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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!
Joint ASA/ASJ Meeting – 2025 Recap
Posted by Acoustics First in Press Release, Uncategorized on January 23, 2026
The joint meeting of the Acoustical Society of America (ASA) and the Acoustical Society of Japan (ASJ), held in Honolulu in December 2025, was a welcoming gathering for the international acoustics community. This joint congress happens only once every ten years, making it a rare opportunity for researchers, consultants, and students from both societies to come together, reconnect, and exchange ideas. Hosting the meeting in Hawaiʻi—literally and figuratively a bridge between the two countries—added to the collaborative and relaxed atmosphere.
Across the week, a wide range of ASA and ASJ technical committees organized sessions that showcased the diversity of acoustics research and practice. These included:
- Architectural Acoustics
- Noise
- Physical Acoustics
- Psychological and Physiological Acoustics
- Speech Communication
- Musical Acoustics
- Computational Acoustics
- Engineering Acoustics
- Education in Acoustics
- Structural Acoustics and Vibration
- Underwater Acoustics
- Signal Processing in Acoustics
- Biomedical Acoustics
- Animal Bioacoustics
- Acoustical Oceanography
Together, these sessions reflected the breadth of the field—from fundamental physics to human perception, from engineered systems to the natural environment.
One such session was organized by the Architectural Acoustics Technical Committee and focused on the Advances in Absorption Measurement and Design. Presentations explored how these materials are evaluated in the lab, how they are specified and integrated into buildings, and how they ultimately shape the acoustic experience of everyday spaces. This session was co-chaired by Jim DeGrandis (Acoustics First Corporation) & Daniel Robinson (Sublime) from the ASA, as well as Toshiki Hanyu (Nihon University) and Tetsuya Sakuma (The University of Tokyo) from the ASJ.
A highlight of the session was a three-part breakdown of a developing Japanese acoustics standard for general spaces—such as lobbies, sporting facilities, and multipurpose rooms. The proposed standard is being informed by the well-established German DIN 18041 room acoustics standard, with thoughtful adaptations for Japanese building practices and cultural expectations. The presentation sparked lively and friendly discussion, illustrating how international collaboration can help refine tools and standards that benefit practitioners worldwide.
Overall, the session captured the spirit of the once-a-decade ASA–ASJ meeting: technically rigorous, openly collaborative, and genuinely enjoyable.
You say “Diffuser,” I say “Diffusor”
Posted by Acoustics First in Diffusion, Q&A, Uncategorized on November 11, 2025
If you’ve spent any time around acoustic treatment—especially sound diffusion—you’ve probably noticed something odd: sometimes the product is called a “diffuser“, and other times it’s a “diffusor“. For newcomers, this can feel like a secret code or a subtle technical distinction – But the truth is much simpler.
Many trace the dual spelling back to Manfred Schroeder, the German physicist who developed the mathematically designed Quadratic Residue Diffusor (QRD).
In German, the word is spelled “Diffusor.” When Schroeder’s work entered the academic world, the spelling likely came with it.
Because his research became foundational in architectural acoustics, the German spelling spread through physics papers, textbooks, and graduate-level acoustics programs. Over time, “diffusor” became a common spelling when discussing mathematical or Schroeder-style diffusors specifically.
As manufacturers began producing these mathematically derived designs—like the ArtDiffusor® line from Acoustics First® (and many other early products)—they retained the “diffusor” spelling as a nod to the academic and scientific origins.
Before long, the industry ended up with two spellings that referred to the same thing:
- Diffuser – the standard English spelling
- Diffusor – the academically inherited, German-influenced spelling tied to Schroeder’s work
Both spellings appear throughout the professional audio world, and both are correct.
Is There Any Practical Difference?
No. None. Zero.
There is no technical difference between a “diffuser” and a “diffusor.” They both refer to devices used to redistribute sound energy and improve the acoustic quality of a space through accelerating the development of sound field diffusion. The spelling variation is purely linguistic.
Think of it like “colour” vs. “color” or “flavour” vs. “flavor.” British English keeps the “u,” American English drops it. (However, if you ask a Brit, they’ll tell you Americans are obviously spelling it wrong.)
The “diffusor/diffuser” split works the same way—just with a German twist.
So Which Should You Use?
Use whichever feels natural or matches the context you’re writing in. Many engineers and academics use “diffusor” when referring to Schroeder-type or other mathematical designs, simply out of tradition. Others stick with the standard English “diffuser.”
Tomato. Tom-ah-to.
The Spellbinding World of Owl Acoustics…
Posted by Acoustics First in Articles, Uncategorized on October 16, 2025
Today we thought we’d take a break from the usual Acoustics First blog topics and talk about owls and the fascinating way in which they experience acoustics. Owls possess some of the sharpest hearing in the animal kingdom. But what makes their hearing so exceptional and how does it differ from our own?
