Nrc Cac Stc and ABC

Nrc Cac Stc and ABC of Acoustics

WHAT is the difference between NRC, CAC and STC and what are the ABC’s of acoustics?
STC rating is the much ballyhooed acronym for sound transmission class that sticks in your vocabulary early on in the acoustics education.


Frequency Spectrum of Musical Instruments
You find that STC has to do with the frequency range from 125 to 4000 Hz which winds up being the range of the human voice…give or take a few Hertz. Stc has been used as reference for walls, but each of us has used this as a measurement for any panel, wall, ceiling and floors. It works, so we use it.

While this is trendy to use STC, it does not help much in a musical environment since the frequency range can go from a low of 20 Hz to a high of 20kHz!

So a better descriptor would be “TL” or transmission loss:

“Transmission Loss is a measurement of a partition’s ability to block sound at a given frequency, or the number of decibels that sound of a given frequency is reduced in passing through a partition. Measuring Transmission Loss over a range of 16 different frequencies between 125-4000 Hz, is the basis for determining a partitions Sound Transmission Class. ”

NRC or noise reduction coefficient is a tad different. This is the measure that a material has to absorb sound. How good it does is rated with a NRC rating. 1.0 is about as good as an NRC can get.

CAC, or ceiling attenuation class, is the measurement for the performance of a ceiling. Obviously, if you were paying attention, the higher a CAC rating, the better.

That leaves us with the ABC’s. Right. Well, it may not be a direct requirement for recording studio construction diy’ers but it never hurts to be able to pop it into conversation at the grand opening of the local Piggly-Wiggly. Never know who is paying attention these days.

C=covering sound