NRC is measured at 250, 500, 1000 and 2000 Hz and is presented as a average value between 0 and 1, or 0 and 100%. Absorption classes include a broader range of frequencies, measured from 250 to 4000 Hz, and are presented in classes ranging from A to E. Absorption class graphically shows how the panels perform in each frequency, whereas NRC displays an average.
Since NRC only shows an average, this value can sometimes be limited in providing the information needed to create a good sound environment.
For example: A conference room needs a high level of speech clarity. To achieve this, low-frequency sound needs to be absorbed. A panel with a high NRC value can have a poor ability to absorb low frequencies, making it unsuitable as a solution for this kind of space.
NRC can also be limiting when working on an acoustic design that requires high frequency absorption, since it is only measured up to 2000 Hz.