Guide: NRC and STC, what is the difference?
In short: NRC measures how much sound a material absorbs, and STC measures how much sound a wall blocks. One does not replace the other: a wall can block a lot and absorb almost nothing, or the reverse.
NRC: absorption
NRC (noise reduction coefficient) is the average of a material's absorption coefficients at 250, 500, 1,000 and 2,000 Hz. It runs from 0, for a surface that reflects everything, to about 1, for a surface that absorbs everything. Because of the test method, a result can slightly exceed 1. An absorbent material keeps noise from bouncing off the wall and travelling elsewhere.
STC: blocking
STC (sound transmission class) sums up in a single number the transmission loss of a wall, measured from 125 to 4,000 Hz. The higher it is, the less sound gets through. It depends mostly on the mass and construction of the wall.
Why a sound barrier needs both
A wall that blocks without absorbing sends noise back toward the equipment and the buildings across from it. A material that absorbs without blocking lets noise through to the neighbour. The Deflection acoustic panel combines the two: a galvanized steel external wall to block, and a perforated internal wall in front of a rock wool fill to absorb. It has an NRC of 1.0 and an STC of 43.
Look at the octave bands too
A single number hides the differences from one frequency to the next. Mechanical equipment noise is often strong in the low frequencies, where every wall is less effective. Here is the panel's performance per octave band:
| Octave band (Hz) | 125 | 250 | 500 | 1K | 2K | 4K |
|---|---|---|---|---|---|---|
| Absorption coefficient | 0.98 | 1.10 | 1.06 | 0.96 | 0.95 | 0.74 |
| Transmission loss (dB) | 24 | 32 | 44 | 55 | 57 | 64 |
Frequently asked questions
Is a high NRC enough to soundproof?
No. A very absorbent but light material reduces echo without stopping sound from passing through. To protect a neighbour, you also need a wall that blocks, so a good STC.
Which panel thickness should I choose?
The Deflection panel is available 2 in or 4 in thick. The choice depends on the noise level to be reached; an acoustic study determines it.
Further reading: acoustic panels and sound barriers, acoustic enclosures.