Understand NRC noise rating.  NRC demystified

NRC stands for Noise Reduction Coefficient

NRC is the average of absorption at only four frequencies (250, 500, 1,000 and 2,000 Hertz).

These ratings alone should not be used when selecting acoustic panels for spaces where both low range and high frequencies are important such as music venues or recording studios.

If your space has major reverberation and echo, acoustic absorption panels with a high NRC value are going to help improve the acoustic ambience.

Understanding the rating.  NRC rating is described in decimals. So a rating of .75 means 75% of the sound that reaches your acoustic panel is absorbed and is not reflected.
The NRC figure ranges from 0.00 -1.00   For example, a hard concrete floor may have an NRC of. 00 (highly reflective). Whereas a  25mm thick acoustic panel may rate at .95 -1.00 (very absorbent offering very good acoustic ambience).
The NRC data enables simple comparisons of different materials but ignores performance at frequencies below 250 Hz, or above 2KHz.
250 Hz is bass or low range and 2KHz (2000 Hz) is a high range frequency.

Carpet and curtains help absorb high range frequencies but do little with middle and lower frequencies.

Often rooms with these booming frequencies feature glass panels or facades and benefit greatly from acoustic wall panels on the opposite side of the glass.