NRC vs STC: Which Acoustic Rating Matters for Your Product?

NRC and STC are the two acoustic ratings most often seen on product data sheets, specifications and submittals. Both appear as single numbers, both relate to "sound," and both are used to market acoustic products. Yet they measure completely different things.

NRC describes how much sound a material absorbs. STC describes how well a partition blocks airborne sound. A product with an excellent NRC can have almost no blocking performance, and a wall with a high STC can be acoustically harsh inside the room. Choosing the wrong rating for the job is one of the most common causes of acoustic disappointment.

AcouStick manufactures both absorptive and sound-blocking systems in KSA, and its engineering workflow selects products against the rating that actually matters for each space. This guide explains what NRC and STC mean, how they are tested, their limitations, their international equivalents, and which one matters for your product or project.

What Is NRC?

NRC stands for Noise Reduction Coefficient. Under ASTM C423, the reverberation room test method for sound absorption, NRC is the average of a material's sound absorption coefficients at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05.

• Scale — values near 0 mean highly reflective; values near 1 mean highly absorptive.

• What it describes — how much sound striking the surface is absorbed rather than reflected back into the room.

• Typical products — acoustic panels, ceiling tiles, baffles, wool panels, sprays and plasters.

• Companion rating — the Sound Absorption Average (SAA), from the same test, averages twelve one-third octave bands from 200 to 2500 Hz, rounded to the nearest 0.01.

What Is STC?

STC stands for Sound Transmission Class. It is a single-number rating for airborne sound insulation, derived from laboratory transmission loss measurements to ASTM E90 using the curve-fitting procedure of ASTM E413.

• Scale — higher numbers mean better airborne sound blocking.

• What it describes — how much airborne sound a wall, floor-ceiling, door or window prevents from passing through.

• Typical products — partition systems, floating walls and ceilings, acoustic doors, windows, barrier assemblies.

Riverbank Acoustical Laboratories, an accredited test laboratory, describes typical STC ranges:

• STC 25–30 — speech easily understood through the partition.

• STC 50–55 — typical minimum building-code performance for shared walls; speech privacy at normal voice levels.

• STC 60–65 — loud sounds barely audible; suitable for executive offices.

• STC 65 and above — most sounds virtually inaudible; used in recording studios.

NRC vs STC: The Core Difference

NRC

• Function — absorption — controls sound inside the room.

• Test — ASTM C423 reverberation room absorption test.

• Depends on — porosity, thickness, density and mounting.

• Solves — echo, reverberation, poor speech clarity.

STC

• Function — insulation — controls sound passing between spaces.

• Test — ASTM E90 transmission loss test, rated under ASTM E413.

• Depends on — mass, decoupling, damping and airtightness of the complete assembly.

• Solves — noise from neighbouring rooms, corridors, plant and outdoors.

A National Research Council Canada research note captures the difference: sound passing to adjacent rooms is determined by the insulating properties of the walls, while absorbing materials control noise within the room. The mass law explains why — doubling a panel's mass per unit area adds 6 dB of transmission loss (Acoustical Society of America terminology), and lightweight porous absorbers add very little mass.

Simple rule: NRC tells you how a surface behaves inside a room. STC tells you how an assembly behaves between rooms.

International Equivalents: αw and Rw

αw — Weighted Sound Absorption Coefficient

ISO 11654 converts absorption data into a single number, αw, by shifting a reference curve across octave bands from 250 to 4000 Hz and reading its value at 500 Hz. Its informative Annex B groups results into classes:

• Class A — αw of 0.90 – 1.00

• Class B — αw of 0.80 – 0.85

• Class C — αw of 0.60 – 0.75

• Class D — αw of 0.30 – 0.55

• Class E — αw of 0.15 – 0.25

Rw — Weighted Sound Reduction Index

ISO 717-1 defines Rw for laboratory-tested building elements, derived from ISO 10140-2 measurements, with field ratings such as R'w and DnT,w for completed buildings. It also defines spectrum adaptation terms C (a spectrum weighted to mid and high frequencies) and Ctr (a spectrum with relatively more low-frequency energy).

NRC and αw are both absorption ratings; STC and Rw are both insulation ratings. They are calculated differently, so a value in one system should not be treated as identical to a value in the other.

