Absorption controls echo inside a room; soundproofing stops sound between rooms. How NRC and STC ratings differ, and what acoustic plaster can and can't do.
Whether you are an architect specifying finishes for a new conference center or a homeowner trying to make a media room more livable, one of the most common points of confusion in acoustics is the difference between sound absorption and soundproofing. These two concepts address completely different acoustic problems, are measured by different ASTM standards, and require different material strategies. Confusing them is one of the most costly mistakes made in acoustical design, and it is remarkably common.
This page explains what each term means, which measurement standards define them, and how to identify whether your acoustical problem calls for one approach, the other, or a combination of both. Along the way, you will see why the language used to market acoustical products matters enormously, and what to ask before specifying anything.
At the most fundamental level, the difference comes down to this:
No material can substitute for the other. A highly absorbent ceiling tile with a Noise Reduction Coefficient (NRC) of 1.00 will not prevent a conversation in one office from being audible in the next. Conversely, a dense mass-loaded vinyl barrier will not fix the harsh, reverberant sound of a tiled lobby.
Important: in professional acoustics, "soundproof" is widely considered a misnomer. Practical construction can greatly reduce sound transmission, but it cannot achieve absolute silence. More accurate terms are "sound isolating" or "sound attenuating."
When sound is generated inside a room, the energy from those sound waves bounces off every hard surface it contacts. In a room with many hard, reflective surfaces such as concrete, glass, gypsum board, or tile, sound waves continue reflecting back and forth long after the source has stopped. This buildup is called reverberation, and it is measured as the time it takes for sound to decay by 60 decibels after the source ceases, a quantity acousticians call T60 or simply reverberation time (RT).
Excessive reverberation is not just unpleasant. An analysis published in the Journal of the Acoustical Society of America found that full speech intelligibility is possible in classrooms with reverberation times up to about 0.4 to 0.5 seconds, and recommended that range. [2] The effect of longer reverberation compounds in larger or highly reflective spaces, making critical communications harder to understand and placing greater cognitive demand on listeners.
The same problem shows up in workplaces. An analysis of occupant survey data from the Center for the Built Environment found that noise distraction and loss of privacy were the major causes of workspace dissatisfaction in open-plan offices. [3]
Sound absorptive materials are rated using the Noise Reduction Coefficient (NRC) and the Sound Absorption Average (SAA), both derived from laboratory testing conducted in accordance with ASTM C423, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. [4]
Under ASTM C423, a test specimen is placed in a reverberation room and broadband noise is generated. The rate at which sound decays with and without the specimen present is measured across frequency bands from 100 Hz to 5,000 Hz. The absorption coefficient at each frequency describes how much of the incident sound energy the material absorbs at that frequency; higher values mean more absorption. Values can exceed 1.00 in laboratory testing due to edge diffraction effects at the specimen perimeter, not because a material absorbs more than 100 percent of sound. [4]
NRC: The NRC is the arithmetic average of a material's absorption coefficients at four frequencies: 250, 500, 1,000, and 2,000 Hz, rounded to the nearest 0.05. These four frequencies correspond to the primary range of human speech. [4]
SAA: The SAA averages a material's absorption coefficients across twelve one-third octave bands from 200 Hz to 2,500 Hz, rounded to the nearest 0.01. Because it covers more bands, it gives a more detailed picture of performance across the speech range. [4]
A critical caution for specifiers: NRC alone can be misleading. Because it averages only four mid-range frequencies, a product optimized to perform well at 500 Hz to 2,000 Hz can carry a high NRC while absorbing poorly at low frequencies below 250 Hz or high frequencies above 2,000 Hz. Always request a manufacturer's full third-party ASTM C423 test report, including absorption coefficients at every measured frequency band, not just the single-number NRC rating.
BASWA Phon is independently tested per ASTM C423 in both A and E mountings, with NRC ratings from 0.80 at 30 mm to 1.00 at 70 mm, depending on finish and thickness. For a full review of system-specific test data, visit the BASWA Technical Data page. To understand how NRC is calculated and what it means for your project, see our Acoustics 101: Understanding NRC Ratings guide.
Sound absorption controls reverberation, reduces perceived loudness in a room, improves speech clarity, and creates a more comfortable acoustical environment for occupants. It is the right tool when the primary complaint is that a space is too loud, too echoey, or that conversations overlap and become unintelligible.
