Acoustic absorption, high NRC, improves human wellbeing – The evidence is clear.

January 1, 1

When you sacrifice high NRC for other product attributes, you do more harm than good.

Study after study shows that when ceilings with a high Noise Reduction Coefficient (NRC) rating replace ceiling materials with less acoustic absorption, people perform much better.

Regardless of whether the studies focus on learning in classrooms, healing in hospitals or working in offices, when high NRC acoustic ceilings are added, people are more comfortable, safe, productive and satisfied.

The evidence is clear. When selecting acoustic ceilings for your buildings, don’t let anything stand in the way of high-performing acoustic absorption and high-performing interior spaces.

What is acoustic absorption?

Acoustic absorption occurs when an architectural surface – such as a suspended ceiling, wall-mounted panels or carpet – converts energy in sound waves into insignificant heat energy by means of friction inside the pores of the material. The more sound energy that is absorbed by the surface, the less that is reflected back into the room as noise, reverberation, echo or flutter.

How can acoustic absorption be added to a room’s design and what is the benefit?

Optimized Acoustics™ absorption in buildings requires a well-thought-out plan of adding acoustic absorption to all rooms and spaces that people occupy on a regular basis. Adding acoustic absorption into rooms and spaces can be done in a variety of ways. The easiest way to add a lot of absorption, and to position it so that it does not get damaged over time, is to install a suspended, sound-absorptive ceiling or hang acoustic baffles or islands above the space. In tall spaces, acoustic wall panels and carpet also may be needed.

Using acoustic absorption is the best approach to noise control inside rooms. It decreases noise loudness, improving acoustic comfort and overall wellbeing. The noise-reducing ceiling systems should be as sound absorptive as possible and also should be positioned as low as possible to decrease the volume of the room. This moves the absorptive material close to the noise sources. Acoustic absorption decreases the distance that sound travels in open spaces and down corridors, and improves speech privacy. Fewer people are distracted, improving productivity and decreasing stress.

BDO lawyer office, Sonar X-edge 1200x600, lighting Luminex, open-plan

Less Noise. Better Concentration.

In open spaces, a high level of ambient noise can affect people’s health, productivity and ability to learn. With Optimized Acoustics™, you achieve more sound absorption resulting in lower noise levels.

What is speech intelligibility?

Speech intelligibility is one of the measures of good acoustics in spaces where groups of people gather to talk and listen. It is defined simply as how well speech can be heard and understood in a room.

Many factors influence speech intelligibility. These include the strength of the speech signal, the direction of the sound source, the background sound level and the reverberation time (RT) of the room. Using acoustic absorption decreases reverberation in enclosed rooms, such as classrooms and conference rooms, improving speech intelligibility for group communication.


CBS Copenhagen Business School Blågårdsgade, Cosmos, education

Shorter Reverberation. Improved Intelligibility.

Reverberation affects your ability to understand what’s being said. Optimized Acoustics™ provides more sound absorption which lowers reverberation and that means better communication.

Acoustic absorption in building standards, guidelines and rating systems

 More and more building types and room types must now comply with more stringent acoustic absorption criteria in standards, guidelines and rating systems.

  • Guidelines for the design and construction of healthcare facilities, published by the Facilities Guidelines Institute (FGI), require that "all normally occupied healthcare facility spaces shall incorporate acoustic surfaces."
  • The WELL Building Standard typically used for commercial office buildings requires "sound reducing surfaces" (feature 80) for the health of the cardiovascular, endocrine and nervous systems of building occupants.
  • The Collaborative for High Performance Schools (CHPS) emphasizes that "student learning suffers in acoustically poor environments" where "excessive noise and long sound reverberation negatively affect speech communication" (EQ14.0).
Academic buildings often equal noisy occupants. In campus-based offices, acoustic specifications typically are set to ensure privacy and a quiet work environment.

Jamie Borg

AIA, AMR Architects

How is acoustic absorption defined and measured?

The amount of acoustic absorption required in building standards, guidelines, and rating systems is defined in one of two ways – either by the minimum Noise Reduction Coefficient (NRC) of the suspended acoustic ceiling system or by the maximum reverberation time (RT) inside the room.

