NRC Complete Guide — Absorption, Materials & Room Targets

The first line on any acoustic material data sheet is the NRC (Noise Reduction Coefficient) value. NRC expresses how much sound a room-surface material absorbs on a single scale from 0 to 1. Office focus, classroom intelligibility, hotel tranquility — it is the starting point for every room-acoustics design.

What Is NRC?

NRC is a single-number index expressing what percentage of incident sound energy a surface material absorbs, on a scale from 0 to 1. It is the arithmetic mean of the absorption coefficients α measured at four frequencies — 250, 500, 1,000, and 2,000 Hz — covering the range of human speech and general ambient noise.

The meaning of each value range is as follows.

NRC 0 — Complete reflection. Concrete, glass, smooth painted surfaces, etc.
NRC 0.5 — Half of incident energy absorbed. The average for spaces richly furnished with carpet and upholstery.
NRC 0.9+ — Purpose-built absorptive materials. Thick melamine foam, mineral wool, maximum-thickness PET panels, etc.
NRC 1.0+ — Edge diffraction absorbs more than the geometric area. Recognised as a normal result under ISO 354.

NRC = average absorption at 250, 500, 1k, 2k Hz — the speech band in one number.

NRC > 1.0 is not an errorPorous materials can absorb more energy than their facing area implies due to edge diffraction. ISO 354 itself recognises this as normal — values of 1.05 or 1.10 on test reports are not measurement errors.

NRC Measurement — Reverberation Room Method (KS F 2805 / ISO 354)

NRC is measured using the reverberation room method (KS F 2805 / ISO 354). North America uses the equivalent standard ASTM C423; both standards apply the same Sabine equation to convert changes in RT60 into equivalent absorption area.

The test procedure is as follows.

Baseline measurement — RT60 (time for reflected sound to decay 60 dB) of the empty reverberation room is measured per octave band.
Sample installation — A standard specimen (approximately 10–12 m²) is placed on the room floor (Mounting Type A).
Remeasurement — RT60 is remeasured at 250, 500, 1,000, and 2,000 Hz.
Calculation — The absorption coefficient α at each frequency is derived from the before/after RT60 difference → the mean of the four values = NRC.

Lab vs real-room limitsNRC is a laboratory value. Real-world performance shifts with mounting, air gaps, furniture, finish stack-up and occupancy. Large projects should pair average surface NRC with an RT60 simulation.

International Standard Comparison — ISO 11654 vs. ASTM C423

NRC is the single-number output of the North American standard (ASTM C423). European standard ISO 11654 classifies the same reverberation-room data differently — assigning a weighted absorption coefficient α_w and an absorption class from A to E. Understanding the difference between the two systems makes reading material catalogs much easier.

NRC · SAA · α_w at a glance

Different ways to compress the same reverberation-room data into a single number.

기준	지역 / 표준	평균 대역	출력 형식	1.0 초과
NRC (ASTM C423)	US · KR	250 · 500 · 1k · 2k Hz	Single value rounded to 0.05	Reported as measured
SAA (ASTM C423)	US	200 – 2,500 Hz · 12 bands	0.01 step (more precise)	Same as measured
α_w + Class A–E (ISO 11654)	EU	250 – 4 kHz · 6-band weighted	0.05 step + Class A (≥0.90) – E	Capped at 1.0 in reporting

If a European material catalog shows "Class A" rather than an NRC value, there is no need for concern. Class A equals α_w ≥ 0.90, which is effectively the same rating as NRC 0.90+.

NRC Comparison by Material — PET, Melamine Foam, Wood Wool Board

Even at the same NRC value, actual absorption performance varies with thickness, density, and installation method. The following compares three materials commonly used in commercial interiors across their thickness ranges.

◆ = highest NRC per thickness. Typical ranges for commercial interior. Reverberation room method (KS F 2805 / ISO 354).

PET 흡음패널

최저

0.65

9–24 mm 범위 하단

최고

0.95

9–24 mm 범위 상단

멜라민폼

최저

0.80

20–50 mm 범위 하단

최고

0.99

20–50 mm 범위 상단

목모보드

최저

0.50

15–50 mm 범위 하단

최고

0.75

15–50 mm 범위 상단

Five-material overview

Five common acoustic materials for commercial interiors, including in-ceiling glass wool.

