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Room Acoustics 201 - Part 3: Absorption and Diffusion: traditional acoustic treatment products

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Customer email newsletter dated: May 1, 2007.

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Room Acoustics 201 - Part 3

Absorption and Diffusion: traditional acoustic treatment products.

This is the third installment in our three part series on the various approaches to room acoustic treatment. In the first part we wrote about our preferred mid-room speaker placement approach, and also about the basic treatment of the room, which we feel is best accomplished using the Cathedral Sound Acoustic Panels.  You can read this first installment in our Archives.

In Part 2 we addressed products such as Shakti Hallograph, Acoustic System Resonators, and the acoustic products from Shun Mook, all designs which "sympathetically resonate" with the music to create a favorable acoustic envelope around the system. This resonating approach is an alternative to the traditional absorbing/diffusing type of acoustic products we will address now in Part 3.

Step three: The Traditional Use Of Absorption and Diffusion.

Don't throw out the baby with the bath water.
When most people think about acoustic treatment they will picture egg crate foam, or fiber-filled panels of some sort. This is by far the most common approach to room acoustic treatment, and also to recording studio acoustics.

The basic idea of this approach is the elimination of reflections. If you were to set up your music or video system in nature on a flat plain or in the desert, everything you hear would be direct sound from the speakers. No reflections would complicate what we hear. This is an anechoic space: a space without echo or reflection.

But when you are in a room those pesky walls, ceilings, and floors, are all reflecting acoustic energy. Now if it were just six reflections (one from each of the walls, ceiling, and floor) this would be manageable. But in fact these "early reflections" become a series of complex interactions as they rebound across the room, and are reflected back and forth in a confused manner. Add in the corner effect where acoustic pressure is megaphone loaded back into the room and the basic desirability of some sort of acoustic treatment becomes clear.

The recording studio approach to the problem is to fill the room with broad band absorbing material - the more, the better -  to attempt to approach an anechoic space, like you would achieve outdoors in the desert. But in a listening room this dry, studio acoustic can seem sterile, overly dead, and basically unnatural given that most real acoustic spaces are not outdoors in the desert, but rather are concert halls, night clubs, theaters, and other environments which do reverberate.

For this reason the biggest challenge with traditional acoustic treatment products is to acoustically reign in the room without "killing" it.

In one sense this game seems by nature to be rigged against the user. Why? Because when dealing with bass frequencies, both in theory and in practice, more acoustic treatment will usually be better than less. In fact some acousticians would recommend starting by treating 30% of the surface area in the room with "bass traps" and then moving up from there. (You have probably seen photos of recording studios that have 50% or more of the surfaces treated.) One obvious problem is that most of us don't want 30% of the surfaces in our listening or theater rooms covered with acoustic panels. The direct acoustic problem is that "bass traps" will tend to absorb other frequencies as well, making the room too dead in the midrange and in the high frequencies.
Selective Damping Using Absorption/Diffusion
So here is a suggested approach for those of us who don't want our sound rooms to look and sound like a recording studio. Let's selectively add absorption and diffusion to our acoustic treatment mix to try to achieve a balance between control and liveliness.

The biggest advantage we have in terms of recent product developments are those Cathedral Sound Acoustic Panels we keep mentioning. Now their importance becomes clearer because these Panels directly affect the deep bass frequencies that otherwise require so many bass traps to control in the traditional way. One or two of the Cathedral Panels in each corner near the ceiling can have the same impact as a truck load of bass traps. If the room still suffers some bass boom, the careful introduction of a much smaller number of absorbing bass traps can get you the rest of the way there. If still too much echo, a manageable number of broad band absorbing/diffusing products can successfully address the problem.

Let's say that you have already implemented our primary recommendations for use of:

the Cathedral Sound Panels from Part 1,
and the Shakti Hallographs from Part 2.

