What does NRC mean?
NRC stands for Noise Reduction Coefficient. The NRC of a material can be found by using either the Reverberation Room Method (ASTM C423) or the Impedance Tube Method (ASTM C384). We use the Reverberation Room Method.
1) Approximately 72 square feet of material is rested on the floor of a reverberation chamber. This chamber usually has all hard concrete surfaces.
2) The change in absorption from the empty room to the room with the material is measured.
3) The changes in sound are measured for octave bands ranging from 125Hz to 4000Hz, recording the differences in Sabin absorption coefficients. The overall NRC rating is the calculated average of frequencies 250Hz, 500Hz, 1000Hz and 2000Hz.
NOTE: The higher the NRC rating, the more sound the material can absorb.

What does STC mean?
STC stands for Sound Transmission Class. This rating is used to compare the acoustic isolation of different materials. The method used to calculate the STC rating is ASTM E413.
1) There are two rooms, a "source" room which contains a speaker and a "receiver" room which contains a microphone. Between the two rooms is an opening typically sized 8 by 9 feet.
2) In decibels, the sound is recorded in 1/3-octave bands from 125Hz to 4000Hz.
3) The next step is to record the sound again in decibels with the material(s) completely covering and sealing the opening.
4) Then, the difference between the before and after test is measured and recorded. These measurements are the transmission loss or "TL" of the sound.
5) Lastly, the transmission data is drawn across an axis of frequency in Hz and transmission loss in dB. This transmission data curve is referenced to standardized STC curves. The standardized curve that most closely matches the transmission data curve gives the transmission data curve its STC rating. The requirements used to match the curves are that the reference curve shall not exceed the measured transmission loss by more than 8 decibels in any 1/3 octave band and that the sum of all "negative discrepancies" shall not exceed 32. Once these requirements are met, the value of the closest-matching reference curve at 500Hz is read as the STC of the material.
NOTE: Lab and field measurements may vary 1 to 5 STC depending on application. This should be taken into consideration when choosing materials for acoustic isolation projects.