Archive for category Absorption
Anechoic Chambers – Standards for Acoustic Measurement
Posted by Acoustics First in Absorption, Anechoic Chambers, Product Applications, Products, School & Educational Facilities, Uncategorized on September 3, 2026
Everything you wanted to know about anechoic chambers… but were afraid to ask.
An anechoic chamber is a specialized room engineered to eliminate sound reflections. Its walls, ceiling, and—in the case of a full anechoic chamber—the floor are covered with wedge-shaped foam or fiberglass absorbers that prevent sound waves from reflecting back into the test space. This creates a highly controlled acoustic environment that approximates free-field conditions, allowing the sound source to behave as though it were suspended in open air with no nearby reflective surfaces.
Such an environment is essential because acoustic measurements—including sound power, directivity, and frequency response—must be performed under known, reflection-free conditions to ensure repeatable, standards-compliant results. In the absence of these conditions, room reflections influence the measurements, causing the results to depend on the characteristics of the test room rather than the performance of the product being evaluated.
Here is a list of common measurement standards and the chamber/test requirements for each.
Precision Sound Power Measurement (ISO 3745)
ISO 3745 is the reference standard for precision sound power determination. It requires testing to be conducted in either a full anechoic or hemi-anechoic chamber that has been qualified to satisfy stringent free-field deviation requirements over the frequency range of interest. Chamber qualification requires demonstrating that the inverse-square law is maintained within ±1 dB at all specified measurement locations.
The typical chamber cut-off frequency ranges from 80 to 200 Hz, depending on the chamber dimensions and the depth of the acoustic wedges. Below the cut-off frequency, the chamber no longer provides true free-field behavior.
Engineering Sound Power Measurement (ISO 3744)
ISO 3744 establishes engineering-grade methods for sound power determination. While less stringent than ISO 3745, it still requires testing to be performed in a hemi-anechoic environment. The standard permits the application of environmental correction factors when the test environment does not achieve ideal anechoic performance, making it well suited for production-floor test cells and other facilities that approximate, but do not fully satisfy, free-field conditions.
Qualification of Free-Field Environments (ISO 2610)
ISO 26101 specifies the procedures for verifying that a test environment satisfies free-field performance requirements. It is the standard used to qualify an anechoic or hemi-anechoic chamber by confirming that the chamber’s acoustic characteristics meet the required free-field criteria and deliver the level of performance claimed for the facility.
Other Standards
- ECMA-74: IT equipment noise measurement (uses ISO 3745 or ISO 3744 as the underlying acoustic method)
- ANSI S12.55 / S12.56: North American equivalents of ISO 3744/3745
- ISO 11201-11205: Various sound pressure level determination methods, some requiring free field conditions
Chamber Size and Absorber Performance
The physical dimensions of an anechoic chamber determine its lowest usable frequency. As a general guideline, the chamber must be sufficiently large so that the distance between the sound source and each measurement microphone is at least one wavelength at the lowest frequency of interest.
For a chamber with a 100 Hz cut-off frequency, the minimum source-to-microphone distance is approximately 3.4 meters. Consequently, the internal chamber dimensions (excluding the absorber wedges) should be approximately 7–8 meters on each side for a hemi-anechoic chamber.
The depth of the acoustic absorber wedges governs the chamber’s low-frequency performance. Increasing wedge depth improves absorption at lower frequencies.
| Wedge Depth | Approximate Low-Frequency Cut-off |
| 200 mm | ~500 Hz |
| 500 mm | ~200 Hz |
| 1000 mm | ~80–100 Hz |
Common absorber materials include melamine foam (https://www.acousticsfirst.com/acoustical-foam-anechoic-chamber-wedges.htm , which is lightweight and inherently fire-retardant, and fiberglass, which provides superior low-frequency absorption but has a greater mass and higher cost.
Chamber Ambient Noise Floor
In addition to controlling sound reflections, an anechoic chamber must provide effective isolation from external noise sources. The ambient noise level within the chamber, measured with no test source operating, should be at least 6 dB—and preferably 15 dB—below the sound pressure level produced by the test object at the specified measurement positions.
Achieving these background noise levels typically requires a chamber constructed with multiple layers of high-mass materials, such as concrete and steel, combined with vibration-isolated mounting systems to minimize the transmission of structure-borne noise.
Alternatives to an Anechoic Chamber
Not every organization can justify the investment required for a purpose-built anechoic chamber, which typically ranges from $500,000 to more than $2 million. Fortunately, several practical alternatives are available, depending on the measurement objectives and required level of accuracy.