Sound absorption does not prevent sound from traveling from one room to the next. Adding absorptive material to the walls of a bedroom will not stop the noise from an adjacent apartment from entering. That is a sound isolation challenge, governed by entirely different physical principles and different ASTM standards.
Soundproofing, more accurately called sound isolation or sound attenuation, is the practice of reducing the transmission of airborne sound through the building envelope: walls, floors, ceilings, doors, and windows. Where sound absorption converts sound energy into heat at a surface inside a room, sound isolation prevents sound energy from traveling through a building partition to an adjacent space.
The physics of sound isolation depend on three primary mechanisms: mass (heavier partitions transmit less sound), decoupling (breaking the structural continuity that lets vibration travel through a wall assembly), and damping (converting vibrational energy into heat within the partition itself). No amount of surface-applied absorptive material substitutes for these mass-and-decoupling strategies in the partition assembly itself.
The standard metric for sound isolation in the United States is the Sound Transmission Class (STC), a single-number integer rating defined by ASTM E413, Classification for Rating Sound Insulation, derived from laboratory measurements conducted per ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements. [1, 5]
Under ASTM E90, a test specimen, typically at minimum 2.4 meters wide by 2.4 meters high for wall assemblies, is installed between two acoustically isolated rooms. Sound is generated in one room and measured in both rooms simultaneously across 16 one-third octave frequency bands from 125 Hz to 4,000 Hz. The transmission loss values at each band are plotted and compared to a standard STC reference contour defined in ASTM E413 to derive the single-number STC rating. [1, 5]
A useful interpretive reference from ASTM E413 directly: the single-number STC ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television, and similar sources of noise in offices and buildings. Critically, this standard also warns that STC is not appropriate for sound sources with spectra significantly different from speech, including machinery, musical instruments, and transportation noise, for which a detailed analysis in frequency bands is required. [1]
STC 25–30: can be heard and understood through the partition.
STC 35–45: Loud speech is audible but not readily intelligible.
STC 50+: Required by the International Building Code for walls, floors, and ceilings separating dwelling units in new multi-family residential construction (tested per ASTM E90). [6]
STC 60+: Loud sounds are faintly audible; typical target for high-performance private offices, recording suites, and medical facilities requiring strong acoustic privacy.
For an in-depth explanation of how STC ratings are determined and what they mean for your project, visit our Acoustics 101: Understanding STC Ratings page.
STC, like NRC, is a single-number simplification of a complex frequency-dependent phenomenon. Because STC is optimized to reflect the transmission of mid-frequency speech sounds, it can overstate the isolation performance of a partition against low-frequency noise such as bass music, mechanical equipment vibration, or traffic. When low-frequency transmission is a concern, specifiers should also review full transmission loss curves and consider the Outdoor-Indoor Transmission Class (OITC), standardized in ASTM E1332, which weights frequencies down to 80 Hz. [5]
Additionally, STC ratings are laboratory values. Real-world performance is consistently lower than the rated STC due to flanking paths, unsealed penetrations, back-to-back electrical boxes, and construction tolerances. Specifiers should anticipate a field reduction of several STC points below the tested assembly value in most practical installations.
In many commercial buildings, sound isolation between offices is undermined not by the wall itself but by the plenum above it: the cavity between the suspended ceiling and the structural deck. When walls do not extend from floor to the deck above, sound travels over the wall through the plenum and re-enters the adjacent space through the ceiling, regardless of how well the wall itself is constructed.
This pathway is rated by the Ceiling Attenuation Class (CAC), defined in ASTM E413 via test method ASTM E1414. A ceiling system's CAC should ideally match the STC rating of the surrounding walls to prevent the plenum from becoming the weakest link in the acoustic enclosure. Seamless, monolithic ceiling systems without penetrations offer more consistent attenuation than modular tile systems, which rely on a grid framework that introduces gaps and flanking paths. For a full explanation, see our Acoustics 101: Understanding CAC Ratings guide.
A question BASWA receives regularly from architects and homeowners is whether an acoustical plaster system can improve sound isolation between rooms. The answer requires honesty about what an absorbent surface finish can and cannot do.
