What Is Ultrasonic Acoustic Absorption Material? (NRC Ratings)
Ultrasonic acoustic absorption is a material’s ability to reduce reflected sound above 20 kHz, but there is no universal rating for it. NRC cannot prove ultrasonic performance: it averages absorption at 250, 500, 1,000, and 2,000 Hz. For ultrasonic claims, request frequency-specific impedance-tube results for the exact material, thickness, backing, and mounting you plan to use.
In a smart home, ultrasonic sound may come from sensors, animal deterrents, cleaning devices, or other equipment. If reflections matter, choosing a material by its room-acoustics rating alone can lead to the wrong result. I treat the question as a measurement and assembly problem: first define the sound to control, then check test data for the proposed installation.
Define the Frequency Band and Select the Correct Metric
This section distinguishes ultrasonic absorption from ordinary room absorption. The key is to specify the frequency range and sound conditions you care about, then use a test method that reports performance in that range. A material name or a high NRC is not enough to predict how it will behave above 20 kHz.
Ultrasound is sound above 20,000 hertz (20 kHz). Absorption describes how much sound energy a surface does not reflect under stated test conditions. It is not the same as sound isolation, which concerns how much sound passes through a wall or barrier.
What does NRC measure?
Noise Reduction Coefficient, or NRC, is a single-number summary of sound absorption in the normal range used for room acoustics. Under the standard rating convention, it averages absorption coefficients at 250, 500, 1,000, and 2,000 Hz, then rounds the result to the nearest 0.05.
ASTM C423 measures absorption in a reverberation room. That result helps compare room-acoustic products under the test conditions, but it is not a measurement of ultrasonic performance. The NRC range stops at 2,000 Hz, well below the 20 kHz ultrasonic threshold. Do not extend an NRC value beyond its stated range.
Which test applies above 20 kHz?
ASTM E1050 and ISO 10534-2 describe impedance-tube methods for measuring normal-incidence sound absorption by frequency. A test report should show results at the actual frequencies of interest, including the conditions and specimen setup used.
Normal incidence means sound travels toward the sample along the tube’s axis. This controlled test is useful for comparing samples, but it does not represent every sound field in a room. Results from different methods or mounting setups should not be treated as interchangeable.
At 20 °C, sound travels in air at about 343 metres per second. Dividing that speed by 20,000 cycles per second gives a wavelength of about 17.2 millimetres. Wavelength gets shorter as frequency rises, so small changes in surface structure, thickness, or mounting may matter. That fact does not, by itself, predict a material’s absorption.
| Question | Useful metric or evidence | What it does not show |
|---|---|---|
| How does a product absorb ordinary room sound? | ASTM C423 results and NRC | Absorption above 2 kHz |
| Does it absorb at a target ultrasonic frequency? | Frequency-by-frequency ASTM E1050 or ISO 10534-2 data | Performance under every room condition |
| Will the tested result match my installation? | Reported thickness, backing, air gap, facing, and mounting | Results for a changed assembly |
| Will the finish meet fire rules? | Applicable listing and local code review | Ultrasonic absorption |
Takeaway: Set a target frequency band before selecting a product. Ask for data at those frequencies, not a general claim that the material is “sound absorbing.”
Survey the Surface, Access, and Ventilation Clearances
A site survey checks whether the proposed assembly can fit without blocking building systems or changing the tested setup. Record the surface, available depth, access needs, airflow paths, and nearby equipment before ordering material. This step also helps identify conditions that make a lab result a poor match for the actual installation.
Start by defining the sound source and goal. Note the frequency band, whether sound reaches the material head-on or from several directions, and whether the goal is to reduce reflection or limit sound passing through a partition. These are different problems and may require different evidence.
Measure the surface and available cavity depth. Record the substrate, existing finishes, and any planned facing or air space. A porous or fibrous material may respond differently when compressed, covered, or placed against a backing. A resonant or layered product may depend on a specific cavity or mounting detail. These are reasons to check the report, not rules for predicting performance.
Also check what must remain clear and usable. Do not cover HVAC outlets, detectors, sprinklers, electrical access, service openings, or other building systems. Preserve required equipment clearances and ventilation paths. If an installation could affect a listed assembly or a required fire-protection feature, pause and get the appropriate design or code review.
| Survey item | Record before installation | Why it matters |
|---|---|---|
| Target sound | Frequency range and source | A test must cover the frequencies of interest |
| Surface | Substrate and condition | The specimen may have been tested on a different backing |
| Geometry | Material depth and any air gap | Changing either may change performance |
| Facing | Type, if any | A facing can alter how sound reaches the material |
| Building services | Vents, access points, and safety devices | These must stay clear and functional |
Takeaway: A product report is useful only when the installation can follow its tested conditions without interfering with building systems.
Install the Tested Material Assembly
Installation means reproducing the assembly described in the laboratory report, not simply attaching a product with the same brand name. Thickness, density, backing, air gap, facing, and mounting can all affect the result. If one of these details changes, the report may no longer describe the installed assembly.