Sound Intensity Method (ISO 9614)
The sound intensity method, defined in ISO 9614, is inherently less sensitive to room reflections because sound intensity is a vector quantity. Unlike sound pressure, it differentiates between outgoing acoustic energy radiated by the test source and incoming energy reflected from surrounding surfaces. As a result, sound power can be determined in ordinary rooms without the need for anechoic treatment.
The primary trade-off is that this method requires specialized sound intensity probes and more complex measurement procedures than conventional sound pressure measurements.
Acoustic Test Boxes
For small products, such as electronic devices, individual components, and transducers, a desktop acoustic test box provides a controlled, low-noise environment that approximates anechoic conditions over a defined frequency range. These systems offer a significantly lower-cost alternative to a full-sized anechoic chamber and can be integrated directly into production-line testing.
Portable Acoustic Arrays
Modern acoustic imaging systems, commonly referred to as acoustic cameras, use portable microphone arrays to identify and localize noise sources in situ, whether on the factory floor, along a production line, or in field applications, without requiring an anechoic environment. Although these systems are not a substitute for standards-compliant sound power measurements, they provide a rapid and effective means of diagnosing and visualizing noise sources that would previously have required dedicated chamber testing.
Contact Acoustics First about your acoustic chamber project to find what sort of treatment is best suited for your application!
From Utility to Engineered Performance: Comparing Armstrong® 1B Perforated Tiles Vs. HiPer Panels®
Posted by Acoustics First in Absorption, Diffusion, Music Tracking Room, Product Applications, Products, Recording Facilities, Recording Studio, School & Educational Facilities, Studio Control Room, Vocal Booth, Voice Over on June 30, 2026
For decades, perforated acoustic panels have played an important role in shaping the sound of recording studios, broadcast facilities, civic buildings, and public spaces. While the iconic Armstrong® 1B perforated mineral fiber tile represented the standard for practical noise control throughout much of the twentieth century, modern acoustic engineering has transformed what perforated panels can accomplish. A comparison between the Armstrong® 1B and today’s Acoustics First® HiPer Panel® family illustrates just how far acoustic technology has advanced.
The Armstrong® 1B tile was designed as a utilitarian acoustic treatment. Manufactured from thin mineral fiber, the 1′ x 1′ panels featured either linear or random perforation patterns that allowed sound energy to enter the porous material for absorption – which increased the NRC of these tiles to a 0.55-0.60. Originally developed during WWII and used in military barracks and installations, these tiles eventually found their way into countless municipal buildings, schools, government facilities, and even legendary recording environments such as RCA and Sun Recording Studios. Their modest absorption characteristics, ease of installation, and economical construction made them a practical solution for reducing reverberation in a wide variety of spaces.
Despite their historical significance, the Armstrong® 1B tiles were fundamentally simple acoustic absorbers. The mineral fiber substrate behaved much like compressed fiberboard or heavy paper, relying solely on the porous material itself for sound absorption. While the perforations exposed more surface area to incoming sound, there was no engineered reflective face, no constrained barrier layer, and no intentional diffusion characteristics. Their primary function was simply to absorb a portion of the reflected sound energy.
Modern critical listening spaces demand considerably more sophisticated performance.
The Acoustics First® HiPer Panel® family builds upon decades of acoustic research using high-performance fiberglass substrates and engineered panel construction. Rather than functioning as simple absorbers, HiPer Panels® are carefully designed hybrid acoustic devices that combine broadband absorption with controlled reflection and diffusion. Where the Armstrong tiles had an NRC of 0.55 – 0.60, the HiPer panels increase this to a 0.90 NRC – but they also introduce tuned absorption and diffusion.
The original laminated HiPer Panel® incorporates a perforated fiberglass bonded to a constrained barrier septum layer. This engineered assembly allowed portions of the sound spectrum to be reflected while some acoustic energy passed into the fiberglass absorber, creating a more balanced acoustic response than a conventional porous panel. The result is an environment that maintains clarity, spaciousness, and natural musical energy instead of becoming acoustically “dead.”
The Hiper Panel® Impact takes a slightly different approach by utilizing a durable perforated reflective face over the fiberglass core. Its optimized perforation pattern and reflective surface are specifically engineered to provide controlled diffusion on top of tuned absorption, making it well suited for performance venues, mastering rooms, critical listening spaces, and recording studios where preserving acoustic life is just as important as controlling excessive reflections. This tuned absorption allows the low frequencies to be absorbed, while leaving the high frequencies so that the diffusion characteristics of the device (over 4Khz) will be more effective in creating a feeling of openness in the room.