기준	NRC 범위	표준 두께	강점	주 용도
PET acoustic panel	0.65 – 0.95	9 / 12 / 24 mm	Recycled PET · 107 colors · CNC	Office walls · meeting rooms · design finish
Melamine foam (VIXUM)	0.80 – 0.99	20 – 50 mm	Best NRC per mm · light · paintable	Studio · ceiling cloud · design form
Wood-wool board	0.50 – 0.75	15 – 50 mm	Cement-bonded · moisture-tolerant · natural finish	Gym · auditorium · lobby · natural-look space
Glass wool / mineral wool	0.70 – 1.00	25 – 100 mm	High performance · also insulates	In-ceiling / in-wall hidden core
Carpet tile	0.20 – 0.40	6 – 10 mm	Floor absorption · impact reduction	Open office floors · hotel corridors · meeting rooms

The key point is — NRC alone cannot determine the right material. Installation location (wall vs. ceiling vs. floor), space use, aesthetic requirements, fire rating, and budget all play a role.

Example: open office = PET wall panels + ceiling melamine cloud + carpet-tile floor. A combination of finish materials almost always outperforms any single material used alone.

Walls + ceiling + floor. Three surfaces combined always beat any single material.

Recommended NRC Values by Space Type

There is no single "correct" NRC. The target average-surface NRC shifts depending on the primary function of the space (focus, conversation, music, rest). The following is a six-scenario guide based on ISO 3382, KS F 2863, ANSI S12.60, and DIN 18041.

SC 01

NRC 0.70–0.90

Open-plan office

Speech privacy · focus

Based on ISO 3382, KS F 2863, ANSI S12.60. Target average across walls, ceilings, floors, furniture. Individual panels may exceed.

SC 02

NRC 0.65–0.80

Meeting room

Speech clarity (STI > 0.60)

Reverberation control to achieve STI > 0.60. Over-absorption sounds unnatural — balance with volume and occupancy.

SC 03

NRC 0.60–0.75

School classroom

Teacher–student speech

ANSI S12.60 classroom acoustic standard. Korean instruction prioritizes 250–1,000 Hz absorption.

SC 04

NRC 0.55–0.70

Library / reading room

RT60 < 0.6s · minimal background

RT60 below 0.6s and minimal background. Critical for managing discrete noise like page turns and footsteps.

SC 05

NRC 0.50–0.70

Restaurant / cafe

Balance liveliness and conversation

Over-absorption deadens the space. Balanced reverberation creates lively yet comfortable conversation. Ceiling treatment is most effective.

SC 06

NRC 0.80–0.99

Studio / practice room

RT60 < 0.3s · maximum absorption

Maximum broadband absorption with RT60 below 0.3s. Melamine 50mm (with air gap) plus PET 24mm covers full band.

It is worth emphasising that these figures are target average-surface NRC values. The combined absorption of walls, ceiling, floor, and furnishings must fall within these ranges. An individual panel's NRC may be higher or lower; what matters is the overall acoustic outcome for the space.

NRC vs. STI — Material Metric or Room Metric?

Material absorption

Per-material absorption measured in a lab — KS F 2805 / ISO 354.

Room speech clarity

How clearly speech is heard in the real room — IEC 60268-16.

NRC and STI (Speech Transmission Index) are closely related but measure different things. In one sentence: NRC = the material's performance; STI = the room's performance. Their relationship and differences are summarised in the table below.

NRC vs STI on five axes

Both are 0 – 1 scales, but the subject, standard and use differ.

기준	NRC	STI
Subject	Material absorption	Room speech clarity
Scale	0 – 1.0 (material)	0 – 1.0 (room)
Where measured	Lab reverb room	On-site (real room)
Primary use	Product selection · material compare	Classroom · auditorium · PA system verification
Standard	KS F 2805 / ISO 354 / ASTM C423	IEC 60268-16

Applying sufficient high-NRC material reduces reverberation and background noise, which improves STI. However, excessive absorption deadens the space — producing a "dead room" quality. Finding the right balance based on room volume, occupancy, and use is the essence of acoustic design.

