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Absorption & Diffusion – The Construction Specifier

For the May 2022 edition of “The Construction Specifier,” Acoustics First was asked to illustrate the use of absorption and diffusion in creating optimal acoustic spaces. The article is a great reference for understanding the types of acoustic absorbers and diffusers, as well as some use scenarios like offices, critical listening spaces, and larger communal spaces.

Note: This version has been edited and the advertisements are removed. The full published version of the May 2022 digital edition can be found on The Construction Specifier’s website here.

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“If a tree were to fall … and there was no one to hear it, would there be any sound?”

The philosophical thought experiment of “does something need to be perceived to exist,” has been around since the beginning of time.  This allows for human extrapolation into concepts such as quantum mechanics (Schrödinger’s cat) and advanced Artificial Intelligence principals.  Albert Einstein was effectively “unfriended” for asking the question of his colleague, Abraham Pais,

“Do you really believe that the moon only exists if you look at it?”

Albert Einstein to Abraham Pais

Pais prescribing to “the majority view of the quantum mechanics community then (and arguably to this day) that existence in the absence of an observer is at best a conjecture, a conclusion that can neither be proven nor disproven.”

But the question still exists. “If a tree falls in a forest and there is no one to hear it, does it make a sound?”

Fallen tree in forest in Haukkamäki district, Jyväskylä. The forest is situated between the streets Nuutinkorventie and Koppalankaari. – Tiia Monto (Tiia was not on site when the tree fell and was unable to verify that the tree made a sound.)

The eminently interesting, Dr. Irving Lirpa asked the burning question…

“If observation is proof, can we calculate the Amplification Coefficient of Human Perception upon something that is ‘likely’ into ‘truth’ versus the amplification of ‘hogwash’ – which will always remain ‘hogwash?’  Because if something is able to be perceived, it must exist in some degree, as amplifying the perception of the non-existent is akin to multiplying by zero.”

Dr. I. Lirpa further posited that: “while the observation of sound proves its existence, the lack of observation does not disprove it… it merely has not been amplified by the scrutiny of human perception.” 

He further affirmed that the tree would indeed create sound, but with a much lower intensity due to the Human Perception Amplification Coefficient… henceforth, there would still be sound because it exists – but it would fail to be amplified by human perception.

Seminal work on the Human Perception Amplification Coefficient by Dr. I. Lirpa.

This seminal work calculated a Maximum Reverb Time of only 0.04 @ 1000 Hz “Without Audience” in a full-leafed, deciduous biome common to the vernal mid-temperate zone. Further calculations found that the Human Perception Amplification Coefficient is equivalent to the reverb time being amplified by 42 TIMES per frequency band upon being observed – which coincides exactly with the calculation made by the supercomputer DEEP THOUGHT on the “Ultimate Question.

Coincidence? We think not.

TL:DR
“If a tree was to fall in the forest and there was no one there to hear it, would there be any sound?”
“Yes, but there would be 42 Times more sound if there was someone there to hear it.”

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Sound Diffusion vs. Absorption in Worship Environments

There are many examples of diffusing architectural elements in this synagogue.

Cathedrals, mosques, synagogues and temples are often decorated with an abundance of architectural details (deep coffers, arches, columns, sculptures, intricate engravings etc.). These features are not only beautiful to look at, but also serve the vital acoustic purpose of sound diffusion. Large, uninterrupted spans of hard, flat surfaces reflect sound in a singular, specular wave, which creates discrete echoes and comb filtering (In acoustics, comb filtering is when a delayed reflection interferes with, and distorts the original sound wave). These conditions can contribute to an acoustically uncomfortable environment, in which speech is difficult to understand, and music can be hard to perform and enjoy. Irregular surfaces, on the other hand, scatter these reflections, minimizing comb filtering and distracting echoes. In a “diffused” worship environment, speech is intelligible, music is clear and warm, and there is a sense of envelopment which greatly enhances the congregants’ worship experience.

Absorptive treatment can also be used to control echoes and harmful reflections. Instead of redistributing reflections, these “fluffy” materials (drapery, padded pews, acoustic panels etc.) reduce the overall sound energy from the reflections in the room. However, using to too much sound absorption in a room can often make a space sound ‘dry’ or ‘dead’. Determining whether your space needs absorption, diffusion, or a combination of both is dependent upon the acoustic properties of the space, as well as the type of worship service being conducted.

