoustics or building acoustics. Room acoustics challenges are usually addressed through absorptive and diffusive materials—often by adding more. Building acoustics challenges are usually addressed by attention to both the construction and background sound levels in the space. Keeping these in mind should help you sort through the various solutions and material options available.

More on Acoustics and Design

If you are interested in learning more about acoustics and office acoustics solutions, check out our Acoustic Design Guide.


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