Posts Tagged astm

Acoustic Material Testing: What it is and what it isn’t.

A 64 ft2 sample of material to be tested on the floor of a reverb chamber. Notice how the pieces are grouped together.

Your room works as a system with many variables – some of which will affect the performance of your acoustic treatments. However, a laboratory test chamber for absorption is a reverb chamber – and they sound terrible! The test parameters have limits for placement of sources (speakers), microphones, and samples in the room to facilitate a more repeatable measurement. Incidentally, none of these parameters are made to use the material at its highest efficiency – quite the opposite. When you spread the materials around the space, they will perform differently than if you bunch them all together (which incidentally, is exactly how most are tested!) If you place materials at certain positions in a room, relative to the source and listening positions, you will optimize the performance of those materials in the space.

Still, if you are looking to buy some acoustic material (an absorber, sound diffuser, barrier, isolator, etc.), you will likely look into the tested performance of these materials. If you delve deeper into the different test numbers, like Sound Absorption Coefficients, Noise Reduction Coefficients (NRC), and Sound Transmission Class (STC), you will learn more about how they are measured and calculated. We had a previous blog (here) about mounting methods for testing, and how they simulate installations in different environments. However, in addition to different mounting methods – there are also different tests.

Different test standards with the same reported data?

Let’s say that you are looking for NRC or Sound Absorption numbers for a material – you may not know that there is both an ASTM C423 and an ISO 354 test that will give you this information. While they are comparable in many ways, there are a few variables that allow subtle differences in the measurement and calculation of these numbers. One major difference is that they allow for a different amount of material to be used in the test. The minimum test sample size for ASTM C423 is 60ft2, while the minimum in ISO 354 is 10m2 (or 107.5ft2.) The maximum amount of material is also different, with the ISO 354 maximum set to 12m2 (with an allowed increase of V/200 for rooms with a volume (V) > 200m3) and no maximum on C423.

While this variability may seem a little confusing, just remember that these tests can be run in different sized rooms with differences in setup and configuration. There are also variations in “when” the analyses begin – with the C423 starting 100-300ms after the signal is turned off and ISO 354 starting after a 5dB drop in level.

What does this mean to you?

It means that these tests don’t give you an absolutely-perfect, solid (or stable) number. The material performance values are an imperfect, but still useful, measure of performance. Even if you retest the same sample in the same room, you will likely see some variation in the results – This variation is used to calculate “Repeatability” of a test method. If you test the same material in different rooms, you are definitely going to get variation, and this variation is used to calculate “Reproducibility” of a test method. Counter-intuitively, this means that these different numbers are both accurate, even though they are not the same. From these different values, a test’s “Uncertainty” can be calculated – this is a way to show how reliable the test values are. (For information about reliability and uncertainty, read the article here.)

Other variables.

We know that the test results are not “pin-point” accurate values, however, they are a measure of material performance in an acoustic environment. The mounting methods will also contribute to real-world variations in performance. The E400 is a standard mounting method/test for ceiling treatments placed in a grid. This method tests the tiles or panels with a 400mm air gap behind them to simulate the plenum, or dead space, in the ceiling behind the tiles. 400mm is a very particular number equating to roughly 15.75 inches. What happens if your air gap is a different depth? Simple, you will get different results. The ISO 354 recommends 400mm for North America, 200mm for Europe, and 300mm for Japan – while the ASTM C423 test recommends 400mm, with other depths allowed as specified.

There are many potential variables here. For instance, “A” mount is mounted flat to a wall, while “D-5” mount is with a 5mm gap behind, and “J” mount is for free-hanging baffles. When comparing product performance, it is best to use results from the same test method with the mounting type closest to the real world installation method. While there may be variability in the results, you can adjust for the variation by learning about the uncertainty of a measurement.

So what exactly are these numbers?

The test results are a guide to help you select material for a space. This guide helps you to approximate how much of what type material you will need to address a problem. Acoustic problems will have a massive impact on the listening experience, and should therefore be minimized. However, you still need to pay attention to how and why you are using test numbers.

For example, if there is a 12 dB drop below 125 Hz at the listening position, you may have a node, or dead spot, that can be fixed with bass traps. Research products that are tested to work down below 125 Hz and calculate how much you need based on the absorption numbers. If, after you install the products, you have a 2 dB difference, even with all your calculations, you can chalk that up to the variability of the performance in different environments and the certainty of the test. So, you can just add a little more acoustic treatment or use an EQ to fine tune it.

Did you calculate wrong? Not exactly. The calculations may be correct, but if the uncertainty of a measurement is +/- 0.2 below 125Hz, that means that you may need to account for that in the calculation. (Many tests, and laboratory environments have a great deal of uncertainty below 125Hz.)

Sometimes, it’s better to figure high and have some extra pieces of material. How do you do that calculation? Well… if the uncertainty is +/- 0.2 @ 100 Hz and the test results say that the performance is a 0.8 @ 100 Hz – then re-run the calculation assuming that it does a 0.65 or 0.7 @ 100 Hz and you should get a little more square footage of material. That is still within the performance certainty of a measurement. You can say – with about 95% accuracy – that if this material was tested in a different lab, on a different day, under different conditions… it could have tested with those performance results as well.

So be informed!

Some quick points to remember…

  • Test results vary from lab-to-lab, test-to-test, and day-to-day – even with the same material.
  • Test results are not random, but vary within a range based on test reliability and certainty calculations.
  • When calculating, you can compensate for these variations (to some degree.)
  • When comparing products, account for the variability.  If two products are similar in build, materials, and performance – but one is slightly higher or lower – there may be no practical difference at all… and the variation could simply be due to the test uncertainty.