The differences between these products extend well beyond appearance. While both feature perforated surfaces, the Armstrong 1B’s small perforations simply exposed a thin mineral fiber absorber. By contrast, the HiPer Panel® systems use perforation geometry, reflective facings, engineered barrier layers, and high-density acoustic fiberglass to create a far more efficient and balanced acoustic device. Instead of merely reducing reverberation, they help shape the acoustic character of a room.
More than fifty years after their introduction, Armstrong® 1B perforated tiles remain recognizable symbols of an earlier era of architectural acoustics. Many continue to serve in municipal offices, schools, police interview rooms, and other institutional settings where functional noise control is sufficient. In modern recording studios, listening rooms, and performance spaces, however, engineered systems like the Acoustics First® HiPer Panel® and Hiper Panel® Impact represent the evolution of acoustic treatment—combining advanced materials, acoustic science, and carefully tuned performance to meet the demands of today’s most critical listening environments.
Acoustic Transformation at New Harmony: Preserving Art, Improving Sound
Posted by Acoustics First in Absorption, Art Galleries, Articles, Customer Feedback, Diffusion, Multipurpose Rooms, Museums, Music Rehearsal Spaces, Product Applications on April 27, 2026
In New Harmony, Indiana, there is a former Odd Fellows Lodge repurposed as a private residence—the main hall doubles as an event and performance space. Measuring 80 by 40 feet with a 14-foot ceiling, the room features a mezzanine, raised stage, large windows, and an extensive collection of artwork. While visually striking, the space presented serious acoustic challenges.
Acoustics veteran, John Gardner was engaged to address these issues after experiencing a VIP performance tied to a blues festival. The goal was clear: improve the sound without disturbing the artwork or compromising the room’s aesthetic.
The Challenge: Excessive Reverberation and Harsh Reflections
Initial assessment and measurements revealed a highly reverberant and reflective environment:
- Reverberation times:
- ~3.5 seconds at 500 Hz
- Over 4 seconds at 1 kHz
- A pronounced “chatter” or flutter echo that degraded clarity
- Strong reflections from walls, mezzanine face, and windows
- Poor intelligibility for both speech and live music
Further analysis showed:
- Extended decay times in mid frequencies
- A rising frequency response:
- +12 dB from 63 Hz to 6.3 kHz
- High-frequency roll-off beginning near 8 kHz
- Noticeable slap-back echoes from rear wall surfaces
The Solution: Integrated, Art-Conscious Treatments
Given the requirement to preserve the room’s visual identity, all treatments were carefully selected and adapted to blend seamlessly into the environment.
Key treatments included:
- Mezzanine Face – Diffusion
- Installed a series of ArtDiffusor® Model F diffusors
- Arranged in a continuous matrix across the mezzanine face
- Positioned against existing molding for a clean, intentional look
- Purpose: break up reflections and reduce flutter echo without deadening the space
- Rear Wall – Absorption
- Installed Tone Tiles®
- Artist-painted to match the room while maintaining acoustic performance
- Purpose: reduce slap and high-frequency reflections
- Reflective Wall Treatment – Absorption + Aesthetic Matching
- Covered a large reflective wall with Sound Channels® wall fabric
- This material is acoustically absorptive, not transparent.
- Original paintings were reinstalled over the treated surface
- Artwork Enhancement – Distributed Absorption
- Added Sonora® panels (1-inch thick) behind existing canvas artwork
- Turned each piece into a functional absorber
- Created slight diaphragm damping effect due to the air gap behind canvases
- Maintained full visual integrity of the collection
- Window Treatment – Removable Absorption
- Installed custom-fit Sonora® panels within window frames
- Panels secured with minimal hardware and used only during performances
- Addressed reflections from large glass surfaces near the stage
Results: Balanced Acoustics Without Visual Compromise
Post-treatment measurements showed clear improvement:
- Reverberation reduced to:
- ~2.1 seconds at 500 Hz
- ~3.4 seconds at 1 kHz
- Reduced flutter echo and slap-back reflections
- More controlled and even frequency response
Performance Outcome: Proven in Practice
The ultimate validation came during the following year’s festival:
- The returning headline performer commented on how good the room sounded
- Performers were able to clearly hear themselves on stage
- Audience members and owners noted significantly improved clarity and warmth
Conclusion
The New Harmony project highlights how thoughtful acoustic design can coexist with architectural and artistic priorities. By using targeted solutions like ArtDiffusor® Model F Diffusors, Tone Tiles®, Sound Channels®, and Sonora® panels, John Gardner successfully transformed a challenging space into an acoustically balanced performance environment—without compromising its character.
