Understanding Acoustical Engineering and Why it Matters
What architects and homeowners need to know before specifying acoustic materials, including how to find and work with an acoustical consultant.
What architects and homeowners need to know before specifying acoustic materials, including how to find and work with an acoustical consultant.
Poor room acoustics are not merely an inconvenience. Research published in Frontiers in Psychology confirmed that chronic exposure to excessive noise and reverberation is associated with reduced verbal task performance and diminished reading ability, with effects that are more pronounced in children than adults because their executive functions are still developing. [1] A separate scoping review published in the Journal of Speech, Language, and Hearing Research found that increased reverberation time, reduced signal-to-noise ratio, and decreased speech clarity each independently produce negative effects on student listening, learning, and well-being. [2] In commercial settings, a study involving 50,000 workers in 351 office buildings found that the absence of adequate speech privacy was the single greatest source of workplace dissatisfaction. [3]
These outcomes are predictable, measurable, and entirely preventable. That is the purpose of acoustical engineering, and it is why choosing the right acoustical materials requires understanding how performance is measured, what the numbers actually mean, and how to avoid being misled by claims that cannot be verified.
Acoustical engineering is a broad discipline encompassing the study and control of sound and vibration across a wide range of environments. Its sub-disciplines include mechanical vibration analysis, industrial noise control, audio electronics, and architectural acoustics. It is this last sub-discipline, architectural acoustics, that is relevant to building design, and it is the area in which BASWA operates.
Architectural acoustics focuses on creating an optimum listening environment through deliberate design of room geometry, volume, surface materials, and building assemblies. The professionals who specialize in this field hold advanced degrees in audio, mechanical, electronic, or architectural acoustics, and are typically referred to as Acoustical Consultants.
Acoustical Consultants are typically engaged by owners, developers, or architects to assess and optimize the sound environment within an existing or planned space. Their work addresses three primary categories of concern:
Sound Transmission is the passage of sound energy from one space to another, through walls, floors, ceilings, or doors. It is measured using the Sound Transmission Class (STC), a single-number rating derived from laboratory measurements of airborne sound attenuation through a building partition, tested across sixteen standard frequencies from 125 Hz to 4,000 Hz per ASTM E90. [4]
Sound Absorption is the conversion of sound energy into heat by room surfaces and treatments, which reduces the build-up of reflected sound energy within a space. It is measured using the Noise Reduction Coefficient (NRC), a single-number rating representing the average of a material's sound absorption coefficients at 250 Hz, 500 Hz, 1,000 Hz, and 2,000 Hz, derived from reverberation-room measurements conducted under ASTM C423. [5] NRC values range from 0.00 (fully reflective, such as bare concrete) to 1.00 and above (highly absorptive). The NRC is being supplemented in current practice by the Sound Absorption Average (SAA), which averages absorption coefficients across twelve frequencies from 200 Hz to 2,500 Hz and provides a broader picture of a material's performance across the speech frequency range. [5]
Reverberation Time (RT60) is the most foundational metric in room acoustics. It represents the time, measured in seconds, required for a sound to decay by 60 decibels after its source has stopped. RT60 is calculated using the Sabine equation: RT60 = 0.161 x V / A, where V is room volume in cubic meters and A is total absorption in sabins, derived by summing the products of each surface area and its corresponding absorption coefficient. [6] Shorter RT60 values produce cleaner, more intelligible speech. Longer RT60 values add warmth and sustain appropriate for music but become detrimental to speech clarity when excessive. For speech-focused environments such as classrooms and conference rooms, a target RT60 of 0.4 to 0.7 seconds is widely accepted in the literature and reflected in published standards. [7]
Speech Privacy is assessed using the Articulation Index (AI), a weighted signal-to-noise ratio that predicts the intelligibility of speech at a listener's position. Per ASTM E1130, the AI ranges from 0.00 (speech is generally unintelligible) to 1.00 (all individual spoken words can be understood). [8] Its complement, the Privacy Index (PI), expresses the degree to which speech remains private: PI values approaching 1.00 indicate confidential privacy. These metrics help consultants evaluate open-plan offices, healthcare environments, and institutional spaces where speech privacy is both a comfort and a legal consideration.
In the course of a project, an Acoustical Consultant will typically measure existing sound levels using precision-grade equipment, build digital room models to predict how sound will behave in a proposed design, test multiple treatment scenarios computationally before construction begins, identify problematic sources of noise, and recommend specific materials and assemblies. Their recommendations carry weight only when the manufacturers of those materials can substantiate their performance claims with independent third-party laboratory data.
Acoustical performance requirements are not suggestions. Multiple national and international standards bodies have codified minimum performance thresholds for spaces where speech communication is essential.