Limitations of NRC

• No low frequencies — NRC does not include the 125 Hz band, so it says nothing about bass or low-frequency hum.

• Rounding — the 0.05 rounding can hide differences between products.

• Mounting — ASTM E795 states that absorption depends on how a material is mounted; NRC from one mounting does not apply to another.

• No blocking information — NRC tells you nothing about transmission.

Limitations of STC

• Limited frequency range — STC is based on laboratory bands that do not fully describe low-frequency plant noise or bass-heavy music; ISO 717-1 similarly rates over 100–3150 Hz in one-third octaves unless supplementary terms are used.

• Laboratory vs field — NRC Canada warns that flanking along floors, ceilings and abutting walls can seriously impair a wall's insulation on site; building codes recognise this — the 2021 IBC, for example, accepts a lower field rating than laboratory rating for impact insulation between dwellings.

• Assembly-specific — STC applies to the tested assembly, not to a single layer.

• Airborne only — STC does not describe impact sound such as footsteps.

Other Ratings You May Need

• IIC (Impact Insulation Class) — for footsteps and impact on floors; measured to ASTM E492 and classified to ASTM E989 — higher is better.

• Impact levels (Ln,w, L'nT,w) — the international impact ratings, where lower is better; England's Approved Document E sets a maximum L'nT,w of 62 dB for floors in purpose-built dwellings.

• Insertion loss — for enclosures, louvers, silencers and barriers. Research at the Australian Acoustical Society's Acoustics 2018 conference found louvre manufacturers use different metrics, so confirm which is quoted.

Which Rating Matters for Your Product?

• Acoustic wall panels, ceiling tiles, baffles, sprays, plasters — NRC / SAA or αw — they are absorbers.

• Partition walls, floating walls and ceilings — STC or Rw of the complete assembly.

• Acoustic doors and windows — STC or Rw of the complete door or window set, including frame and seals.

• Mass loaded vinyl and barrier layers — STC or Rw of the assembly they form part of.

• Floating floors and acoustic underlays — IIC or impact level, plus STC or Rw for airborne sound.

• Acoustic enclosures, louvers and attenuators — insertion loss with octave-band data.

• Desktop screens and acoustic booths — absorption data for surfaces; isolation or attenuation data for enclosed booths.

Which Rating Matters for Your Problem?

• Meeting room sounds echoey — NRC — add absorption.

• Conversations heard in the next office — STC — upgrade the partition, door and ceiling details.

• Footsteps from above — IIC — treat the floor above.

• Generator noise at the boundary — insertion loss — enclosure and louvers.

• Open plan distraction — NRC for the ceiling plus screens and layout.

How to Read a Data Sheet Correctly

1. Identify the rating — absorption (NRC, SAA, αw) or insulation (STC, Rw)?

2. Check the test standard — ASTM or ISO, and which edition.

3. Check the mounting or assembly — does it match your installation?

4. Look for frequency data — especially for low-frequency problems.

5. Check the laboratory — accredited laboratories and full test reports give confidence.

6. Consider site conditions — flanking, doors and penetrations will affect insulation.

NRC and STC in Saudi Projects

Saudi specifications regularly reference both ASTM and ISO standards, so submittals may show NRC alongside αw, or STC alongside Rw. Clear rating requirements in the specification — naming the metric, the standard and the assembly — avoid confusion during approvals and prevent products being substituted on the strength of an irrelevant number.

Common Mistakes

Mistake 1: Specifying NRC for a Soundproofing Problem

High-NRC panels will not stop sound passing through a wall.

Mistake 2: Comparing NRC With STC

They are different properties on different scales.

Mistake 3: Quoting a Layer's STC as the Wall's STC

STC belongs to the tested assembly.

Mistake 4: Ignoring Low Frequencies

Neither NRC nor STC fully describes bass or plant rumble.

Mistake 5: Ignoring Doors and Flanking

A high-STC wall can be undermined by a weak door or flanking path.

Checklist

• Is the problem absorption, insulation, impact or attenuation?

• Is the right rating specified for each product?

• Are test standards and assemblies stated?

• Is frequency data needed?

• Are doors, penetrations and flanking addressed?