Frequently Asked Questions (FAQ) — NRC Sound Absorption Coefficient

Q1What is the difference between NRC 0.9 and NRC 0.5?

A material with NRC 0.9 absorbs 90% of incident sound energy, while NRC 0.5 absorbs only 50%. In practice, NRC 0.9 panels reduce reverberation and echo far more effectively. For noisy open offices with many hard surfaces, NRC 0.9 panels (e.g., 24mm PET) are recommended. NRC 0.5 can achieve similar results with greater coverage area, but this significantly increases cost.

Q2How is NRC measured?

NRC is measured using the reverberation room method (KS F 2805 / ISO 354). A material sample is installed in a reverberation room, and the absorption coefficient (α) is measured at 250 Hz, 500 Hz, 1,000 Hz, and 2,000 Hz. NRC is the arithmetic mean of these four values. Domestic certified testing institutions include KCL (Korea Construction Living Environment Institute) and FITI Testing & Research Institute.

Q3What is the recommended NRC for an office?

For open-plan offices, an average surface NRC of 0.70–0.90 is recommended. This is typically achieved by combining NRC ≥ 0.80 wall panels (e.g., PET 24mm, NRC 0.90) with ceiling treatment and carpet flooring. Screen panels at individual workstations provide localized acoustic privacy but alone are insufficient — room-level treatment is essential.

Q4Can NRC exceed 1.0? Is it a measurement error?

Yes, NRC can exceed 1.0 (e.g., 1.05, 1.10) — this is not a measurement error. Porous materials, especially when installed with an air gap or near edges, exhibit a diffraction effect that absorbs more acoustic energy than their physical area suggests. ISO 354 officially recognizes this phenomenon. Values above 1.05 are uncommon in practice and often indicate optimal installation conditions.

Q5What are the NRC values for Tornex PET absorption panels?

Based on KCL certified test results, Tornex PET absorption panels have the following NRC values: 9mm — NRC 0.65, 12mm — NRC 0.75, 24mm — NRC 0.90. These values apply to panels installed directly against the wall surface. Adding an air gap (e.g., 50mm) can increase NRC by approximately 0.05–0.10 at low frequencies.

Q6Is NRC (absorption) the same as soundproofing (insulation)?

No. NRC measures sound absorption (reducing echo and reverberation within a space), while soundproofing (insulation) refers to blocking sound transmission between spaces (measured by STC or Rw). High-NRC absorption panels reduce indoor noise but cannot block sound from passing through walls. For inter-space noise isolation, a separate structural insulation solution is required.

Glossary

All abbreviations and standard numbers appearing in this guide are consolidated here.

NRC (Noise Reduction Coefficient) — Average absorption rate at 250 / 500 / 1k / 2k Hz. Range: 0–1.
SAA (Sound Absorption Average) — ASTM C423 12-band average from 200 to 2,500 Hz. More precise than NRC.
α_w / Class A–E — ISO 11654 weighted absorption coefficient + classification from A (≥ 0.90) to E.
RT60 (Reverberation Time) — Time for sound pressure to decay 60 dB after a source stops. The core indicator of room reverberation.
STI (Speech Transmission Index) — IEC 60268-16. Room speech intelligibility, 0–1.
STC (Sound Transmission Class) — Sound isolation rating. A separate metric, distinct from sound absorption (NRC).
ISO 354 / KS F 2805 — International and Korean standards for reverberation-room sound absorption measurement (equivalent).
ASTM C423 — US reverberation-room sound absorption standard. Outputs both NRC and SAA.
ISO 11654 — European sound absorption rating standard. Outputs α_w + Class A–E.
ANSI S12.60 — US classroom acoustics standard. RT60 ≤ 0.6 s (rooms < 283 m³), background noise ≤ 35 dB(A).
DIN 18041 — German indoor acoustic quality standard. Group A (classrooms, lecture halls) / Group B (offices).
IEC 60268-16 — International standard for STI measurement. Includes STIPA and STITEL variants.
KCL · FITI — Korea Conformity Laboratories · FITI Testing & Research Institute. Accredited domestic acoustic testing bodies.

References

The standard numbers, measurement principles, and figures in this guide can be verified against the following primary sources.

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