ANSI/ASA S12.60, the American National Standard for Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools, Part 1, specifies that core learning spaces with volumes up to 566 cubic meters must maintain a maximum one-hour average background noise level of 35 dB(A) and a maximum RT60 of 0.6 seconds for rooms under 283 cubic meters and 0.7 seconds for rooms up to 566 cubic meters. [9] These thresholds exist because research consistently shows that students at every grade level are harmed by conditions that fall below them. One frequently cited finding is that thousands of students nationwide are unable to understand 25 to 30 percent of what is spoken in their classrooms due to inadequate acoustic treatment.
Acoustical performance requirements also appear in healthcare design guidelines, courtroom standards, and occupational noise control regulations, reflecting a broad scientific consensus that the acoustic environment is not a cosmetic concern but a functional and public health one.
One of the most common mistakes made by architects and homeowners during material selection is accepting a manufacturer's NRC or RT60 claims without scrutinizing how those numbers were obtained.
Under ASTM C423, a valid test requires a specimen size of at least 64 to 72 square feet, tested in an accredited reverberation room under controlled temperature and humidity conditions. [5] Diffraction effects at the edges of a test specimen can cause reported absorption coefficients to exceed 1.00, which does not indicate that a material absorbs more sound than arrives at it, but rather reflects a measurement artifact of edge geometry. [11] Reproducibility between different testing laboratories at two standard deviations is approximately plus or minus 0.15, which means that small differences in NRC between competing products should be interpreted cautiously unless those products were tested at the same facility. [11]
The mounting condition used during testing also affects the result significantly. Acoustical ceiling tiles are commonly tested in Type E400 mounting, which simulates a 16-inch deep plenum. A product tested with a deep air cavity behind it will often show better low-frequency absorption than the same product installed directly against a substrate. When comparing products, always confirm that the mounting conditions in the test report match the installation conditions in your project.
BASWA acoustic provides third-party independent laboratory acoustical test data that reflects actual installed environment conditions. You can review BASWA's full technical data documentation to evaluate NRC, SAA, and absorption coefficient data across individual frequency bands, which is especially important for spaces with critical speech or music requirements.
This is a question that deserves a direct answer, because the acoustical plaster category is subject to significant marketing confusion.
Ordinary plaster, including gypsum, lime, and cement-based finishes, is a hard, reflective surface. A bare plaster ceiling typically carries an NRC near 0.05, meaning it reflects approximately 95 percent of the sound energy that strikes it. Installing ordinary plaster in a space where you need acoustic absorption makes the acoustic problem worse, not better.
True acoustical plaster systems, such as BASWA, are porous, seamless, mineral-based products that absorb sound through their microstructure rather than reflecting it. BASWA systems achieve NRC ratings that are independently verified and documented in laboratory reports conducted per ASTM C423. The seamless surface of an acoustical plaster system also eliminates the visible tile grid lines, suspension hardware, and modular panel edges that define conventional acoustical ceiling tiles, allowing designers to achieve a smooth, monolithic appearance without sacrificing acoustic performance.
If you are evaluating products in this category, ask every manufacturer for their ASTM C423 test report, confirm the mounting condition, and verify that testing was performed by an A2LA-accredited laboratory. Statements like "acoustically designed" or "sound-absorbing finish" without accompanying laboratory data are not substitutable for verified NRC or SAA values.
For a deeper look at BASWA's product families and their specific absorption profiles, visit the Products page or explore real-world project applications in the Portfolio.
The downstream costs of inadequate acoustical design are well documented.
In classrooms, poor acoustic conditions impair speech perception, listening comprehension, literacy skill development, and numeracy performance. The effects are measurably larger for younger students, students with hearing loss, students with learning disabilities, and students whose primary language differs from the language of instruction. [12]
In workplaces, research published in a peer-reviewed study on open-plan office environments found that productivity can decrease by up to two-thirds when employees are exposed to intelligible speech from nearby colleagues. [13] A separate longitudinal study found that workers who moved from private offices to open-plan environments reported significant increases in noise distraction, particularly for cognitively demanding tasks and phone conversations. [14]
In healthcare settings, research from simulated environments has demonstrated that replacing reflective ceiling materials with absorptive panels can reduce reverberation time to 0.8 seconds and lower ambient noise levels by more than 7 dB, with combined strategies achieving noise reductions exceeding 11 dB. [15]
These are not abstract outcomes. They represent measurable differences in patient recovery, student performance, and worker health, and they are within the control of the design team when the correct materials and specifications are selected.
BASWA acoustic works closely with acoustical engineering and consulting firms around the world to help owners and designers achieve their acoustic performance goals before construction begins. Acoustical Consultants use architectural plans in specialized modeling software to identify problem areas, model proposed treatment scenarios, and specify products with documented performance. Engaging a consultant during the design phase, rather than after a space has been built and found wanting, is nearly always more cost-effective.