  • What is a Noise Reduction Coefficient (NRC) rating? NRC is important in areas where people converse in groups, where high levels of noise are present or where sound-sensitive activities occur – classrooms, patient care areas, open plan offices, conference rooms, shops, lobbies and waiting rooms. The acoustic ceiling tile NRC rating refers to how much sound the ceiling material can reduce noise by absorbing sound. A higher number indicates more absorption. NRC generally varies between 0 (no absorption) and 1 (very high absorption) and is the average of the sound absorption coefficients at four frequency ranges, the 250, 500, 1000 and 2000 Hertz (HZ) octave bands. Ceiling noise reduction is measured according to ASTM C423. Using higher NRC ratings can reduce cost because fewer sound-absorbing products need to be installed to reach the goal amount of absorption in the room.
  • What is Reverberation Time (RT)? RT is the time it takes for sound inside a room to decrease 60 decibels – in other words, for a loud sound to fade away until you can no longer hear it. When considering reverberation vs. echo, remember that these are different; an echo being a single, audible reflection delayed in time. RT is affected by the size and shape of the room, and the amount and location of the acoustic absorption in it. Large rooms with little acoustic absorption have long reverberation times. As more acoustic absorption is added, the RT becomes shorter. For high speech intelligibility, RT should be less than 0.60 seconds. This is why most standards require RT to be less than 0.60 seconds.

NRC is a property of sound-absorbing products, while RT is a property of the whole room. They are closely related; as higher NRC materials are used, the reverberation and echoes in the room decrease.

RFN-NA, optimized acoustics, noise reduction coefficient (NRC) graphic

NRC refers to a surface's ability to reduce noise by absorbing sound.

Classrooms in the U.S. typically have speech intelligibility ratings of 75 percent or less, meaning every fourth spoken work is not understood.

Acoustical Society of America

How to select the right Noise Reduction Coefficient (NRC) rating

Selecting the correct NRC acoustics rating is easy. First, consider how much noise is generated by the people and equipment in the room. Next, consider what people will be doing in the space and how sensitive it would be to have disruptive noise or excessive reverberation.

NRC ratings for sound-absorbing, noise cancelling and noise reduction ceiling tiles fall into three categories as seen in the noise reduction coefficient chart below:

RFN-NA, optimized acoustics, absorption categories

NRC Categories: Best – 0.90, Better – 0.80, Good – 0.70, Avoid less than 0.70.

If the room has a lot of people or equipment, and noise or reverberation would interfere with what they are doing in the room, high noise absorption is very important. Use the Best NRC category of 0.90. Some examples of rooms where high NRC sound absorption is required are open office areas and patient care areas in healthcare facilities.

If one person usually occupies the room, or if the main function of the room is not sound sensitive, use the Good NRC category of 0.70. Some examples requiring a Good NRC sound absorption are private offices and public waiting rooms.

See the NRC difference: Why to avoid acoustic ceiling panels with NRC below 0.70

There are several reasons why ceiling panels with NRC ratings below 0.70 should not be used. The main reason is that requirements for acoustic absorption in building standards, guidelines, and rating systems have become more stringent. It is unlikely that low-performing ceiling panels will comply with them or with user expectations. To compensate for the deficiency in the ceiling, acoustic wall panels may need to be added to reach the required amount of acoustic absorption in rooms. This is an avoidable cost.

Laboratory measurements conducted by Rockfon using a high-definition acoustic camera and a sound intensity probe show visually the difference between low-performing and high-performing acoustic ceilings. Similar to a thermal imaging camera, this visualization method shows louder sound reflections off the ceiling in red and yellow colors, and effective acoustic absorption in light blue. The return-air grille on the right side acts as a perfect absorber or “open window.” Any noise that hits it passes into the plenum above the ceiling and never comes back into the room. The metal housings of the light fixtures (center) and supply-air diffuser (left) efficiently reflect noise back down toward people. This can be seen in red. The low-performing ceiling (top) with an NRC rating of 0.60, and predominately yellow in color, reflects a lot of noise back down toward people. The high-performing ceiling (bottom) with a NRC rating of 0.95, and predominately light blue in color, performs as well as a perfect absorber, the open return-air grille.

