RealTraps - Measuring Absorption

And The Numbers Game


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The tests shown below were performed in IBM's Hudson Valley Acoustics Lab in Poughkeepsie, NY on April 19, 2004. Note the large, motorized boom stand that supports the test microphone. Each product is tested 100 times - automated, of course! - while the microphone moves around the room in all three planes. Then the test results from all the microphone positions are averaged together so no one location in the room dominates the results.

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Testing MiniTraps in the room corners. Click the image for a larger version.


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Testing LE MiniTraps. Click the image for a larger version.


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Testing corner foam in corners. Click the image for a larger version.







"Who wouldn't prefer a product that offers 6.0 sabins of absorption compared to one having an absorption coefficient of only 0.32?"











"A single test takes about 30 minutes to run because one hundred separate noise bursts are played."










"It is rare to see absorption specifications for frequencies below 100 Hz."

By Ethan Winer

NOTE:This article explains how absorber products are tested in layman's terms. For a more technical explanation see the newer article Alternative Test Methods from Sound & Vibration magazine.

The standard way to specify the effectiveness of absorbent materials is with an absorption coefficient. This number ranges from zero (no absorption) to 1.0 where 100 percent of the sound is absorbed. Since all materials absorb more at some frequencies than at others, the absorption coefficient values are also accompanied by a frequency. This frequency is simply an average of all frequencies within the stated octave or third-octave band. Material that has an absorption coefficient of 0.5 at a given frequency absorbs half of the sound and either reflects or passes the rest.

Some vendors specify absorption using sabins instead of absorption coefficient, perhaps because it obscures the results and gives a larger number that is more impressive looking! After all, who wouldn't prefer a product that offers 6.0 sabins of absorption compared to one having an absorption coefficient of only 0.32? However, specifying absorption in sabins is sometimes justified when testing non-standard materials or when using unconventional mounting where the standard test methods do not apply. You can convert sabins to an equivalent absorption coefficient by simply dividing the sabins by the square feet of front surface area. Top

Besides giving the absorption values at different frequencies, many product specifications also include the Noise Reduction Coefficient, or NRC. This is an average of just the midrange "speech frequency" bands (250 Hz through 2.0 KHz) and is not particularly useful when comparing materials for music purposes. For example, one material may absorb mainly low frequencies while another works best at higher frequencies, yet both can have similar NRC values. So for music and audio purposes the NRC value is mostly irrelevant.

Absorption is typically measured in a special room (see the photos at left) that is very reverberant at all frequencies. The reverb decay time is measured at each frequency of interest with the room empty, and then again with the test material present. By comparing the difference in reverb times with the room empty and with the test material in place, the amount of absorption can be computed. Some minimum amount of material is required for testing so that the difference in decay times is large enough to ensure accurate measurements. In the tests I've observed at IBM's acoustic labs, at least 64 square feet of material is required in order for the test results to be certified. Top

The standard way to measure reverb time is to play an impulse sound, such as a burst of pink noise through loudspeakers, then measure how long it takes for the sound to fall by 60 dB. This type of test is called RT60, where RT stands for Reverb Time and 60 indicates the time it takes to fade by that many decibels. But it's difficult to measure levels that low because of ambient noise in the measuring room. So more often these tests measure the time it takes the reverb to decay by 30 or even 15 dB, and the time it would have taken to fall the full 60 dB is calculated from that.

At IBM's lab broadband pink noise bursts are played through loudspeakers, and a high quality microphone records each burst and its decay. A single test takes about 30 minutes to run because one hundred separate noise bursts are played. The results of all the tests are separated into 1/3 octave bands and then averaged. While the noise is playing the microphone that records the sound is constantly moving around the room. Instead of just placing a mike in one location, a special motorized boom stand rotates slowly in all three dimensions. That is, it swings around the room in a circle and also goes up and down from a few feet off the floor to seven or eight feet high. This way the reverberation at many places in the room is averaged into the measurements. Top

Another type of absorption test places the material being tested in a device called an impedance tube. This is a long narrow sealed chamber made of concrete or brick in which standing waves along the length of the tube are measured with and without the test material present. When using either test method, the ambient temperature and humidity must be constant and known precisely, as these affect the absorption of air and thus must be factored into the measurements.

US labs that perform acoustic tests are certified by NVLAP, a department of the US Government's National Institute of Standards and Technology. Testing of acoustic materials is defined by ASTM, a US organization that establishes standards and practices used by acousticians and their companies. By ensuring that its members follow exactly the same rules and guidelines, materials tested to ASTM standards in different facilities can be compared with confidence. Top

Most absorption measurements are taken with the test material mounted directly to a wall. But since spacing absorbent material away from a wall improves its low frequency performance, absorption figures that include spacing are often included in published specifications. In this case the specs indicate the type of mounting and also the spacing. The "A" mounting method means the material is flat against a wall, and E-### means the material was spaced, where the number (###) indicates the size of the air gap in millimeters. E-400 is common, which is about 16 inches. When "E" mounting measurements are made according to ASTM standards, a reflective skirt is applied around the edges of the material that extends to the mounting surface. This prevents reflections from bouncing off the surface at an angle and entering the material from behind, which could wrongly increase the absorption measurements.