The secret lies in the unique structure of their faces and ears. Owls have flat, circular faces which gives them an incredible ability. Their facial discs, a set of specialized feathers arranged in a ring around their face, act like a natural sound collector, similar to a satellite dish picking up signals. These feathers are flexible, allowing them to adjust their position for optimal sound gathering. The sound they collect is then funneled into their ear openings located on the sides of their head. Imagine if we could move our ears to “zero in” on a particular sound!
Now, you might be wondering about those “ears” that stick up from an owl’s head. These aren’t actually ears at all! Those feathers are called plumicorns, and while they help with camouflage and communication between owls, they don’t contribute to hearing. The true ear openings are located on the sides of the owl’s head, much like humans. These openings are protected by a layer of feathers, and in some species, they even have movable flaps that can cover the ears. These flaps don’t interfere with hearing; they help reduce the sound of air turbulence when the owl is in flight.
What makes an owl’s hearing even more extraordinary is the position of its ear-holes. Unlike most animals, owl ear openings are asymmetrical, meaning one ear sits higher than the other. This unique design allows them to pinpoint sounds not only left or right, but also above or below. Thanks to this setup, owls can triangulate the source of a sound with incredible precision—sometimes within millimeters! This ability allows them to swoop down and catch prey they’ve never seen. The degree of asymmetry varies among owl species—some, like the Northern Saw-Whet, have a noticeable difference in ear placement, while others have more subtle variations. Either way, it’s an impressive adaptation!
Humans, like many other animals, have symmetrical ear-holes, making it more difficult for us to pinpoint whether a sound is coming from above, below or directly in front of us. This is why central clusters of speakers installed above a lectern effectively make the sound feel like it’s coming directly from the orator, not from the ceiling speakers.
Owls also have a “sound-location memory” that further enhances their hearing. When they hear a sound, their brains create a mental map of its location relative to the owl’s position. Special cells in their brain help process sounds from different directions, allowing them to track and locate the sound later.
Finally, like dogs, owls have a broader range of hearing than humans, and they can detect finer details within sounds. According to researchers, owls can hear sounds much faster than we can. While humans process sounds in increments of about 50 milliseconds, birds can discern sounds as short as 5ms. This means that where humans might hear a single note, owls may hear up to 10 distinct notes. Their auditory skills are truly out of this world—and it makes you wonder what we might be missing in our own world of sound!
Don’t Knock the Knock: Acoustics and the Pursuit of the Perfect Watermelon
Posted by Acoustics First in HOW TO, Uncategorized on July 3, 2025
After being outside on a hot summer’s day, nothing quite hits the spot like a slice of a cool, crisp, sweet watermelon. Unfortunately, not every watermelon is the same, and everyone has different methods for selecting the best ones.
There are number of visual clues that one can rely on to help identify a good watermelon. A large, yellow field spot on the bottom indicates that the watermelon was on the vine longer and is probably sweeter. Also, the coloring of a “ripe” melon will have strong, consistent stripe pattern; dull dark green stripes alternating with light yellow/pale stripes.
However, even when I followed these visual tells, I would often cut into my purchase only to be dismayed by a Styrofoam-like, flavorless inside or an over-ripened, mushy mess.
This melon melancholy haunted me until a few years back when I saw a middle-aged woman kneeling on the concrete floor of the supermarket with 5 watermelons circling her. I watched as she bent over and carefully knocked on each one, listening and nodding her head like she was holding a séance with the “other side” of the produce aisle. She repeated this process, rearranging the melons in front of her, until she picked up “the one” and put it in her cart, returning the other watermelons to the display bin.
I greeted the woman, trying not to startle her, and admitted that I often struggled to pick out watermelons. Clearly, she knew what she was doing and I was curious if she might share her system with me.
She kindly told me that she was listening for a “hollow” sound, that was full, but not too deep in pitch. I told her that I had heard that this “knock” test is a good way to judge the water content, but I never had much luck. She said that people will make the mistake of holding onto the melon when knocking, which quickly dampens the sound, so you can’t hear much of a tonal difference between melons (like palming the string of a guitar will change its sound to a shorter, more percussive, note).
Her strategy is to pick out 4-6 similar sized melons that have a large, yellowish sugar spot and strong striping, then sets each down so the only point of contact is with the hard floor. Without external dampeners, the melon can really “sing” when knocked, telling her how far along the fruit is. Too deep of a sound and the fruit is over-ripe/mushy, too high-pitch (or not hollow at all) and the watermelon is not ripe enough. She organizes the melons from lowest to highest in pitch and she simply selects from the middle melons to find one or two that are “just right”.
She said that this ritual, as ridiculous as it might appear to other shoppers, is the best way to guarantee a good watermelon and it’s worked for me ever since; so, don’t knock the knock!
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