BASWA Phon is an acoustical plaster system engineered for high-performance sound absorption within a space. Sound passes through the micro-porous finish and into the mineral wool panel beneath, where it dissipates as heat. At lower frequencies, the finished surface vibrates slightly against the mineral wool, which converts that energy to heat as well. The 70 mm system achieves NRC ratings up to 1.00. In terms of sound isolation, BASWA Phon can contribute an additional 5 to 7 points to the STC rating of an existing wall or ceiling assembly when installed in conjunction with the partition, because the added mass and damping of the system layer raises the transmission loss of the total assembly. This is a meaningful contribution to an overall isolation strategy, but it is not a standalone solution to a partition-transmission problem.
In most projects, both absorption and isolation are needed. A home theater needs walls with high STC to prevent bass from disturbing the rest of the house, and it also needs highly absorptive interior surfaces to prevent flutter echo and achieve the right reverberation time for film dialogue and music. A hospital patient room needs high-STC walls for privacy, and acoustically controlled ceiling surfaces to reduce the noise buildup that can elevate stress, disrupt sleep, and impair communication between clinicians and patients.
BASWA Phon is also rated Class A for fire per ASTM E84, contains no VOCs, has achieved no mold growth per ASTM D3273, and provides an average light reflectance value of 0.91 per ASTM E1477. For residential and commercial applications where health, sustainability, and aesthetics must align with acoustic performance, visit our Products page and Portfolio to see installed examples.
Before specifying any acoustical product, answer these questions:
Is the problem that your space sounds too loud, too echoey, or that speech is hard to understand inside the room? You need sound absorption. Target NRC and SAA ratings per ASTM C423, and request full frequency-band test data.
Is the problem that you can hear noise from an adjacent room, floor, or outdoor source? You need sound isolation. Focus on STC-rated assemblies per ASTM E90 and E413, evaluated in full assembly, not as individual materials.
Is the problem both? A layered strategy is required: isolation in the building assembly and absorption in the room surfaces. These two strategies are complementary and should be designed together.
Are you unsure? Consult an Acoustical Consultant or contact a BASWA technical representative for a project-specific review.
The acoustics market is populated with products that use terms like "soundproof panels," "sound barrier foam," and "noise-cancelling tiles" in ways that are technically inaccurate. Foam panels and similar lightweight absorptive products do not block sound in any meaningful way; their low mass and thin profile offer essentially no transmission loss. Products marketed as soundproofing that carry only an NRC rating should be evaluated skeptically.
Before specifying any acoustical product, request:
BASWA publishes third-party test data on the Technical Data page. For a broader foundation in acoustical terminology, the BASWA FAQ page addresses the full range of ratings and concepts you are likely to encounter during specification.
Sound absorption and soundproofing are not interchangeable terms, and they are not interchangeable solutions. Understanding the distinction, reading ASTM test data critically, and asking the right questions before specifying are the most effective ways to avoid costly acoustical mistakes.
If you are evaluating acoustical plaster as part of a broader acoustical strategy for a commercial or residential project, BASWA technical representatives and our network of certified installers are available to assist. Use our Continued Education resources for AIA-approved learning units on acoustical design, or contact us directly to discuss your project.
[1] ASTM International. E413-22, Classification for Rating Sound Insulation. ASTM International, West Conshohocken, PA, 2022. www.astm.org.
[2] Bistafa, S.R., and Bradley, J.S. "Reverberation time and maximum background-noise level for classrooms from a comparative study of speech intelligibility metrics." Journal of the Acoustical Society of America, 107(2) (2000), pp. 861–875. https://pubmed.ncbi.nlm.nih.gov/10687696/
[3] Kim, J., and de Dear, R. "Workspace satisfaction: The privacy-communication trade-off in open-plan offices." Journal of Environmental Psychology, 36 (2013), pp. 18–26. https://doi.org/10.1016/j.jenvp.2013.06.007
[4] ASTM International. C423-22, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. ASTM International, West Conshohocken, PA, 2022. www.astm.org.
[5] ASTM International. E90-23, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements. ASTM International, West Conshohocken, PA, 2023. www.astm.org. See also: ASTM E1332-21, Classification for Rating Outdoor-Indoor Sound Attenuation.
[6] International Code Council. 2021 International Building Code, Section 1206, Sound Transmission. Minimum STC 50 (per ASTM E90), or NNIC 45 (per ASTM E336) if field tested, for walls, floors, and floor-ceiling assemblies separating dwelling units and public or service areas.
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