NRC vs. RT60: How to Evaluate Acoustic Performance

NRC helps compare tested absorptive materials and assemblies. RT60 describes how sound decays in a room. Good acoustic design connects the two—without treating either metric as the whole story.

Why NRC Alone Does Not Tell the Whole Acoustic Story

Explore Feltlite acoustic lighting solutions designed to coordinate sound absorption with commercial illumination.

Ask about an acoustic product and one number often appears first: its Noise Reduction Coefficient, or NRC. The number is useful, but it cannot tell you whether a conference room will support clear conversation, whether an open office will limit speech distraction, or whether a restaurant will feel lively rather than exhausting.

Those are room-level outcomes. They depend on the amount and location of absorption, room volume, geometry, furnishings, occupancy, background sound, and—depending on the space—sound isolation and speech propagation. NRC and RT60 therefore answer different questions:

• NRC asks: How absorptive was this material or assembly under a specified laboratory test?
• RT60 asks: How long does sound take to decay in this particular room?

Neither metric replaces the other. A high-NRC panel installed in too small a quantity may have little effect on a large room. A moderate absorber used in sufficient area and in the right locations may produce a much better result.

What Is NRC?

A single-number absorption rating

NRC is the arithmetic average of the sound absorption coefficients measured at 250, 500, 1,000, and 2,000 Hz, rounded to the nearest 0.05. In the United States, NRC is commonly reported from testing under ASTM C423, which measures sound absorption in a reverberation room.

NRC is often described on a scale from 0.00, highly reflective, to 1.00, highly absorptive. That is a useful shorthand—not a hard physical limit. Under ASTM C423, edge and diffraction effects can produce apparent absorption coefficients, and therefore ratings, above 1.00 for highly absorptive specimens.

How NRC is tested

ASTM C423 compares the decay rate of a qualified reverberation room before and after a specimen is installed. The change is used to calculate absorption by one-third-octave band. ASTM E795 provides standardized mounting practices because the same material can perform differently when it is direct-mounted, backed by an airspace, or installed in another configuration.

For a continuous surface such as an acoustic ceiling, an absorption coefficient can be applied to the installed area. For a discrete object—such as a baffle, cloud, screen, or acoustic pendant—the laboratory may report equivalent sound absorption area per object, commonly expressed in sabins. That object value is often the more appropriate input for room calculations.

What Is RT60?

A room-level decay metric

RT60 is the time, in seconds, required for sound energy in a room to decay by 60 dB after the source stops. A hand clap can reveal an obvious flutter echo or a highly reverberant room, but it is only a listening demonstration. Professional RT measurements use calibrated equipment and standardized procedures.

RT60 is frequency-dependent. A room may have acceptable decay at 1,000 Hz while remaining boomy at 125 Hz. Design reports should therefore identify the frequency bands and averaging convention used rather than presenting one unexplained number.

Estimating RT60 with the Sabine equation

RT60 = 0.161 × V / A

For SI units, V is room volume in cubic meters and A is total equivalent absorption area in square meters (sabins). The equation shows the basic relationship: increasing room volume tends to increase reverberation time, while adding absorption tends to reduce it.

NRC and RT60: The Practical Difference

The most useful workflow is to select products using verified absorption data, calculate the quantity needed to approach the room target, coordinate placement with the architecture and lighting plan, and then verify the completed room where required.

Why Acoustic Lighting May Not Have a Conventional NRC Rating

An acoustic luminaire is not simply a flat piece of felt. It combines an LED system, an absorptive structure, suspension hardware, and an architectural form. Its acoustic performance can change with fixture size, exposed surface area, edge condition, spacing, orientation, and suspension height.

A conventional NRC may be meaningful for a flat felt component tested as an area specimen, but it may not describe the complete suspended luminaire. For discrete products, ask for test data that matches the way the product will actually be installed.

For more context, read why a conventional NRC rating may not fully represent a suspended acoustic luminaire.

Relevant product configurations include acoustic linear pendant lights, acoustic baffle lights, and coordinated acoustic blades.

What to request from the manufacturer

• • The test standard, laboratory, report date, and specimen description
  • One-third-octave absorption data—not only a marketing headline
  • Mounting type, suspension height, array spacing, orientation, and quantity tested
  • Equivalent absorption area per object or validated array data for discrete luminaires
  • NRC or SAA when the tested configuration makes those ratings appropriate
  • Photometric files and lighting data, including lumens, efficacy, distribution, CCT, CRI, controls, and glare information

How to Evaluate an Acoustic Luminaire

Acoustic material and assembly

Review thickness, fiber structure, surface treatment, backing, airspace, fire performance, and the exact tested configuration. Greater thickness or a deeper airspace often improves absorption at lower frequencies, but density alone does not guarantee better performance across every band.

Product geometry and installation

Count the absorptive area actually exposed to the sound field. Curves, fins, and multiple faces may increase exposed area, but geometry should be evaluated through relevant test data rather than assumed to improve absorption. Coordinate fixture spacing and suspension height with both the test configuration and the lighting layout.

