Posts Tagged 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!
Eat with your Eyes (and Ears): Acoustic Treatment for Restaurants
Posted by Acoustics First in Absorption, Products, Restaurants on March 21, 2025
When evaluating a restaurant, guests will often look at four factors: food quality, service, price point and atmosphere. The first three are fairly obvious in terms of how they influence the customers satisfaction, but the link between atmosphere and guest satisfaction is a bit murky.
Atmosphere is a sort of “catch-all” term for the various room and design elements that contribute to the overall experience of the patrons. “Atmosphere” is usually associated with visual elements, like lighting, table setting and decorations, but “atmosphere” can literally refer to the air in the room (is the restaurant properly ventilated, are there distracting smells from the kitchen?) or more functional/operating elements like the layout of the tables and, our focus in this article, sound management.
Sound Management – How many times have you been to a busy restaurant that is so loud you can’t hold a conversation with those at your table? It’s difficult to understand speech when background noise and reflections from other sources cover up new information. This causes patrons to elevate their voices to be heard, further exacerbating noise level issues.
Studies show that patrons spend more time and money at restaurants that properly address sound management, ensuring their guest don’t feel overwhelmed at peak hours. Key considerations include:
- Background music: Choose music that complements your restaurant’s concept. Adjust the playlist and volume based on the time of day and desired energy level—soft jazz for a relaxed dinner or upbeat tracks for a lively lunch.
- Comfortable conversation levels: Keep music and ambient noise at a volume that allows guests to speak without straining to hear each other. The right balance creates a welcoming buzz without becoming disruptive.
- Consider layout impacts: Open kitchens, high ceilings, or closely packed tables can amplify noise. Think about incorporating design features like partitions and high booths to help “break up” sound that is traveling from table to table.
- Acoustic design elements: Sound will build up most in spaces that have a lot of hard/reflective surfaces. Use sound-absorbing materials like acoustic panels to reduce reverberation/echoes and create a more intimate atmosphere.
Tone Tile Panels – In restaurants, it’s especially important to minimize visually obtrusive acoustic treatment so it does detract or conflict with the carefully constructed aesthetic of the dining room. Tone Tiles are a perfect solution for restaurants that require “invisible” sound absorption as they can be field or factory painted to match the wall or ceiling color precisely. They also have a white, lightly textured surface that resembles drywall (Tone Tiles can be used as projector screens). Keep in mind, the more you paint an acoustic panel, the less sound reaches the absorptive substrate. We recommend light passes with water based paint to ensure the surface of the panel remains as acoustically “transparent” as possible
Silent Picture Panels – Another popular treatment option is our Silent Picture panels. Silent Picture panels can be wrapped with customer supplied artwork, images or branding. Restaurants love the double utility of full-color images and premium sound absorption.
Can’t I just put foam under the tables? We do not typically recommend acoustic material underneath restaurant tables. Treating the underside of tables will only “take the edge” off overall sound buildup, primarily attenuating sounds produced below the table (shuffling feet, chair slides, etc.). Sound absorptive treatment is much more effective when in the direct “line of sight” of primary sound sources. Also, installing acoustic foam or felt under tables, where it is likely to be picked at or possibly soiled, present durability and sanitary concerns.
We’ve all heard the expression that we “eat with our eyes”; if the meal is not appealing to look at… it is less pleasing to eat. However, we also “eat with our ears”; if the environment is not conducive to comfortable conversation, then the customer will leave with a bad taste in their mouth, even if the food quality and service is exceptional.
Acoustic Treatment in the 1920’s – A look at the Guardian Building
Posted by Acoustics First in Absorption, Offices, Uncategorized on January 14, 2025
The bank wanted the building be a “show-piece” and communicate its principles of “security” and “fidelity” (remember, this was at a time before the FDIC), to impress customers and convince them to store their valuables at their bank. Incredibly, the building was completed in just one year; construction began in 1928 and finished just before the Stock Market Crash of 1929 (so much for fidelity).
Head designer Wirt C. Rowland had far from a subdued vision for the building. Blending Native American, Aztec, and Arts & Crafts designs, Rowland wanted to make an indelible impression on anyone who walked in. He said “We no longer live in a leisurely age…the impression must be immediate, strong and complete. Color has this vital power.”
Colorful, luxury materials grace every surface of this building. Italian Travertine marble was used for steps and wall surfaces, contrasting with deep-red Numidian marble imported from Africa. Brilliantly colored tiles fill the lobby’s vaulted ceiling, and a massive multi-colored mosaic adorns the vestibule wall. Monel metal was used in the large ornamental gate dividing the banking hall and main lobby, supporting a pair of Tiffany & Co. Glass clocks. Even the office corridors and restrooms are lined in a Tavernelle marble from Tennessee.
