Learn the critical difference between sound absorption and soundproofing before you choose acoustical materials. Understand NRC, STC, and ASTM-tested ratings so you can make informed decisions and avoid costly specification mistakes.
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, the term "soundproof" is widely considered a misnomer. As ASTM E413 notes, practical construction can significantly attenuate sound transmission but cannot achieve absolute silence. A more accurate term is "sound isolating" or "sound attenuating." [1]
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). [2]
Excessive reverberation is not just unpleasant. Research published in the Journal of the Acoustical Society of America found that reverberation times above 0.4 to 0.5 seconds can meaningfully reduce speech intelligibility in occupied classrooms, even when background noise is otherwise well controlled. [3] The effect compounds in larger or highly reflective spaces, making critical communications harder to understand and placing greater cognitive demand on listeners.
A large-scale study of 50,000 workers across 351 buildings found that inadequate speech privacy, heavily influenced by excessive reverberation and noise buildup, was the single greatest source of workplace dissatisfaction, with nearly 30 percent of private-office occupants identifying acoustics as a factor that actively interfered with their ability to perform their jobs. [4]
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. [5]
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 what fraction of incident sound energy the material absorbs at that frequency, on a scale from 0 (perfectly reflective) to 1.00 (fully absorptive). Values can occasionally 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. [5]
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. [6]
SAA: Adopted into ASTM C423 in 1999, the SAA is an average across twelve one-third octave bands from 200 Hz to 2,500 Hz, rounded to the nearest 0.01. Because it covers more frequencies, the SAA provides a more complete picture of low-frequency absorption performance, which is important when HVAC equipment or low-frequency music is a concern. [7]
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 acoustical plaster systems are independently tested per ASTM C423 using both A and E mountings, providing NRC ratings from 0.80 to 1.00 depending on system thickness, with the 70mm system achieving NRC ratings at the top of the market. 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 primarymechanisms: mass (heavier partitions transmit less sound), decoupling (breakingthe structural continuity that lets vibration travel through a wall assembly),and damping (converting vibrational energy into heat within the partitionitself). No amount of surface-applied absorptive material substitutes for thesemass-and-decoupling strategies in the partition assembly itself.
The standard metric for sound isolation in the United Statesis the Sound Transmission Class (STC), a single-number integer ratingdefined by ASTM E413, Classification for Rating Sound Insulation, derived fromlaboratory measurements conducted per ASTM E90, Standard Method for LaboratoryMeasurement of Airborne Sound Transmission Loss of Building Partitions andElements. [1, 8]
Under ASTM E90, a test specimen, typically at minimum 2.4meters wide by 2.4 meters high for wall assemblies, is installed between twoacoustically isolated rooms. Sound is generated in one room and measured inboth rooms simultaneously across 16 one-third octave frequency bands from 125Hz to 4,000 Hz. The Transmission Loss values at each band are plotted andcompared to a standard STC reference contour defined in ASTM E413 to derive thesingle-number STC rating. [8, 9]
A useful interpretive reference from ASTM E413 directly: thesingle-number STC ratings correlate in a general way with subjectiveimpressions of sound transmission for speech, radio, television, and similarsources of noise in offices and buildings. Critically, this standard also warnsthat STC is not appropriate for sound sources with spectra significantlydifferent from speech, including machinery, musical instruments, andtransportation 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: Loudspeech is audible but not readily intelligible.
STC 50+: Requiredby the International Building Code for walls, floors, and ceilings separatingdwelling units in new multi-family residential construction (tested per ASTME90). [10]
STC 60+: Loudsounds are faintly audible; typical target for high-performance privateoffices, recording suites, and medical facilities requiring strong acousticprivacy.
For an in-depth explanation of how STC ratings are determinedand what they mean for your project, visit our Acoustics 101: Understanding STC Ratings page.
STC, like NRC, is a single-number simplification of a complexfrequency-dependent phenomenon. Because STC is optimized to reflect thetransmission of mid-frequency speech sounds, it can overstate the isolationperformance of a partition against low-frequency noise such as bass music,mechanical equipment vibration, or traffic. When low-frequency transmission isa concern, specifiers should also review full transmission loss curves andconsider the Outdoor-Indoor Transmission Class (OITC), standardized in ASTME1332, which weights frequencies down to 80 Hz. [8]
Additionally, STC ratings are laboratory values. Real-worldperformance is consistently lower than the rated STC due to flanking paths,unsealed penetrations, back-to-back electrical boxes, and constructiontolerances. 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 officesis undermined not by the wall itself but by the plenum above it: the cavitybetween the suspended ceiling and the structural deck. When walls do not extendfrom floor to the deck above, sound travels over the wall through the plenumand re-enters the adjacent space through the ceiling, regardless of how wellthe 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 andhomeowners is whether an acoustical plaster system can improve sound isolationbetween rooms. The answer requires honesty about what an absorbent surfacefinish can and cannot do.
