An article titled “Specifying Ceiling Panels with High NRC”4 elaborates on the research supporting a connection between highly absorptive ceilings and improved human performance and health. More information about the building standards and guidelines that require high NRC ceilings is also provided.
Seeing the NRC 0.90 difference
A sound intensity probe was used to scan acoustic ceiling systems ranging between NRC 0.60 and 0.95 while loud, broadband, noise was played through a loudspeaker in the space under them. A high-definition camera and analysis software tracked the probe’s location and the sound intensity levels it measured. These location-specific sound intensity data were then processed into colour sound maps, which were overlaid onto the digital image of the ceiling (Figure 2). The process allows visualization of ceiling NRC rating.
Yellow and red colours in Figure 2 indicate loud noise reflecting off the acoustic ceilings while blue indicates noise being absorbed. Red areas mostly are caused by noise reflecting off the hard, painted, metal air diffuser and light fixtures. Note the open return air grille on the right side of the images (blue) acts as an effective sound absorber because the noise passes through the opening into the plenum and is not reflected. The base question is, at what NRC rating does an acoustic ceiling stop behaving like a reflector (red and yellow) and behave more like an effective absorber (blue)? Based on the series of images in Figure 2 , the answer is NRC 0.90.
The perception of what constitutes high-performance sound absorption has slipped over time. Some specifiers have come to believe NRCs as low as 0.70 to 0.75 are acceptable, but as the sound intensity scans in Figure 2 show, at that level of performance, the ceiling is still acting more as a noise reflector than absorber.
Key takeaways: Room acoustics
The primary role of the acoustic ceiling is to provide sound absorption above occupied rooms and spaces. Research has shown that human performance improves when the ceiling NRC is high (0.90). As a result, building design standards require or recommend high NRC ceilings. Specifications for normally occupied rooms should always include NRC ratings. Typically, NRC 0.90 is required for open spaces with multiple people and noise sources. Some standards do permit ceilings ratings of NRC 0.80 in enclosed rooms where people gather for communication, such as classrooms, conference rooms, and training rooms. Some standards also permit NRC ceiling ratings as low as 0.70-0.75 in small private rooms where there is typically one person and no loud noise sources.
Second role: Privacy between rooms
Sound isolation between rooms can be important for speech privacy or limiting noise transmission to avoid annoyance or distraction. Achieving sound isolation between rooms relates to the construction of the overall envelopes of the rooms including the walls, floors, windows, doors, and sometimes, the ceilings. Figure 3 shows three different sound isolation approaches. The overall level of sound isolation often depends on the weakest link in the room’s boundary construction.