Obviously, these beautiful materials are also extremely sound reflective. Having worked on similar buildings, Rowland understood the need for acoustic treatment in the banking hall as there would be hundreds of customers, tellers and their managers trying to conduct important business in this large, cathedral-like space. If they used the same ceramic tiles they used on the ceiling of the lobby, conversations would be drowned out by a cacophony of typewriters. In lieu of the tiles, the banking hall has an incredibly appointed, intricate system of stretched canvas over wood frames backed with sound-absorbing horse hair. The canvas was hand painted with real gold and silver and requires regular maintenance. In fact, the same Italian family that made the ceiling nearly 100 years ago has been caring for it ever since!
I recently had the pleasure of touring the Guardian building, and walking through the Monel gate from the lobby to the banking hall, you can hear the difference. Though the banking hall is much larger, it feels much more intimate and comfortable, in large part because of the ceiling. Although the horse hair and canvas materials may not meet fire code today, modern stretched-fabric acoustic assemblies owe a lot to this sort of early innovation.
The Guardian Building is a symbol of creativity and achievement. Designed for the future, it is no surprise that Rowland’s masterpiece still dazzles and inspires visitors to this day.
For more information on the Guardian Building’s long history, visit https://www.guardianbuilding.com/history
Similar, yet different: HiPer Panel® vs. HiPer Panel® Impact
Posted by Acoustics First in Absorption, Diffusion, Product Applications, Products on December 11, 2024
While the HiPer Panel® and the HiPer Panel® Impact may appear to be identical on the surface, there are some key differences that may change which one you would use, and why you would use it. They are both layered, flat-panel diffuser products, with perforations, and they are both covered in fabric. However, their construction, below the surface, is drastically different. One is a broadband absorber with a modified frequency response which focuses on reduction of specular energy, and cancellation of noise – where the other is a high frequency diffuser and reflector with a tuned bass absorption which is constructed to maintain acoustic energy in the space.
Construction
The HiPer Panel® was originally designed to optimize the capabilities of a standard broadband absorber. Its internal membrane and perforations create a material that works to modify the range of absorption, and create high frequency diffraction… but that isn’t all. The cavities are backed up to the membrane, which changes the reflection characteristics, where high frequencies can be reflected, and higher energy waves are absorbed more than if it was just fiberglass. This extended range is random, as the perforation density is gaussian in nature, but the membrane is also randomly backed by more cavities.
This design creates 4 different physical conditions that acoustic energy has to contend with… in a gaussian distribution.
- areas of the panel with 2 layers of fiberglass and a membrane in the middle.
- one layer of fiberglass with a rear membrane over a cavity.
- a cavity with a membrane back… sitting on fiberglass.
- a cavity with a membrane back… stretched over another cavity.
The random distribution of multiple acoustic obstacles is what gives this device its unique characteristics. It’s an absorber that changes its performance depending on where sound hits it, and at which frequency. Some frequencies pass into the cavities and reflect off the membrane, while others are dampened by the membrane… while longer wavelengths see the membrane as a stretched diaphragm or limp mass.
The HiPer Panel® Impact has a very different construction and may be used for a very different reason. The HiPer Panel® Impact uses the same pattern of holes, but the holes aren’t cut into an absorber… they are cut out of a reflective face, which is attached to an absober. Unlike the first HiPer Panel®, the “Impact” can be used to maintain more of the energy in the space, break up some of the higher frequencies with that gaussian hole pattern, and be a low frequency bass trap. The design is simple and effective, but is not necessarily used in the same places where you would use the first HiPer Panel®.
Use cases.
The first Hiper Panel® is often used in theaters, and listening spaces where focusing on the source is of primary importance. Its broadband absorption, gentle high frequency diffusion, and smooth mid frequency control are ideal for critical listening environments such as mixing rooms, media rooms, theaters, or even voice over spaces. The performance is about removing the acoustic elements that could interfere with the focus on the source speakers.
The HiPer Panel® Impact is often used in performance spaces, where you want to maintain energy, break up high frequency flutter, and remove low bass. The reflective face doesn’t remove as much energy from the space, however it does change the characteristics of the space. This helps break up some frequencies, reduce bass, and keep the energy moving around the room. Music halls, churches, auditoriums, and any space that relies on the room helping to reinforce the sound will benefit from these taking the edge off the highs and dampening the lows – which is how the HiPer Panel® Impact controls the sound… while helping it maintain its “impact.”
In summary, while these two products are in the same family, they have a different core construction, which changes their performance. There are scenarios where you may use them both, however since they address different problems in a space, they are not always interchangeable. Contact Acoustics First® if you have questions about any of our products.
Acoustical Considerations for Classrooms
Posted by Acoustics First in Absorption, Articles, Classrooms, Product Applications, Products, School & Educational Facilities, Teaching Rooms on September 19, 2024
Poor classroom acoustics has long been the invisible problem that has the farthest reaching implications for learning. Excessive noise and reverberation degrade speech intelligibility, resulting in reduced understanding and therefore reduced learning. In many classrooms in the United States, the speech intelligibility rating is 75% or less. That means, in speech intelligibility tests, listeners with normal hearing can only understand 75% of the words read from a list. Imagine reading a textbook with every fourth word missing. Wouldn’t that make comprehension near impossible? Fortunately, poor classroom acoustics can usually be remedied with some basic knowledge and commercially available treatment. But before getting into specific treatment, let’s go over some basic acoustic principles.