BASWA Phon is an acoustical plaster system engineered for high-performance sound absorption within a space. The 70mm system achieves NRC ratings up to 1.00, meaning it absorbs effectively the full spectrum of incident mid-frequency sound energy. 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 transmissionloss of the total assembly. This is a meaningful contribution to an overallisolation strategy, but it is not a standalone solution to apartition-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 restof the house, and it also needs highly absorptive interior surfaces to preventflutter echo and achieve the right reverberation time for film dialogue andmusic. A hospital patient room needs high STC walls for privacy, andacoustically controlled ceiling surfaces to reduce the noise buildup thatresearch has linked to elevated stress, disrupted sleep, and impaired communicationbetween 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 anaverage light reflectance value of 0.91 per ASTM E1477. For residential andcommercial applications where health, sustainability, and aesthetics must alignwith acoustic performance, visit our Products page and Portfolio to see installed examples.
Before specifying any acoustical product, answer thesequestions:
Is the problem that your space sounds too loud, tooechoey, or that speech is hard to understand inside the room? You needsound absorption. Target NRC and SAA ratings per ASTM C423, and request fullfrequency-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 certified third-party test data for all products which are available 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 interchangeableterms, and they are not interchangeable solutions. Understanding thedistinction, reading ASTM test data critically, and asking the right questionsbefore specifying are the most effective ways to avoid costly acousticalmistakes.
If you are evaluating acoustical plaster as part of a broaderacoustical strategy for a commercial or residential project, BASWA technicalrepresentatives and our network of certified installers are available toassist. Use our Continued Education resources for AIA-approvedlearning units on acoustical design, or contact us directly to discuss your project.
[1] ASTM International. E413-22, Classification for Rating SoundInsulation. ASTM International, West Conshohocken, PA, 2022. www.astm.org. (Sections 1.3, 4.1, 4.2)
[2] Nature / Scientific Reports. Optimizing acoustic design fordual-function concert and speech halls. Scientific Reports, 2025.https://doi.org/10.1038/s41598-025-96139-8. (Definition and role ofreverberation time, T60)
[3] Bistafa, S.R. and Bradley, J.S. Reverberation time and maximumbackground-noise level for classrooms from a comparative study of speechintelligibility metrics. Journal of the Acoustical Society of America,107(2):861-875, 2000. PMID: 10687696. (Optimal reverberation times for speech intelligibilityof 0.4 to 0.5 seconds)
[4] Cited in: Kang, S. et al. The impacts of room acoustic quality levels on speech intelligibility in open-plan offices: A laboratory study.Intelligent Buildings International, SAGE Publications, 2025.https://doi.org/10.1177/01436244251361333. Original survey data (50,000 workers, 351 buildings) widely attributed to BOSTI Associates workplace studies. See also: Haworth workplace acoustics research summary, 2024.
[5] 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. (Sections 1.1,5.2, 5.3 re: absorption coefficient definition and edge diffraction)
[6] Wikipedia / NRC (sourcing ASTM C423). Noise Reduction Coefficient.https://en.wikipedia.org/wiki/Noise_reduction_coefficient. Citing ASTM C423 for NRC calculation methodology: average of absorption coefficients at 250, 500,1,000, 2,000 Hz, rounded to nearest 0.05.
[7] Intertek. ASTM C423: Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. https://www.intertek.com/building/standards/astm-c423/. (SAA definition: average of 12 one-third octave bands 200 to 2,500 Hz, rounded to nearest 0.01; adopted into ASTM C423 in 1999)
[8] ASTM International. E90-23, Standard Test Method forLaboratory 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-IndoorSound Attenuation.
[9] Riverbank Acoustical Laboratories. Understanding Sound Transmission Class(STC) Ratings and Testing. https://riverbankacoustics.com/news-full-article/understanding-sound-transmission-class-stc-ratings-and-testing-what-is-it-and-what-do-the-results-mean.(STC calculation procedure per ASTM E413; reference to 1/3 octave bands 125 Hz to 4,000 Hz)
[10] International Building Code (IBC) 2021. Section 1206, Sound Transmission. Minimum STC 50 (per ASTM E90) or NNIC 45 (per ASTM E336) required for walls, floors, and ceilings separating dwelling units and public/service areas in new multi-family construction. Cited in: Wikipedia, Sound Transmission Class. https://en.wikipedia.org/wiki/Sound_transmission_class.
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