There are two common concerns when discussing acoustics within a defined space:
- The build up of noise within the space.
- The transmission of sound between spaces.
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?
- By perforating skins you will dramatically reduce the STC rating and thus allow the transfer of sound through the partition. Think about the drum! Does it make sense to drill holes in a perfectly good STC 52 panel so that you reduce the STC to maybe 46 resulting in more of the drum noise travelling through the wall while the holes do absolutely nothing to reduce the noise build up in the source room? The only thing that has been accomplished is that more people will hear more of the drum. Materials with a high NRC will help to absorb some of the sound and stop it from reflecting back into the room. The same material and mounting system will have a low STC and allow a large amount of sound to pass through it and into the adjacent room.
- By adding sound absorbent panels to the partition skins you increase stack depth bay an average of 2”/panel thus using up valuable and often unavailable space.
- NRC ratings only take into account a mid range of frequencies and not all of the range of sounds that will occur in a typical meeting facility that uses operable partitions. Not all sounds will be dissipated. Think about the drum!
- The perforated skins will require a special and more expensive acoustically transparent vinyl or fabric wall covering. It will not have the wear characteristic of conventional wall coverings & will deteriorate quicker & require recovering sooner. And when the initial covering is applied, special care must be taken to ensure that adhesive does not block the perforated skins. During a recover it is unlikely that the same care will be taken any absorption capabilities will be lost.
- The only time that the partitions may have some NRC value is when extended. It is likely that the acoustical designer when determining the amount of absorption needed did so without taking into account the partitions but is concerned that the additional reflective surface of the partition may increase sound build up: thus the specified NRC feature. However it is reasonable to assume that when the partition is extended the room will be populated with people who are “sound absorbent” and by their movement & presence will reduce sound build up. But their presence will dramatically increase the possibility of sound transference through the partition thus making a high STC more desirable.
- The NRC rating, as it is with STC ratings, is based on laboratory tests under ideal conditions. The lab results will not be duplicated in the field.
- Read manufacturer’s test results. NRC is based only on absorptive characteristics at 250, 500, 1000 & 2000 Hz. If any other frequencies are noted then the test is suspect.
- Read the fine print. For example a carpet manufacturer may say his product provides an NRC of .80. What it may not say is that this rating was obtained by carpet installed over fiberglass. In this configuration it is the fiberglass not the carpet that acts as the primary sound absorber. Without the fiberglass the carpet alone will probably only achieve an NRC of .20.
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.