THE ALPHABET SOUP OF ACOUSTICS

As promised this is the first of several articles and links to information regarding acoustical solutions for commercial spaces.  First of all, and this is very important, I AM NOT an acoustician nor do I claim to be.  However over the course of the last eleven years I have learned a great deal about acoustics and one of the biggest things I have learned is that it often gets overlooked in the design process for one reason or another.  I have also learned it is much more difficult and expensive to fix the problem after the fact.

The best place to start with anything is with the basics, so for those of you that are very familiar with this I apologize.  After having conversations with young interior designers that have no idea what NRC means I felt it was best to start at the beginning. 

Terms:

NRC – noise reduction coefficient

This measures the amount of sound absorbed by a surface and the amount reflected back, this is “echo” within the room.  The scale is an easy one: 0% – 100% absorption

·       drywall NRC .06

·       standard ceiling tile NRC .55

·       1” fiberglass wall panels with the right fabric NRC .80 -.85 

STC – sound transmission class

This measures the sound that transfers through a wall, from space to space.  The scale of measure for this is not as simple as NRC, an STC <35 is considered low while an STC >55 is high.

The STC is the result of an assembly and not based on a single component.

CAC – ceiling attenuation class

This is the rating that measures sound going through a ceiling in an open plenum design, sound from office to office.  A ceiling system with a CAC <25 is low while > 38 is high performance.

AC – articulation class

For those spaces with cubicles; measure for rating the attenuation of reflected speech passing over the top of wall partitions or furniture.  A ceiling system with an AC<150 is considered low whereas an AC> 180 is considered to be high performance.

SABIN – okay, it’s a whole word but there are always exceptions

This is always a confusing one for me, here is the official definition.  A measure of sound absorption provided by a material when installed within an architectural space.  The number of Sabin per unit is approximately equal to the total surface area of the unit (in sq.ft.) that is exposed to sound, multiplied by the absorption coefficient of the material.  This is typically used in reference to hanging baffles.  Asked a supplier for an explanation:  I guess the first thing to remember is that NRC (Noise Reduction Coefficient) is defined as "sabines of absorption per unit area". In other words, if a material has an NRC of 0.85 it means there are 0.85 sabines of absorption per square foot. If there's 10 square feet of material then there would be 8.5 sabines of absorption. So essentially sabines is NRC.

PI – privacy index

This is for rating the speech privacy, or lack of speech intelligibility of a space.  A privacy level of PI>95% represents confidential speech privacy, a PI between 95-86% represents normal or non-intrusive privacy and PI <80 is poor.

IIC – impact insulation class

It is a measure of the floor-ceiling assembly to isolate impact noise (footfall) and other impact noises such as furniture being drug across the floor.  The IIC rating typically ranges between a low of around 20 to a high of 80, with 20 being very poor and 80 being excellent.  This typically applies to multiple family housing.

Next article I’ll get into a couple of the basic issues and the best way to solve them.  If there is some aspect of acoustics you would like to see addressed please let me know and I’ll see about getting input from those that know more than I do.