There are two common concerns when discussing acoustics within a defined space:
One cause of noise build up within the space is reverberation (echoes or continued multiple reflections) such as in a large space that has hard reflective surfaces using non porous construction materials like brick, concrete, glass, gypsum, tile etc. The other primary cause is focused reflections of sound energy resulting from curves such as domes, peaks or concave surfaces. The solution to the noise build up, other than design change, is to provide absorptive materials that will reduce reverberation time (the time it takes for the sound to become inaudible after the source stops transmitting) and limit potentially distracting reflections caused primarily by the shape of interior surfaces. Enter NRC!
NRC (Noise Reduction Coefficient) is a single number index used to rate the absorptive efficiency of materials (acoustic ceiling tiles, baffles and banners, office screens, acoustic wall panels etc.) that may be used to address & hopefully solve the anticipated noise build up. The absorptive coefficients of various materials and assemblies are determined through the use of a standardized ASTM testing procedure (ASTM C423) in reverberation rooms of accredited acoustical laboratories & using samples of materials and specific mounting methods.
After the material and mounting method has been tested and results tabulated, an NRC rating is calculated. The NRC rating is an average of how absorptive the material is for the mid-frequencies of sound (250, 500, 1000 & 2000 Hz) and expressed as a decimal rounded to the nearest 5%. Since the frequency range – therefore hearing range – of audible sound is 16Hz to 16,000 Hz or higher, the NRC testing procedure and the NRC number provides no information as to how absorptive a material is in the low (below 250 Hz) or high frequencies (above 2000 Hz) that might be generated by music, mechanical equipment or other non tested frequencies. Examples: a typical sound system will generate sound ranging from 16 Hz to 18000 Hz: a tuba 50 Hz to 500 Hz: a drum 70 Hz to 200 Hz (remember this drum as you read on): a piano 32 Hz to 4000 Hz: normal speech from 125 Hz to 8000 Hz. Note in these examples that none of the sound generated by a drum and only portions of the other common sounds will be absorbed by the medium.
The NRC rating can range from 0.00 (perfectly reflected) to 1.00 (perfectly absorbed). For example a material with an NRC of .90 will absorb 90% of the sound within the tested range of 250 to 2000 Hz that comes into contact with it and will reflect 10% of the sound back into the space.
So what does NRC have to do with operable partitions? The primary function of an operable partition is to divide sight, sound and people and be aesthetically pleasing while doing so. Acoustically the partition is primarily designed to contain sound within a space (STC) but not to prevent the build up of the noise (NRC). But every once in awhile a specification will call for a partition to have an NRC rating in addition to an STC rating.
There are two ways to accomplish this: make the partition sound absorbent by perforating the steel skin thus allowing access to a sound absorbent core (provides an NRC rating but significantly lowers the STC rating) or attaching independent sound absorbent panels to the panel skins (provides an NRC rating, maintains the partition STC rating but increases the panel thickness and stack depth.) If you want details as to how these methods are accomplished please contact Moderco. But the big question is whether or not there is any advantage to having a partition with an NRC rating or should this be a separate issue or requirement & addressed by more conventional means?
We believe that there are far better alternatives to solving noise build up within a space then perforating an operable partition panel and reduce it’s effectiveness in reducing sound transmission from one side to the other.