Room conditions

Document room volume, ceiling height, surface finishes, furnishings, occupancy assumptions, and other absorptive elements. Glass, concrete, exposed structure, and hard flooring can increase the amount of treatment required. Furniture and occupants contribute absorption, but design assumptions should be explicit.

Lighting performance

Acoustic lighting must still perform as lighting. Confirm maintained illuminance for the visual task, luminaire efficacy, distribution, color quality, controls, flicker performance, and glare. Values such as 4000 K or UGR below 19 may be appropriate for some office projects, but they are not universal requirements; apply the standards and owner criteria relevant to the project.

A Better Way to Estimate Acoustic Requirements

Use the following process during design:

1. Define the room use and the acoustic outcome. Identify whether the priority is speech clarity, controlled reverberation, low distraction, privacy, music quality, or a combination.
2. Select the applicable criterion. RT targets depend on room volume, use, occupancy, and frequency. Confirm the governing code, standard, rating system, or acoustic consultant’s criteria.
3. Estimate existing absorption by frequency band. Include room surfaces, fixed treatments, furniture, and any justified occupancy assumption.
4. Calculate the additional absorption required. For area materials, use absorption coefficient × installed area. For discrete products, use tested equivalent absorption area per object or applicable array data.
5. Coordinate quantity and placement. Distribute treatment to control reflections and avoid relying on a small cluster of high-rated products.
6. Model and verify as appropriate. Use acoustic software for complex spaces and field testing when project requirements call for it.

For a complete worked example, see Feltlite’s guide to calculating how many acoustic lights a space needs.

Illustrative meeting-room calculation

Consider a 10 m × 8 m × 3 m meeting room. Its volume is 240 m³. If the design target at a selected mid-frequency band is 0.60 seconds, the Sabine equation gives a target total equivalent absorption area of:

A = 0.161 × 240 / 0.60 ≈ 64.4 m² sabins

This is the total target—not automatically the amount of new product required. The designer must first subtract the absorption already provided by the room surfaces and furnishings. If the room already contributes an estimated 22 m² sabins at that frequency, the remaining requirement is approximately 42.4 m² sabins. The number of luminaires or panels then depends on their tested absorption in the intended mounting configuration.

Do Not Use RT60 as the Only Metric for Every Space

RT60 is central to many enclosed-room designs, but it is not a complete measure of acoustic comfort. It does not directly describe background noise, mechanical-system noise, sound isolation between rooms, speech privacy, or the spatial propagation of speech.

Open-plan offices are a particularly important example. ISO 3382-3 evaluates parameters related to the spatial decay of speech and speech intelligibility over distance. Screens, workstation layout, background sound, and sound masking may matter as much as—or more than—a single reverberation-time value.

Three Common Acoustic-Design Mistakes

Mistake 1: Selecting by NRC alone

A small quantity of a high-rated product may add less total absorption than a larger quantity of a moderately rated product. Compare both the tested performance and the absorption delivered by the proposed quantity.

Mistake 2: Ignoring test configuration

A felt data sheet, a direct-mounted panel test, and a suspended-luminaire array test are not interchangeable. Match the evidence to the installed product and clearly document any engineering assumptions.

When choosing a system type, compare acoustic blades, baffles, and integrated acoustic lighting before finalizing the ceiling strategy.

Lighting professionals can also review Feltlite’s acoustic LED lighting compliance guide for fire, acoustic, and specification considerations.

Mistake 3: Treating acoustic lighting as ordinary lighting

A successful system must satisfy the acoustic plan, photometric requirements, controls strategy, visual comfort goals, architectural intent, and applicable fire and material requirements. Optimizing only one dimension produces an incomplete design.

Conclusion

NRC and RT60 are complementary—not competing—metrics. NRC helps designers understand tested absorption performance. RT60 helps them predict and verify how sound decays in a room. The connection between the two is total equivalent absorption area, evaluated by frequency and based on the actual installation.

For acoustic lighting, the most reliable specification goes beyond a single rating. It combines relevant third-party acoustic testing, a room-level design calculation, a coordinated lighting plan, and clear installation criteria.

Feltlite develops acoustic LED lighting solutions that integrate absorptive PET structures with commercial lighting performance. When evaluating a project, ask our technical team for the acoustic and photometric data applicable to the product, configuration, and installation under consideration.

Frequently Asked Questions

Browse more Feltlite acoustic-design articles or contact the Feltlite acoustic lighting team for project-specific data.

Technical References

• ASTM C423-23e1: Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method — source
• ASTM E795-23: Standard Practices for Mounting Test Specimens During Sound Absorption Tests — source
• ISO 3382-1:2009: Measurement of room acoustic parameters—Performance spaces — source
• ISO 3382-3:2022: Measurement of room acoustic parameters—Open plan offices — source
• WELL Building Standard v1, Feature 78: Reverberation Time (historical project criterion; confirm the version applicable to each project) — source

Editorial note: Target values and compliance requirements vary by jurisdiction, building program, room volume, frequency band, and standard edition. Confirm current project criteria with the authority having jurisdiction and a qualified acoustical professional.