Eric,
No worries, bro. I have a pet interest in this topic.
Of course, I could provide another - perhaps a fresher? - perspective on THE NUMBERS GAME:
From http://www.ethanwiner.com/acoustics.html and http://www.realtraps.com/art_measure.htm
THE NUMBERS GAME
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.
Theoretically, this is true. But in the contexts of acoustical treatments and the "commonly" used ASTM C423 test, the absorption coefficient can never be 0.00 and can exceed 1.00...which does
not mean 100% 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.
For normal incidence absorption coefficients, this would certainly be the case. But, again, in the contexts of acoustical treatments - almost always tested for random incidence absorption - it is not the case.
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.
While there is potential for confusion, numbers greater than 1.00 do not make comparisons difficult. All other factors being equal, an absorber with a 1.10 absorption coefficient at 500 Hz is better than an absorber with a 1.00 absorption coefficient at 500 Hz. This is not difficult. And the values larger than 1.00 are completely legitimate.
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.
Data manipulation is not part of the laboratory method. Of course, in the context of THE NUMBERS GAME, "manipulated fairly" would be an oxymoron.
Acoustic absorbers are tested using methods defined by the ASTM, an international organization that establishes standards and practices used by acousticians and testing labs.
The ASTM is a US-based organization. Absorbers in other countries will be tested in accordance with the standards those countries have adopted. Just so there's no confusion, the ASTM sets testing standards for many industries besides acoustics.
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.
Edges are discussed in ASTM C423, Section 9. To summarize, test specimens are to be mounted "in a way that simulates actual installation." A subsequent Section states that edges of materials intended for ceiling installation (i.e., a T-bar grid) should be sealed. A good lab usually asks a client whether the edges of the test specimen should be sealed. Since the edges of wall panels are typically exposed, this is not often done for foam and fibrous panels. Since, to the best of my knowledge, the edges are rarely - if ever - sealed for wall panel tests, it is usually safe to compare the results of absorption tests, provided all other factors are equal to one's satisfaction. If there are any doubts, the manufacturer should be contacted. Section 12 of the C423 test method requires a full test report, which must include a detailed description of the exact conditions of the test specimen in the lab.
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.
A minimum sample size is provided in Section 9. Again, a full test report will show and/or describe the size and layout of the test specimen. (Most labs recommend at 72 ft² sample size; smaller than 64 ft² is typically not recommended).
The test does "ignore" the area of the exposed edges. But, as mentioned above, most every absorber on the market reports absorption coefficients that omit the edge area. Thus, it is largely moot with respect to fair comparisons.
Since the subject at hand is edge effects and how they're related to absorption coefficients greater than 1.00, it might be worth discussing a product mentioned above: the MiniTrap. If memory serves, 8 MiniTraps were tested in the "A" configuration (specimen directly on test floor; no airspace). This yields 64 ft² of face area. Add the edges (of the perimeter - assumes panels are abutted in a square pattern) and you get about 74 ft² of area. The absorption for the array of MiniTraps at, say, 500 Hz is roughly 112 sabins (data all extrapolated from
here). So, if the standard method is used, the absorption coefficient at 500 Hz would be
112/64 = 1.75
If the additional area is included - which is
not in accordance with the standard method, btw - the absorption coefficient at 500 Hz would be
112/74 = 1.51
which is still greater than 1.00. Thus, it is once again shown that the added area of exposed edges cannot fully account for absorption coefficients greater than 1.00.
Even if all four edges of the panels were exposed (panels spread out), the absorption coefficient comes out to about 1.17. Which is still greater than 1.00.
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.
This final sentence seems to be insinuating that there is some unfairness going on. If virtually every absorption coefficient is calculated using the method in question, then all the numbers are equally inflated. So, again, the subject is largely moot. The method in question is not perfect, but it is the
standard method. If it is not followed, then the data are meaningless outside of their own context.
sidebar on RealTraps page; in main body text of ethanwiner.com page wrote:This panel (figure could not be linked - see above URLs for graphics) 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.
During the test, all four edges would not be exposed. The edges of the perimeter of the test specimen would be, and would be omitted from calculations. If the edge area were to be included, the results would no longer be in accordance with the standard method.
When many panels are mounted adjacent to each other on a wall, the configuration is closer to the test method and thus the published numbers (not "specs," btw) will be more applicable. The edges did not contribute to the published "specs." They were omitted from the calculation. Since most - virtually all - wall panels are tested this way, the point is largely moot.
Further, most acoustic panels are meant to be installed adjacent on the wall in a cluster.
This is a generalization. The needs of the space dictate the method of installation. How an acoustical panel is "meant to be installed" is largely up to the designer and installer (and owner...and spouse...). The manufacturer may have its own suggestions, but the needs of the space will dictate how
most acoustical panels get installed.
In this case the edges are not available to absorb even though they were when the material was tested.
Most likely, they were not "available" during testing. As mentioned in THE NUMBERS GAME (
), panels are typically placed edge-to-edge for the ASTM test in order to minimize edge effects.
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 area of the "entire wall" is not given, so the statement about the absorption being "2/3 what the published numbers indicate" cannot be verified. If the area of the entire wall is roughly equivalent to the entire exposed area of the test specimen, the absorption could be very close to 1x the published numbers...
Of course, these last few snippets seem to be hinting at the impact the test method has on the reliable use of absorption coefficients in predictive architectural acoustics calculations. Most acoustical professionals are well aware of the limitations of using lab test data in their models and will typically take those limitations into account when selecting acoustical treatments for a room design.
For readers of THE NUMBERS GAME, it is unlikely that there will be much of a need to calculate, e.g., reverberation time. This leads back to the point that the absorption coefficients are fine for comparisons, all factors being equal to one's satisfaction.
The same thing occurs with corner absorbers, as shown in the figure below. (Again, figure can be found in above links.) 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 the devices in question, total absorption, or absorption per unit, could provide the necessary information for comparison, but would require modification to the standard test methods to accommodate. Otherwise, comparisons of devices such as these using data obtained from the reverb room method are difficult. I have mentioned (opined) in other threads that manufacturers should follow the lead of RPG and perform low frequency specific measurements on their low frequency devices.
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.
While the intent of this paragraph is understandable, the "65 percent" comment has no basis. The devices would have to be retested with the edges in question sealed in order to determine what the actual difference in absorption would be.
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.
First, the unit is the
sabin (small "s"). As is standard with units derived from a person's name, the unit - spelled out - is not capitalized, but the abbreviation - in this case, "Sab" - is capitalized. (Sorry - pet peeve...
)
Second, the last sentence here would seem to contradict claims made elsewhere in THE NUMBERS GAME. If such comparisons can be made (and they
can be made), then this statement with some supporting instruction to the reader as to how to find information on the absolute absorption of a panel or product would suffice. One short paragraph on this worthwhile concept could replace a lot of the misleading content that preceded.
************
The above is for clarification purposes only - constructive criticism. The author of THE NUMBERS GAME is asked to kindly consider it as such.
All the best!
...........Please........... Not again.