Before work begins, compare the product submittal with the test report. Confirm the product identity and the specimen details. Check that the report names the test standard and includes frequency-by-frequency results at the target ultrasonic frequencies. If the report covers only NRC or frequencies up to 2,000 Hz, it does not verify ultrasonic absorption.
Follow the manufacturer’s installation instructions and the tested mounting details. Do not substitute a thinner layer, different backing, added facing, or altered air gap while claiming the original test result. There is no universal thickness, density, air gap, or airflow rating that proves ultrasonic absorption. Performance must be tied to the tested material and assembly.
A useful project record includes the product and batch identification, material dimensions, backing, facing, mounting method, and any departures from the report. Photograph concealed work before closing it. If a deviation is unavoidable, describe it plainly and seek new test data or qualified engineering advice rather than assuming the original result still applies.
Takeaway: Install to the report’s assembly details. Document any change, and do not label an altered assembly as equivalent without evidence.
Verify Ultrasonic Performance and Fire Compliance
Verification checks two separate issues: whether the installed assembly meets the acoustic goal and whether its use meets applicable fire and building rules. Lab absorption data and fire-test evidence answer different questions. Neither one replaces the other, and both must match the proposed product and use.
For acoustic verification, first review the laboratory report. It should identify ASTM E1050 or ISO 10534-2, show absorption by frequency across the target band, and state specimen thickness, backing, air gap, facing, and mounting. Check that the report covers the material you received and the way you installed it.
Impedance-tube results are normal-incidence measurements on a specimen. If performance in a room or installed system matters, ask an acoustics professional what field method is suitable. A field result depends on the source, receiver, room, and test setup; it should not be presented as the same measurement as a tube result.
For fire performance, verify the locally adopted building code and applicable requirements for the interior finish and location. The International Building Code (IBC) is adopted with local changes in many places, so check the rules enforced by the local authority. Where required, confirm the relevant ASTM E84 or UL 723 listing for the product and its intended use. These are surface-burning tests; they do not establish ultrasonic absorption or, by themselves, a fire-resistance rating.
Keep the acoustic report, product listing, installation record, and any approvals with the project documents. If the product, backing, or mounting differs from the approved details, ask the building official or qualified design professional whether review is needed.
Takeaway: Use acoustic test data to assess absorption and the correct fire documentation to assess code compliance. Do not use one as proof of the other.
Installation Scenarios: How to Read the Evidence
These examples are hypothetical decision scenarios, not reports of measured project results. They show how the same product claim can mean different things depending on the stated test range and assembly. The practical lesson is to match the evidence to the actual question before buying or installing material.
Scenario 1: A room panel has a high NRC. A homeowner wants to reduce reflections from a device operating above 20 kHz. The panel’s C423 report and NRC may help assess ordinary room sound, but they do not answer the ultrasonic question. The next step is to request frequency-specific impedance-tube data at the target frequencies.
Scenario 2: A test report covers the target frequency, but uses a different assembly. A report tests a sample with a backing and air gap that the installer cannot reproduce. That result does not establish performance for a directly mounted product. The installer should find data for the intended setup or have the proposed assembly tested.
Scenario 3: Acoustic data look relevant, but the location has building services. A proposed wall treatment would cover a vent or detector. The material must not be installed in a way that blocks or impairs the system. Revise the layout and check code requirements before proceeding.
Takeaway: A relevant frequency range is necessary, but not sufficient. The test setup and the permitted installation must also match.
Frequently Asked Questions
Is NRC an ultrasonic absorption rating?
No. NRC averages absorption at 250, 500, 1,000, and 2,000 Hz and rounds to the nearest 0.05. It does not measure absorption above 20 kHz.
What test should I ask for?
Ask for frequency-specific normal-incidence absorption data measured under ASTM E1050 or ISO 10534-2. The report should include the frequencies that matter and the sample’s test setup.
Does “acoustic foam” mean it absorbs ultrasound?
No. A product category or marketing description does not prove ultrasonic absorption. Request relevant test data for the specific product and assembly.
Does a thicker material always absorb more ultrasound?
No general rule applies to every material and frequency. Thickness can affect performance, but the result depends on material properties, frequency, backing, and mounting. Use test data rather than assuming.
Can I use an air gap to improve the result?
Only if the test data support that assembly. An air gap can change performance, so a result for one gap should not be assumed to apply to another.
Is an impedance-tube result the same as room performance?
No. The tube measures normal-incidence absorption under controlled conditions. Room performance can differ because the sound field, geometry, and installation are different.
Does a high NRC mean a material blocks sound?
No. NRC concerns absorption, not sound transmission through a wall or barrier. Sound isolation needs suitable transmission-loss or related data for the full assembly.
What fire documentation should I check?
Check the locally adopted code and the requirements for the material’s location and use. Where applicable, confirm an ASTM E84 or UL 723 listing that matches the product and intended application.
Can I cover a vent or detector with absorption material?
No. Keep vents, detectors, sprinklers, electrical access, and other building systems clear and functional. Check required clearances and access rules before installation.
What should I keep in the project file?
Keep the test report, product identification, installation instructions, dimensions, backing and mounting details, photos, fire documentation, and records of any deviations or approvals.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