Sabins, SAC, & NRC — a practical guide.
Posted by Acoustics First in Absorption, Articles, HOW TO on September 18, 2025
When optimizing a room’s acoustics, you’re often balancing how much sound is absorbed (loss) against how much bounces around (reverberation). Some common ways to describe absorption — sabins, SAC, and NRC — look different, but they’re closely related.
Sabins
A sabin is a direct measure of absorption: One sabin equals the sound-absorbing effect of one square foot of a perfectly absorbing surface (like an open window – sound goes out, but doesn’t come back.) In practice, manufacturers or labs will report a component’s equivalent absorption area in sabins at various frequencies. Sabins are additive: add the sabins of all items in a space to get the room’s total absorption for use in reverberation calculations.
SAC
Sound absorption coefficients (SAC) are used to simplify large square footage calculations. Each SAC itself is derived from the measured equivalent sabins of a test sample divided by the sample’s area. This allows you to multiply the square footage of a certain material by the SAC and it will tell you how many sabins it will absorb at a certain frequency. You may also see an average of all the SACs, or a subset of those values… a specific, often-used subset is the Noise Reduction Coefficient (NRC).
NRC and how it’s calculated
NRC (Noise Reduction Coefficient) is a number that represents a material’s average absorption performance at mid-to-high frequencies. It’s calculated by taking the arithmetic average of the material’s sound absorption coefficients (SACs) at 250 Hz, 500 Hz, 1000 Hz and 2000 Hz (per ASTM C423 or other standard test procedures). NRC is typically reported to the nearest 0.05 and runs from 0.00 (reflective) to 1.00 (very absorbent). Being an average, it isn’t the most accurate method, but it can give you a quick estimate which can be useful in the planning stages.
Practical Mathematic Relationship
- From measured data: SAC = measured sabins ÷ sample area.
- NRC is the average of SACs across four bands (250 Hz, 500 Hz, 1000 Hz and 2000 Hz).
- To convert NRC into a working absorption number for a planar surface:
sabins = NRC × area (ft²). - For discrete units (baffles, clouds): manufacturers often give sabins per unit, so total absorption is sabins per unit × number of units.
Why sabins for baffles and NRC for wall/ceiling panels?
Hanging devices like baffles are three-dimensional, exposed on multiple faces, and their effective absorption depends on orientation, spacing, and edge behavior. It’s more accurate and user-friendly to report their absorption as “# sabins per unit.” Flat-mounted wall or ceiling panels cover a known area and behave predictably per square foot, so SAC or an NRC (per ft²) is a convenient, normalized way to estimate absorption across a room.
Putting it into RT60 calculations
RT60 calculations depict the amount of time it takes for a sound to decay 60dB in a particular space with specific treatments. (60dB is roughly a 1000-fold reduction in sound pressure.) Reverberation-time formulas (like Sabine’s) use the room’s total absorption in sabins in the function. A basic average will use NRC × area for planar coverage and add sabins-per-unit for baffles. Sum everything up to get total sabins, then plug that into your RT calculation to estimate RT60.
If using feet your calculation is…
RT60 = 0.049 x Room Volume ÷ Total Sabins
If using metric your calculation is…
RT60 = 0.161 x Room Volume ÷ Total (Metric) Sabins
In summary:
NRC is an area-based average (for flat-coverage estimates); SAC is a sabins per square foot coefficient (for efficient absorption calculations using area); sabins per unit are direct, measured absorption values (better for discrete, hung, multi-faced items).
Throwback Thursday: Diffusion Vs. Absorption
Posted by Acoustics First in Absorption, Diffusion, Product Applications on May 22, 2025
On this Throwback Thursday, we look back at a popular Sound and Communications article about why sound diffusion is so much more complex a phenomenon than sound absorption. In this article, we discuss some of the different variables associated with the quantification of diffusion – magnitude, phase, frequency, direction, etc. When comparing its properties against absorption, which is simply the reduction of energy at specific frequencies, the argument is cemented as to why a single-parameter coefficient is insufficient to fully express the phenomenon of acoustic diffusion.
Read the article here:
PDF Version of Sound and Communications Article.
Revisit the original post here:
https://acousticsfirst.info/2020/03/24/acoustics-first-talks-diffusion-in-sound-communications/
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