BASWA is a proud long-time supporter of the National Council of Acoustical Consultants (NCAC). The NCAC directory is a reliable starting point for locating qualified firms by project type and geographic region.
To learn more about how BASWA engages with the design community through continuing education, visit our Education page, where AIA-accredited learning units are available for architects seeking to deepen their understanding of acoustical design.
BASWA acoustic is specified by acoustical consultants and design teams across a broad range of project types, including:
Airports, Auditoriums, Ballrooms, Boardrooms, Broadcast Studios, Churches, Classrooms, Concert Halls, Conference Rooms, Courtrooms, Historical Restorations, Home Theaters, Hospitals, Learning Centers, Libraries, Living Spaces, Lobbies, Museums, Music Rehearsal Rooms, Recording Studios, Restaurants, Spas, and Natatoriums.
Whether your project is a new construction or an existing space in need of remediation, BASWA's network of certified installers is available throughout North America and internationally. Contact us to locate a certified installer in your project area, or review FAQ resources for common specification and installation questions.
[1] Klatte, M., Bergstrom, K., and Lachmann, T. (2013), as cited in: Saura-Climent, A., Crespo-Crespo, C., Ferrando-Ferrandis, E., and Mogas-Recalde, J. "The effects of classroom acoustic quality on student perception and wellbeing: a systematic review across educational levels." Frontiers in Psychology, 16 (2025), Article 1586997. https://doi.org/10.3389/fpsyg.2025.1586997
[2] Dockrell, J., and Connelly, V. "A Scoping Review of the Effect of Classroom Acoustic Conditions on University Students' Listening, Learning, and Well-Being." Journal of Speech, Language, and Hearing Research, 67(1) (2024). https://doi.org/10.1044/2023_JSLHR-23-00154
[3] Kim, J., and De Dear, R. (2013), as cited in research reviewed by Haworth Workplace Research. "Decrease Office Noise to Increase Productivity." Haworth, Inc., 2024. Referencing original survey data of 50,000 workers across 351 buildings.
[4] ASTM E90, "Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements." ASTM International, West Conshohocken, PA. Active Standard.
[5] ASTM C423-22, "Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method." ASTM International, West Conshohocken, PA, 2022. https://store.astm.org/c0423-22.html
[6] Sabine, W.C. (1900). Reverberation formula RT60 = 0.161V/A, as reviewed in: Yaman, C., et al. "Evaluation and Improvement of Reverberation Time in the Current Enclosed Spaces with Speech Action." Periodica Polytechnica Architecture, 53(2) (2022), pp. 127-136. https://doi.org/10.3311/PPar.19786
[7] Commercial Acoustics. "RT60 Rating 101: Understanding Reverberation Time." Updated September 2025. https://commercial-acoustics.com/guides/rt60-rating-101/; cross-referenced with ANSI/ASA S12.60/Part 1-2010 (R2020) target criteria.
[8] ASTM E1130-16 (Reapproved 2021), "Standard Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index." ASTM International, West Conshohocken, PA. https://store.astm.org/e1130-16r21.html
[9] ANSI/ASA S12.60/Part 1-2010 (R2020), "Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools, Part 1: Permanent Schools." Acoustical Society of America, Melville, NY. https://blog.ansi.org/ansi/ansi-asa-s12-60-part-1-2010-r2020-school-acoustics/
[11] Wikipedia contributors. "Noise reduction coefficient." Wikipedia, The Free Encyclopedia. Reviewed March 2026. Cites ASTM C423 reproducibility findings from Paul Sabine's foundational reverberation chamber research (1920s-1930s). https://en.wikipedia.org/wiki/Noise_reduction_coefficient
[12] Watson, S.M., et al. "The Effect of Classroom Acoustic Treatment on Listening, Learning, and Well-being: A Scoping Review." Acoustics Australia, 51 (2023). https://doi.org/10.1007/s40857-023-00291-y
[13] Iannace, G., Ciaburro, G., and Trematerra, A. (2018), as cited in: Weijs-Perrée, M., et al. "Open-plan office noise is stressful: multimodal stress detection in a simulated work environment." Journal of Management and Organization, 27(6) (2021), pp. 1072-1097. https://doi.org/10.1017/jmo.2021.17
[14] Kaarlela-Tuomaala, A., Helenius, R., Keskinen, E., and Hongisto, V. "Effects of acoustic environment on work in private office rooms and open-plan offices: longitudinal study during relocation." Ergonomics, 52(11) (2009), pp. 1423-1444. https://doi.org/10.1080/00140130903154579
[15] R. Discovery. "Reverberation Time Research Articles." Citing simulation-based intervention study on healthcare ceiling replacement and combined acoustic strategies achieving 11.3 dB noise reduction. https://discovery.researcher.life/topic/reverberation-time/12459119
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