RFN-NA, optimized acoustics, camera study, noise reflecting

Thermal imaging camera shows sound reflection from low NRC 0.60 ceiling and high NRC 0.95 stone wool ceiling.

The difference between Noise Reduction Coefficient (NRC) and Sabins

Both NRC and Sabins are ways to indicate the amount of acoustic absorption being provided. Either can be used to calculate the RT in the room to check compliance with goal criteria or values required in standards.

  • NRC is used to define the acoustic absorption performance of large surfaces of material, such as acoustic ceilings and carpet on the floor. NRC is not used to define the absorption provided by individual sound absorbers that are placed into a room or suspended over a room, such as ceiling baffles and islands.
  • Sabins are the sound absorption unit for acoustic baffles, islands and other three-dimensional sound absorbers placed or suspended inside rooms. One Sabin is equal to 1 square foot of 100 percent sound-absorptive surface. Products with higher Sabins provide more sound absorption.

Avoiding ceiling panels with NRC ratings below 0.70 is just logical. Many choices in ceiling panels exist with NRC ratings of 0.70 or higher. There are many options for aesthetics, color and light reflectance at no cost premium. It will be easier to achieve a goal reverberation time or a goal amount of acoustic absorption using ceiling panels with higher NRC ratings. Fewer products need to be purchased and installed, which is good for the project budget and the environment.

RFN-NA, optimized acoustics, sound absorption in Sabin graphic

Absorption in Sabin indicates the sound absorption property of discrete elements such as islands and baffles.

When should you choose sound-absorption baffles or islands vs. suspended acoustic ceiling systems?

Optimized acoustic absorption can be achieved by using a suspended acoustic ceiling or by hanging sound-absorption baffles or islands over a space. Design aesthetics or coordination with building systems typically establishes which general approach is used to add acoustic absorption.

An acoustic panel ceiling usually covers 100 percent of the room area from wall to wall. The same amount of acoustic absorption can be achieved with only 50 to 75 percent of the room area using free-hanging sound-absorption baffles and islands. Sound can reach all sides of the elements as opposed to just the underside of a suspended ceiling system.

Get inspired about Rockfon® Islands with the Solar Spectrum case study.

Articulation Class (AC) vs. Noise Reduction Coefficient (NRC)

While NRC is the most-used acoustic absorption metric for materials, Articulation Class (AC) is another absorption measure used for acoustic ceilings.

RFN-NA, optimized acoustics, articulation class (AC) graphic

Articulation Class (AC) indicates the speech privacy performance of a ceiling in an open plan environment.

Articulation Class (AC) indicates the ceiling's ability to attenuate speech that could reflect off the ceiling over partial-height cubicle walls in open office spaces. Higher values are better. An AC rating of 150 is low; meaning that speech reflecting off the ceiling over the cubicle wall would be easier to understand. An AC rating of 180 or higher is good, meaning that the ceiling is promoting speech privacy by attenuating the reflection over the cubicle partition. AC is measured according to ASTM E1110 and E1111 standards.

NRC and AC are highly correlated. A high value for one typically means a high value for the other. AC is best used when the concern is speech privacy inside open spaces and where people are close to one another. NRC can be used for these situations as well, but also when the concern is reverberation and general loudness in rooms. AC is not used nearly as much as NRC because of AC's limited application and its unit-less numeric values that are difficult to understand.

Final words on acoustic absorption and sound absorbing ceiling tiles

Made with naturally sound-absorptive stone wool, our acoustic ceiling panels, baffles and islands can be used as part of an overall acoustic absorption strategy in your building to achieve Optimized Acoustics™ and for compliance with standards.

Did you also learn about these other parts of Rockfon’s Optimized Acoustics™?

Learn about the 2nd part: Sound insulation of the walls

Learn about the 3rd Part: Background sound level

Check out our tools to help you design your next acoustics project

After surveying 65,000 people over the past decade in North America, Europe, Africa and Australia, researchers at the University of California-Berkeley report that more than half of office workers are dissatisfied with the level of “speech privacy,” making it the leading complaint in offices everywhere.

John Tierney

Journalist, New York Times

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