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.

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.

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).

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.

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:

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.

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.

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.