How AcouStick Selects Products by Rating

AcouStick's engineering workflow includes RT60 prediction and absorption sizing for absorptive treatment, and transmission-loss calculation for insulation — so each product is selected against the rating that matters, then documented in specifications, shop drawings and submittals.

• Absorption (NRC / αw) — fabric, PET and wooden panels, acoustic ceilings, baffles, wool panels

• Insulation (STC / Rw) — floating walls and ceilings, mass loaded vinyl, damping products, acoustic doors and windows

• Impact — floating floors and underlays

• Insertion loss — enclosures, louvers and attenuators

Frequently Asked Questions

1. What is the difference between NRC and STC?

NRC measures sound absorption inside a room; STC measures airborne sound insulation between spaces.

2. Is a higher NRC better?

For absorption, yes — but only when absorption is what the room needs.

3. Is a higher STC better?

Yes, for airborne sound blocking. The required STC depends on the use of the spaces.

4. Can a product have both NRC and STC?

Some assemblies are rated for both, but each rating describes a different function.

5. What is the European equivalent of NRC and STC?

αw (ISO 11654) is the absorption rating and Rw (ISO 717-1) the insulation rating. They are calculated differently from NRC and STC.

6. Which rating matters for acoustic panels?

NRC, SAA or αw, with the test mounting stated.

Final Takeaway

NRC and STC answer different questions. NRC asks how much sound a surface absorbs; STC asks how much airborne sound an assembly blocks. Choosing the right rating for each product — and reading it with its test standard, mounting and assembly — is the foundation of good acoustic specification.

Unsure which rating your project needs? Share your specification and drawings with AcouStick for an engineering review.

If you are comparing NRC, STC or other acoustic ratings for a project, AcouStick can help you match the right rating and product to your actual noise problem.

Email: info@acoustick.co | Call: +966 51 130 809 | Web: www.acoustick.co

REFERENCES

• ASTM International. ASTM C423, "Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method" — NRC and SAA definitions as summarised by Intertek: intertek.com/building/standards/astm-c423

• Riverbank Acoustical Laboratories. "Understanding Sound Transmission Class (STC) Ratings and Testing" — ASTM E90 / ASTM E413 procedure and STC ranges: riverbankacoustics.com

• Warnock, A.C.C. "Reverberant Noise Control in Rooms Using Sound Absorbing Materials." Building Research Note, National Research Council Canada, June 1980: nrc-publications.canada.ca

• Acoustical Society of America Standards. Terminology entry "mass-law sound transmission loss": asastandards.org/terms/mass-law-sound-transmission-loss

• International Organization for Standardization. ISO 11654:1997, "Acoustics — Sound absorbers for use in buildings — Rating of sound absorption" — αw derivation and Annex B absorption classes (BS EN ISO 11654:1997 text).

• International Organization for Standardization. ISO 717-1:2013, "Acoustics — Rating of sound insulation in buildings and of building elements — Part 1: Airborne sound insulation" (preview edition, iTeh Standards).

• ASTM International. ASTM E795-16, "Standard Practices for Mounting Test Specimens During Sound Absorption Tests" — scope statement: store.astm.org/e0795-16.html

• Warnock, A.C.C. "Controlling Sound Transmission through Concrete Block Walls." Construction Technology Update No. 13, National Research Council Canada, 1998: publications-cnrc.canada.ca

• International Code Council. 2021 International Building Code, Section 1206.3 "Structure-Borne Sound" — as quoted in Allen, D. "Acoustical Requirements for Ceilings and Floor-Ceiling Assemblies." AWCI, July 2023: awci.org

• ASTM International. ASTM E492, E989 and E1007 (impact sound) — as described in "ASTM E492 – Impact Noise Measurement for Floor-Ceiling Assemblies in the Lab," Noise News International: noisenewsinternational.net

• HM Government. Approved Document E, "Resistance to the passage of sound," 2003 edition incorporating 2004, 2010, 2013 and 2015 amendments (England) — Section 0 performance tables: assets.publishing.service.gov.uk

• Hayne, M., Tan, D., Devereux, R., & Mee, D. "Testing and Performance Quantification of Acoustic Louvres." Proceedings of Acoustics 2018, Australian Acoustical Society, Adelaide: acoustics.asn.au/conference_proceedings/AAS2018/papers/p54.pdf