Note that measurements for RealTraps products are modeled after the ASTM type "J" standard, which allows for other mounting methods as long as they emulate how the products will be actually used and the mounting details are described. For these tests the MiniTraps and MicroTraps were measured when spaced four inches away from the test room surfaces, and without applying a skirt. MiniTraps and MicroTraps are also measured when mounted straddling a corner, and this method does not follow any applicable ASTM standard. However, the corner foam test results on the Product Data page are for corner foam mounted in corners, and the foam was placed identically to the MiniTraps that were tested during the same session. This ensures a valid "apples-to-apples" comparison, regardless of whether the corner mounting follows official standards or not. Top

You may notice that absorption coefficients sometimes have a value greater than 1.0. Although it is impossible for any material to absorb more than 100 percent of the sound, measurements can yield a value greater than 1.0. The main reason this occurs is that all material has a finite thickness, and the edges - which are not included in the stated surface area - absorb some of the sound. So for an acoustic panel two by four feet and four inches thick, the real surface area includes the four-inch thick edge around the material. If included in the measurements, this would add four square feet to the stated surface area of eight square feet - a 50 percent increase! (See The Numbers Game below for a more detailed explanation.) Even when the edges are included in the total surface area, values slightly greater than 1.0 are still possible due to diffraction effects at the material's edges and corners. When the edges are rounded, this effect is reduced.

As you might imagine, the fee to use a lab that performs certified acoustical testing is high because it's very expensive to build a reverberation test room. Such a room must have a very low ambient noise level, which requires isolated structures, special sound proof doors, and a low air flow ventilation system. Building a room large enough for testing very low frequencies is even more expensive. For this reason it is rare to see absorption data for frequencies below 100 Hz - it simply costs too much to build a room that can measure absorption accurately below 100 Hz. Further, most industrial manufacturers - the main customers of testing labs - do not need measurements at very low frequencies. However, even when a room is certified down to only 100 Hz, it is still possible to assess relative absorption. That is, you can test different materials at, say, 50 Hz and see which are more absorbent even if the absolute measurements are not guaranteed accurate. Top

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Figure 1: This panel is 2 by 4 feet and 4 inches thick. During testing the four edges add 50 percent to the total surface area, yet they're excluded from the absorption calculations. And when many panels are mounted adjacent on a wall, the edges are not absorbing even though they contributed to the published specs. Click the image for a larger version.


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Figure 2: Foam blocks like this are meant to be mounted in a corner, stacked one above the other from floor to ceiling. When measured for absorption four of the five surfaces are exposed, but when installed as intended only the front surface absorbs. So in practice, a two-foot corner wedge like this provides only 65 percent of the absorption claimed. The shorter the wedge, the larger the disparity between the published and actual absorption. Click the image for a larger version.




To understand why MiniTraps outperform other acoustic absorbers it helps to know how such devices are measured at a testing lab. As you read in the previous section, acoustic products are commonly specified by their absorption coefficient and this number ranges from zero (no absorption) to 1.0 which means 100 percent of the sound is absorbed. For example, an absorption coefficient of 0.5 means that half the sound is absorbed and the other half either passes through the material or is reflected. Since no material absorbs all frequencies by the same amount, absorption coefficients are usually given for different frequency ranges.

Although 1.0 is the largest legitimate value possible, you may have seen higher numbers claimed for some products. Needless to say, this causes confusion, and makes it difficult to compare published data. Once you understanding how absorption is measured, and how the data can be manipulated - both fairly and unfairly - you'll be able to assess room treatment products and materials more wisely. Top

Acoustic absorbers are tested using methods defined by the ASTM, a US organization that establishes standards and practices used by acousticians and testing labs. By requiring its members to follow the same rules, materials tested to ASTM standards in different facilities can be compared with confidence. However, a flaw in the test method does not take into account the edges of the material.

Although the edges are exposed when the material is tested, the calculation for absorption coefficient considers only the size of the front surface, and ignores the edges completely. For a panel that is two by four feet and four inches thick, the edges add 50 percent to the absorbing surface during testing, yet they are ignored in the coefficient calculation. This is further complicated because there is no standard sample size. Since a small sample has proportionally more edge than a large one, a sample that's 8 by 8 feet will measure better than one that's 10 by 12 feet, even if they're the same thickness and made of the exact same material.

In practice, multiple panels are placed adjacent to each other during testing, to minimize the contribution of the edges. So when 2 by 4 foot panels are tested, typically eight of them are arranged into a larger square. But even when placed to form a single surface area of 8 by 8 feet, four-inch thick edges still inflate the measurements by more than 16 percent. Top

Further, most acoustic panels are meant to be installed adjacent on the wall in a cluster. In this case the edges are not available to absorb even though they were when the material was tested. When an entire wall is covered with four-inch thick panels none of the edges are exposed, so the real absorption is only 2/3 what the published numbers indicate - and those numbers were already inflated!

The same thing occurs with corner absorbers, as shown in Figure 2 at left. Unless the vendor describes how these triangle shaped samples were grouped during testing, there is no way to determine how much of the stated absorption is due to the edge effect and how much is due to its effectiveness as an absorber.

For some products, like a tube trap, it is not practical to specify an absorption coefficient because there is no front surface. In that case the correct way to specify absorption is in sabins, named for acoustics pioneer W.C. Sabine (1868-1919). The sabin is an absolute measure of absorption, independent of surface area, and it can be used to compare any two absorbing devices directly and on equal terms. Top

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