Seeing the NRC 0.90 difference
The foundational studies conducted by NRCC 20 years ago have been corroborated more recently by research conducted by an acoustic ceiling manufacturer and presented at InterNoise 2018, an exposition on noise control engineering (Consult Look, Do You See the Noise Leaking Through that Ceiling? by Gary S. Madaras).
A sound intensity probe was used to scan an acoustic ceiling system with panels of different absorption performance levels of NRC 0.60 to 0.95 while loud, broadband noise was played in the space under it. A high-definition camera and analysis software tracked the location of the probe and the sound intensity levels it measured. These location-specific sound intensity data were then processed into color sound maps, which were overlaid onto the digital image of the ceiling (Figure 2).
Yellow and red colors in Figure 2 indicate loud noise reflecting off the acoustic ceiling while blue indicates noise being absorbed by the acoustic ceiling. Red areas are mostly caused by noise reflecting off the hard, painted metal—plaque-style—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 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. Fortunately, building standards, guidelines, and rating systems are now reinforcing what science has shown for decades.
