NRC Explained & Test Standards for Acoustic Slat Panels

What is NRC (in one minute)?

NRC (Noise Reduction Coefficient) is a 0.0–1.0 number that averages how much sound a material absorbs at mid frequencies (typically 250, 500, 1000, 2000 Hz).

• Higher = less echo & shorter reverberation.

• NRC is about absorption, not blocking. For blocking/insulation through a wall, the metric is STC, not NRC.

How is NRC tested? ASTM C423 vs. ISO 354

Both standards use a reverberation room to measure how a sample changes the room’s decay time.

• ASTM C423 (widely used in North America)

  • Recommends large, flat samples (commonly ~8×9 ft / ~72 ft²).

  • NRC is derived from absorption at four octave bands (250–2000 Hz) and rounded to the nearest 0.05.

  • Mounting is defined per ASTM E795 (e.g., Type A = direct to wall; E-400 = with ~400 mm air gap).

• ISO 354 (international)

  • Also reverberation-room based.

  • Similar goal; report format and details differ slightly.

  • Results are comparable when the sample size and mounting are equivalent.

NRC vs. SAA at a glance

• NRC: 4 frequencies, rounded to 0.05 → fast to compare.

• SAA (Sound Absorption Average): 12 one-third-octave bands from 200–2500 Hz, rounded to 0.01 → more detailed spectrum coverage.
  If your panels excel in certain bands (e.g., with an air gap that lifts low-mid absorption), SAA can reveal that advantage more precisely; NRC remains the most recognized shorthand.

Why the same panel can show different NRC values

1. Mounting method

   • Type A (direct mount): Slim profile, great for tight spaces.

   • E-400 (air cavity ~400 mm): Usually boosts lower-mid frequencies, which can raise NRC/SAA and improve perceived clarity in larger rooms.

2. Air gap depth & backing

   • Thicker felt or deeper air gaps often increase absorption in mid/low-mid bands.

3. Sample size & edge effects

   • Very small or unusual shapes exaggerate edges, skewing results. Stick to standard sample sizing when comparing brands.

Pro tip: Only compare numbers tested to the same standard, with the same mounting and similar sample size.

What does “good NRC” look like for acoustic slat panels?

Quality slat systems with felt backing commonly land around NRC 0.65–0.85 depending on felt density, slat spacing/depth, and whether you add an air gap. Beyond the single number, always look at the frequency curve: offices and meeting rooms benefit most from strong performance in the speech range (500–4000 Hz).

How to read a lab report (without being an acoustician)

• Check the standard: ASTM C423 or ISO 354.

• Confirm mounting: Type A or E-400 (and the air gap, if any).

• Look for the curve: 125–4000 Hz absorption—don’t rely on one number.

• Note rounding: NRC to 0.05 steps; SAA to 0.01.

• Identify the lab, test date, and report ID for traceability.

Buying checklist (copy-paste friendly)

• Tested to ASTM C423 (report available).

• Mounting tested matches how you’ll install (Type A or E-400).

• Frequency curve shows balanced absorption across the speech band.

• Coverage math: verify square footage per box and plan panel layout for even distribution.

• Space goals: open office vs. meeting room vs. home media—choose mounting accordingly.

FAQ

Does higher NRC mean better soundproofing?
No. NRC is absorption (reducing echo). STC measures sound blocking through a partition.

Why does an air gap help?
An air cavity behind the panel shifts/boosts absorption at lower mids, often improving speech clarity and comfort.

Can NRC be above 1.00?
Rarely, yes—due to test geometry and edge effects. It doesn’t mean the material “absorbs more than all sound,” just a quirk of the method.