Sound absorption—your key to speech intelligibility, speech privacy, and reducing distraction distance

Getting started with sound absorption

When it comes to noise control and creating the ideal indoor acoustic environment there is no magic bullet. A comprehensive acoustics plan includes several strategies to manage sound. So far, we’ve addressed the role of blocking interior sound and blocking exterior noise sources. But blocking alone is not enough.

The partner to blocking is sound absorption. It serves an important and necessary function to ensure spaces meet the unique acoustic needs of occupants. At its most basic level, the relationship between the two can be described like this: ceilings are most effective at absorbing noise; walls are best at blocking noise.

There’s obviously a lot more to it which we’ll cover. In the meantime, the important thing to remember is that good acoustics planning should include using sound absorption and noise blocking together to create comfortable spaces.

Let’s jump right in.

What happens to sound in a room?

Sound. It’s one of the universal and ever-present components of our daily environment. We rely on it frequently to communicate with one another.

However, when it’s in the form of noise, sound can cause many undesirable effects on the physiological and psychological health of those who are exposed. We cover more of the basics of sound in our article on the fundamentals of architectural acoustics.

When sound within a space strikes one of the surfaces in a room—be it the wall, ceiling, or another obstruction such as a chair—one of three things can happen:

• The sound can be reflected, bouncing off the walls and being redirected (you may think here about a gymnasium),
• A portion of the sound energy may be transmitted through the surface (e.g. when you can hear conversations from the office next to you), or,
• The sound can be absorbed (when the noise is trapped within the material of the surface).

You can see from above that the materials, acoustic wall insulation, and other everyday items in a room are what absorbs sound in a room. You can make a room sound absorbing by including the right combination of building materials and even looking at sound-absorbing fabrics… but we’ll save that for a separate discussion.

Another important but often overlooked part of your noise control plan are the flanking paths. These are the open routes a sound can travel around acoustic wall and ceiling assemblies. For example, these “noise leaks” can be created by small openings from light fixtures and electrical sockets, air vents, etc., and minimize the effectiveness of blocking materials.

The repercussions can be significant; in addition to irritation to the room occupants from the noise coming into a space, speech from adjoining spaces will suddenly become intelligible, meaning privacy for those in the adjacent space, is lost.

How can you sound deaden a room?

One of the ways to create a well-rounded, high-performance interior space is to consider the role of sound deadening, or absorption. When it comes to sound absorption within a space, not all materials are created equal. There are measurements that help to quantify and evaluate sound attenuation materials for their ability to absorb sound energy within a space.

The first is NRC or Noise Reduction Coefficient. What makes NRC important is that it is used as the rating to indicate the ability of a material to absorb sound. NRC ratings use a scale of 0.00 to 1.00; 0.00 = least absorptive and 1.00 = highly absorptive. Materials with low NRC ratings are considered reflective, the opposite of absorptive.

Related to NRC is CAC or Ceiling Attenuation Class. The CAC rating specifically measures a ceiling panel’s ability to block noise from traveling between rooms when the wall does not extend full height to the underside of the floor or roof above.

Don’t make the mistake of sacrificing acoustic absorption (NRC) for CAC. In fact, you should know that CAC is no longer compliant with most acoustic standards, guidelines and rating systems. This is in response to the increased understanding that ceiling materials are generally ineffective at blocking, and that they should instead be used for sound absorption in a space.

NRC is a rating of the average of the sound absorption coefficients of material at frequencies of 250, 500, 1,000, and 2,000 Hz, rounded to the nearest .05. These evaluations are determined using the ASTM Standard C423: Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method.

Material is usually considered to be sound absorbing if it has an NRC value greater than 0.70 which makes selecting the right material an important consideration during the planning phase to effectively absorb sound within the indoor environment. Materials used for managing sound will have the following properties:

• Sound isolation: the ability of a material to reduce sound energy by blocking; this is generally achieved with mass, or
• Sound-deadening or damping: the ability of a material to reduce sound energy through absorption; typically, these materials are more porous.

What's driving the need for acoustic sound absorption?

With the trend toward open environments—beyond open offices, this movement also includes residential, commercial and retail environments—there is also increased occupant density (i.e. more people in close proximity to one another) which has a direct impact on their acoustic comfort.

When you open up the room you lose the physical barriers that were previously there to help block sound, or that could have provided vertical surfaces where acoustic treatments could be applied. Once there are too many people in a space, speech privacy becomes difficult if not impossible to achieve and noise levels are hard to control.

As a result, there is a greater need for the remaining acoustic and noise- absorptive surfaces to reduce the sound level in the room. Architects, building owners, and designers will save time and money in the long run by considering sound absorption early in the planning process, especially when building open spaces, to ensure they ultimately meet the acoustic objectives.