Acoustic Properties of the Worship Space: Reverberation, a principle acoustic factor, is the sound energy that remains in a listening environment as a result of lingering reflections. The dimensions, construction materials, and furnishings of a given worship space determine its reverberation time (RT or RT60). Large halls with reflective materials (glass, wood, concrete) have longer reverb times, while small rooms with absorptive materials (drop acoustic ceiling, carpet, curtains etc.) will have shorter reverb times. Incorporating sound absorptive materials, Such as fabric wrapped acoustic wall panels, is often the best way to reduce the overall reverberation in a room to a suitable level. However, the target reverb time also depends on the nature of the worship service being conducted in a particular space.

Type of Worship Service: Ideal reverb times for worship environments vary widely.  Non-musical, spoken-word worship requires a very short reverb time (.5-.8s range), ensuring that speech is intelligible. At the other end of the spectrum, cathedrals can tolerate an extremely long reverb time (2s and above) due to the traditional nature of their liturgy. Choir, organ and plainchant worship will actually benefit from longer reverb times that create a sense of ambience and spaciousness by sustaining musical notes. These spaces will often lack a sound system, and instead utilize the hard surfaces to propagate sound throughout the room.

Traditional worship may be enhanced by long reverb times, but contemporary worship requires a significantly shorter reverb time. In these environments, drums, guitars, bass and other amplified instruments are critical to the high intensity worship experience, but have far different acoustical needs compared to the choir and organ in a more traditional service. Contemporary “high impact” churches require a reverb time in the .8-1.3s range to ensure that the music won’t become too “muddy” and indistinct. Contemporary churches must also be more cognizant of late specular reflections (slap echoes) which can inhibit the timing of musicians and contribute to poor music clarity.

Let’s take a look a few common scenarios when it comes to treating traditional and contemporary worship spaces.

While including a mix of absorbing Sonora® Panels and diffusing Double Duty barrels, many people overlook the contribution of the padded chairs and carpet to the sound of this space.

Scenario 1: Conversion from Traditional to Contemporary worship

A growing contemporary church moves into a larger, traditional sanctuary and is confronted by a raucous acoustic environment during their first rehearsal… This “live” space was perfect for traditional music, but is not conducive to a “high impact” contemporary worship service. To reduce the excessive reverberation and distracting echoes, sound absorption should be added to the rear wall (opposite stage) and side walls. Also, if using on-stage monitors, the stage walls should be treated to manage stage volume.  Spot diffusive treatment that provides low-frequency absorption would also be beneficial. A good choice for this would be traditional ‘barrel’ diffusers. These are one of the oldest tried and true solutions for controlling bass issues in a performance space. Also, since these units function as both absorbers and diffusers, you get the benefits of both.

Scenario 2: Mix of Traditional and Contemporary worship services.

A worship facility decides to offer a contemporary service in addition to their traditional services… As more absorption is introduced to cut down on distracting reflections, we want to retain the envelopment and spaciousness which benefits congregational singing and traditional worship music. Sound diffusive treatment would be a great way to control echoes and specular reflections, while keeping the energy in the space. A mix of absorption and diffusion is usually best. Multipurpose spaces can also benefit from variable acoustic treatment which allows the room to “adapt” to each service, but that is a subject for another article.

Scenario 3: Poor Music Clarity in Traditional Worship space

A traditional worship space renovates their facility by adding thicker carpet, padded pews and a drop acoustic ceiling… All of a sudden, their once lively space feels “flat” and dull. Acoustic instruments are more anemic, less distinguishable and choirs have a difficult time blending and tuning.  To “liven up” the space, replace sound absorptive materials with diffusers.  For example, replace 1/3rd of the acoustic ceiling tiles with a combination of gypsum tiles and lay-in diffusers. These days, there are wide variety mid-range quadratic sound diffusers available for drop tile ceiling grids, as well as the more traditional barrel and pyramidal diffusers.  

While this space is more lively, the slap echoes from the back wall are controlled with a mix of diffusers and absorbers.

Sound diffusion can often seem a little mysterious compared to sound absorption. This is at least partly because sound diffusion is a more complex and multi-dimensional phenomenon compared to the more easily quantifiable sound absorption. However, sound diffusion is often times the missing piece of an acoustic puzzle: its benefits can help a bad room to sound good, or a good room to sound great!

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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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Sonora® Panels help Citizen Heights Church

Citizen Heights Church found a great facility to move into, even during a global pandemic. However, a major obstacle was that the facility had a traditional cathedral ceiling and a nearly 6 second reverb time was not compatible with their high-energy modern services. To address this issue, they included custom Sonora® panels in their overhaul. This decision helped take their 6 second reverb time down to an incredible 1.5 seconds – creating a space that maintains a level of intelligibility which would have been impossible otherwise.

Read the full story here.

Original Technologies for Worship article link.

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