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Acoustics First® makes sound Visible!

AcousticsFirstAs many of you know, Acoustics First® invests a great deal of energy in the development of the science of acoustics.  Here are three ways that we are making advances that help people learn and actually Visualize acoustics!

Those interested in sound diffusers  have certainly noticed that Acoustics First® has produced a Diffuser Data book, containing all kinds of test data about how our diffusers contribute to the sound in your space.  This information is a great advancement and we have worked closely with the ASTM committee developing this test method. Some people wonder exactly how the test produces the data that we report, and we have developed a simulation to show exactly how the sound energy is sampled during this test.

If you download the Diffuser Data booklet, you can see a picture of the gigantic test arc at NWAA Labs which is used for the real world tests.

Acoustics First – Diffuser Data Test Demo from Acoustics First®.

We have also made leaps and bounds in using simulations to show the different ways that diffusion develops in a space.  Depending on the type and placement of the diffusers you install, the diffuse field will develop at different speeds, at different frequencies.  We can now show a couple of simulations of the development of a diffuse field to help you visualize how sound moves in room without treatment and with two different sets of diffuser treatments.

Acoustics First – Room Simulations from Acoustics First®.

Bonus video! Imagine being able to see the Untreated room and the Model D room from a different angle – To be able to move around the outside of the room and see how the sound field develops from a different perspective.  Imagine no more!  Here it is!

Acoustics First – Sound Field Development Simulation – 3D Panning from Acoustics First®.

If you prefer to use YouTube – you will find the videos uploaded at our YouTube channel here.

We hope these helped you to “look” at acoustics in a whole new way, and stay tuned – more advancements are coming soon!

Contact Acoustics First® for a high bitrate, presentation quality version of the videos – as well as permission and terms of use.

Bonus! Simulations!  Now in stereo – and with absorption! Click Here!

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Demystifying Acoustic Data: Part 2 – Test Material Mounting

For anyone new to the world of acoustics, there is a multitude of terms, coefficients and numbers that are thrown around.  This flood of information can seem intimidating, especially to beginners. In this series, acoustician Cameron Girard of Acoustics First® hopes to help you distinguish between what’s useful and what’s not.

Part 2: How Mounting in Testing Affects Sound Absorption Data

As I discussed in my previous article, the best way to compare the performance of sound absorbing panels is by referencing the Sound Absorption Coefficient (SAC) and Noise Reduction Coefficient (NRC). However, these coefficients are often used as marketing tools. Be on the lookout for companies that list absorption coefficients and NRCs without mention of a particular testing standard or mounting method. It’s vital to check for this information, as direct comparisons to competitors and other materials can only be made if their testing procedures are the same.

The sound absorption of a material that covers a flat surface not only depends on the physical qualities of the material but also on how the material is mounted during installation. The mountings specified in laboratory tests are intended to simulate conditions that exist in normal use, such as direct wall mounting and installation into a ceiling grid.

Many materials for treatment of walls or ceiling are tested using what is called Type ”A” mounting. Type ”A” mounting means the test specimen was placed directly on the test surface of the reverberation chamber. Lay-in ceiling tiles, on the other hand, are often tested using ”E400” mounting. The ”E” designates a sealed air space behind the specimen (simulating  the air gap between a dropped tile ceiling and the structural ceiling) and the number after the ”E” is the depth of the airspace in millimeters. The airspace behind the acoustic material affects the sound absorption by acting as a bass trap. The deeper the cavity behind the panels is, the lower the fundamental of the “trapped” frequencies will be.

To see what this look like in terms of actual numbers, let’s take a look at how different mounting methods effect the sound absorption coefficients of Acoustics First’s HiPer® Panel (a low-profile, composite absorber/diffuser panel).

Since the HiPer® Panel can be used effectively in multiple applications; we had it tested in accordance to the two most-common mounting procedures, Type E-400 and Type A. The results of the laboratory tests are as follows:

Product Info

Sound Absorption Coefficients

Product Name Thickness Mounting 125Hz 250Hz 500Hz 1kHz 2kHz 4kHz


1″ HiPer® Panel 1″ E-400 0.43 0.28 0.51 0.76 0.99 1.10 0.65
1″ HiPer® Panel 1″ A 0.09 0.28 0.78 0.75 0.94 0.85 0.70


As you can see from the chart, the sound absorption coefficient at 125 Hz varies greatly between E-400 mounting (SAC of .43) and Type-A mounting (SAC of .09). If mounting the HiPer® Panel in a ceiling grid, with a sizable airspace, you can expect significant low-frequency absorption, but mounting it on a wall (Type-A) will result in much less absorption at 125Hz.

Other mounting methods are available, but are not used as frequently. Here are some of the basic mounting designations (See ASTM E795 for more information.)

Type A mounting – Test specimen laid directly against the test surface (wall panel on drywall).

Type B mounting – Test specimen cemented directly against the test surface. Type B mounting is intended to simulate acoustical ceiling tiles or other sound-absorptive products adhered to a hard surface with an adhesive.

Type C Mounting—Test specimen comprising sound-absorptive material behind a perforated, expanded, open facing or other porous material.

Type D Mounting—Test specimen mounted on wood furring strips.

Type E Mounting—Test specimen mounted with an air space behind it (dropped tile ceiling).

As we’ve discussed, acoustical data can vary greatly depending on the mounting method used during testing. Acoustics First tries to include as much information about testing procedures as possible, because we feel an informed client makes the best client.

Contact Acoustics First for your all your sound control needs!

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