Whether you’re building an office, healthcare facility, or school—any project that will provide a workplace for people—remember that noise leads to stress, and over the long term can increase absenteeism and turnover rates for organizations. The initial financial benefit you may gain by increasing the occupant density may be lost later.

There’s now the acoustic occupancy rating for architects, designers, and building owners to think about as a fairly new concept in the industry. It considers how many people can safely be in a space without disruptions to their productivity or worsening their health.

How to use absorption to reduce the noise in a room

Sound absorption is an important tool for designing good acoustic spaces because it helps reduce airborne sound. That’s the sound generated from conversations, radio, or that person eating lunch next to you, transmitted by air and atmosphere.

When we’re looking to achieve speech privacy in open offices, waiting rooms, or other sensitive areas of a building, consider distraction distance. When sounds are created in open spaces they will radiate out and distract people within a certain distance; consequently, an increase in distraction distance predicts an increase in disturbance by noise.

The Acoustical Society of America published an article in 2017 on the topic of distraction distance which proved the need to look at room acoustic design using absorption, blocking, and masking together to reach good working conditions in open-plan environments.

It’s also important to remember that when sound from the interior or exterior passes through a wall or roof assembly with ROCKWOOL stone wool insulation inside the cavity, you are already gaining a level of sound absorption. This is called cavity absorption. So when you think about it, you absorb noise just by insulating your interior walls to break the path of the sound.

What happens inside the room when sound is present—from either interior or exterior sources that hasn’t been controlled by the building envelope—is called room acoustics absorption. Here, either ceiling tiles or other materials with absorption properties are controlling the sound.

Overhead sound absorption demands can vary for the room, but the ROCKWOOL/Rockfon approach uses a “good, better, best” scale. Good = an NRC of 0.70, Better = an NRC of 0.80 and Best = NRC of 0.90 as outlined below, to create a good acoustic design, in particular for open environments.

This is because the ceiling is a critical element in open offices as well as meeting rooms, patient operating rooms, and classrooms. Adding to that, reaching the required sound absorption also shouldn’t limit your ability to develop a contemporary design. The NRC can be provided by any combination of baffles, islands, banners, acoustic metal decks, deck treatments. But based on our experience, ensure that you uniformly distribute the absorption across the space.

Why are ceilings so effective for sound absorption? Sound waves that rise above 5 ft. (roughly the space above chairs, desks, equipment and other surfaces that impact the path of noise) have to interact with the overhead surface before coming back down.

When the surface of that ceiling is absorptive, it is able to capture a significant portion of the energy. Stone wool acoustic ceiling tiles are especially effective at absorbing the reverberating sounds.

Physical barriers are another option for reducing noise in open spaces. This is not an argument in support of cubicles! Rather, consider using acoustic treatments on walls separating two spaces or interior zones.

This could include sound-damping panels (sound panels for walls that are installed after the room is finished to provide additional sound absorption) to reduce speech from traveling across the space. Or, when space is limited, using free-standing acoustic screens, furniture, or other sound-absorbing products that complement the room design might be a better choice.

Also, remember that to meet the criteria established for some high-performance settings, you’ll also need to consider how every structure, surface, fixture, material, and flanking path plays a role in the way noise is experienced. For the best results, this means thinking beyond walls for blocking sound and focusing on the true strength of ceiling panels—noise absorption.

What questions should you consider for your next project?

1. Noise Sensitivity: Is there potential for occupants to experience noise sensitivity? Do you require certain levels of speech privacy or intelligibility or both?
2. Noise Potential: Is noise from inside or outside the room expected to be a concern? What about adjoining rooms?
3. Requirements: What are the requirements and standards that will apply? Building codes should be viewed as the bare minimum and often fall short of meeting the specific needs of the occupants using a space. As such, it is always recommended that when planning you consider any higher standards that have been developed for various applications.  In addition, evaluate the specific needs of your project to ensure you’re getting the best solution that fits.

Look to Rockfon for direction on sound-absorbing ceiling solutions

As part of a complete acoustics strategy, sound deadening—or acoustic absorption—is an important factor that shouldn’t be overlooked. It has practical applications in both rooms and hallways.

When designing your acoustics solution, don’t forget to look up. Selecting the right ceiling will have a significant impact on the acoustic environment. Use a ceiling system that will deliver high NRC-rated levels of absorption to complement the blocking properties of other materials and assemblies in the space.

Systems that include stone wool insulation, like the ones offered by Rockfon, are ideally suited for this purpose.