Noise
Obviously, it’s difficult to understand what the instructor is saying when there is a lot of naturally occurring noise in the room. A glut of factors can be considered noise sources, including HVAC “rumble”, traffic outside the building and students moving in their chairs. These sources contribute to a “noise floor” that makes understanding speech very difficult. Since there is no one “cure-all” for an excessive noise floor, it is often best to seek the assistance of a professional acoustical consultant to properly diagnose and find a solution to these issues.
Reverberation: Undesirable vs Useful Reflections
When not attributed to a noise issue, the culprit of poor classroom acoustics is often excessive reverberation. In simple terms, reverberation is the sound energy that remains in the listening environment as a result of lingering reflections. As mentioned before, these reflections can easily interfere with speech intelligibility. As you may have experienced at some point, it can be difficult to understand what is being said when reflections from old information cover up what is newly spoken.
The reverberation time (RT or RT60) is used to determine how quickly sound decays. The RT is dependent upon the volume and surface materials of a given room. Large spaces with hard materials (tile, drywall, etc.) have longer reverberation times, while small rooms built with “softer” materials sound more “dead”. Ideally, classrooms should have relatively short RT’s, somewhere in the .6-.8 second range.
A long reverberation time is not the only factor that should be considered when treating a classroom with poor acoustics. Flutter echo is a particularly significant problem when it occurs between the side walls at the front of the classroom where the teacher is speaking. This condition can be heard as a “ringing” sound (when one claps) as the sound rapidly bounces back and forth between two parallel walls. Flutter and other discrete echoes are considered “undesirable reflections” and should be controlled with absorptive or diffusive materials.
Not all reflections are bad though. There are “useful reflections” that reinforce spoken word, rather than cover it up. The teacher’s voice can be propagated throughout the room by shaping a sound reflecting gypsum board ceiling over the front of the room or by making the center of the ceiling a hard, reflecting surface (see figure 1). This will help project the speaker, so they don’t have to strain their voice to be heard over the students.
Reducing Reverberation
Often reducing the dimensions of a classroom to attain a more suitable reverberation time is not feasible, but one can improve the acoustics by introducing sound absorptive materials. Typical classrooms usually have a dropped “acoustical” ceiling that has some absorptive qualities. In classrooms that don’t have this ceiling, reverberation can be reduced by installing an acoustical ceiling or a number of fabric faced fiberglass panels, like Sonora® Ceiling Clouds. Likewise, if there isn’t carpeting in the room, you can marginally reduce the reverberation time by installing sound absorptive flooring.
Wall treatment: Acoustic Panels
If the ceiling and floor are at least rudimentarily treated, then hard walls are usually at fault for poor speech intelligibility. Absorptive wall panels, like Acoustics First Sonora® panels, are a common treatment to control lateral reflections and reverberation.
These panels are popular because they can be customized with a variety of colors, edge designs and fabric facings. They also can come with a high-density fiberglass adder that improves durability. In classrooms, these “Hi-impact” panels are particularly useful because the adder allows for the panels to be used as tack boards. This brings an extra level of functionality to the panels outside of their absorptive properties.
Though wall panels are a perfectly suitable treatment, uncovered areas between the panels can sometimes allow a few hard reflections and/or flutter echo to still occur (although full treatment of the walls would likely result in a room sounding too “dead”). For these situations, Acoustics First often recommends Sound Channels® acoustic wall fabric.
Acoustical Wall Fabric
In many instances, acoustic wall fabric is actually a viable alternative to traditional wall panels. Unlike a typical “wall carpet”, Sound Channels® is made of 100% recycled content and has ridges to increase surface area and absorption. Perhaps most importantly, the uniform coverage you get by treating the walls with acoustic wall fabric eliminates the flutter/slap from reflective parallel walls (without making the space too “dead”). Acoustic wall fabrics are generally light weight and most can be put up just like any other wallcovering.
Also of note are the additional benefits when using Sound Channels® in early education classrooms. The effective range that this wall fabric controls is the higher speech frequencies, which is the ideal range for classrooms with younger children (there are not many bass/baritone kindergarteners). Another advantage is in keeping the treatment clean. Wall panels may suck up sound, but they can also absorb fluids (like the occasional juice box). Sound Channels®, on the other hand, is resistant to moisture, mildew and rot. It is also is non-allergenic, easy to clean, and is highly resilient to common wear.
Acoustical Considerations for Classrooms
Although this knowledge has been around for decades, classrooms across the country continue to be plagued by a lack of acoustical forethought. Perhaps as this information becomes more readily available to architects, contractors, administrators and teachers we will begin to see (and hear) better sounding classrooms. School is challenging enough on students and teachers as it is, let’s not compound their daily obstacles by continuing to overlook classroom acoustics.
(Originally published in Christian School Products Magazine – November, 2015)
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