If so, you may be done. Adding absorption/diffusion in this case may only begin to drain some of the life out of your system. But in many rooms, (or if you don't want to use the Shakti Hallographs at all), the exclusive use of resonating products leaves the room too live and echoey. In this case selectively adding absorption/diffusion can be a good thing.

The first place to start is the middle of the wall behind the speakers. In the diagram above the suggested placement for absorption/diffusion products are in the numbered circles and this particular location is #1.  In Part 1 we called this a "secondary pressure zone" - the only pressure zone we are not already using to "activate" our resonating Cathedral Sound Panels and Shakti Hallographs, or using to reinforce our music system (i.e.. the speaker placement positions and listening positions). Some absorption/diffusion at the middle of the wall behind the speakers can serve to anchor the center image. For the same reason placement of your component rack in this location can be a good thing acoustically.

Additional absorption/diffusion as indicated in diagram #3 can also serve to tame an overly live acoustic. Note that with absorption/diffusion products trial and error is essential. Introduce the panels one or two (usually symmetrically in pairs) at a time to evaluate. If you reach a point where the introduction of more product is not certainly better, then stop. Send us a room diagram; we can offer our help to help you optimize the room.

Note 1: if you are not using Cathedral Sound Panels to help control the bass frequencies, you must locate bass traps - the bigger the better - in the four corners of the room. A triangular absorbing panel in the upper corners of the room would also be recommended in this case. But it is hard to understand why you wouldn't just get the Cathedral Sound Acoustic Panels instead - smaller, lower cost, more effective...

Note 2: Diffusion panels vs. Absorption panels: Diffusion works best in larger rooms, with a much smaller effect in smaller rooms. Absorption tends to work best in small rooms. Products that have reflecting/diffusing sides, as well as soft, absorbing sides can combine both attributes.

As for a choice of absorption/diffusion product, aesthetics come into play. The pictured
ASC Studio Trap ($758/pr - $667/pr on special), which is also height adjustable, with one hard and one soft side, is particularly flexible. The affordable "Double Buster" from Echobuster ($195 each - $172 on special) is a flat panel with hard and soft sides. Eighth Nerve, Furutech, and Michael Green also have products like this. We are happy to discuss options with you. In general, however, you will orient the hard, reflective side towards the listening position, and the absorbing side to the nearest wall.

Note 3: We often get questions about how to deal with windows in listening rooms. Here are two ideas for you:

a. Marigo Audio makes resonance tuning "dots" (40mm or 1.5" diameter) that can make windows (and other glass surfaces in the listening room) better sounding reflective surfaces. Harmonix has more expensive products using the same tuning concept.

b. We have had pretty good results obscuring the bright splashy sound of large windows by using Venetian blinds on windows. This is preferred over using drapes.
As we move in this series of articles from Part One, to Part Two,  to Part Three, we are also introducing less certainty about the acoustic treatment. EVERYONE will benefit from step one, the Cathedral Sound Acoustic Panels, regardless of the type of system, or the speaker setup approach. It is also the most cost effective of all the treatment options, the smallest, doesn't take up floor space, and the most effective for difficult to manage deep bass frequencies. If you haven't already, and you want to improve everything about the way your system or theater sounds, DO THIS NOW.

Steps two and three are offered with two channel stereo (only) in mind. There are also applications for these products for home theater, but with a few twists not covered here. Call to discuss your theater acoustics.

For two channel stereo, the Hallographs are a sure thing, just like the Cathedral Sound Acoustic Panels, but more expensive, requiring a little floor space, more visually intrusive, and requiring a little experimentation with the amount of the toe in. But, it is a powerful product, with an entirely complementary effect to the Cathedral Sound Acoustic Panels, and highly recommended. If you want to max out your room acoustically, you need the Hallographs, too.

Step three, selective introduction of damping/diffusion to the room, is the traditional approach to room acoustics. But it is also the most challenging approach, in terms of ultimate costs, floor space requirements, and the need for experimentation. You may or may not need damping/diffusion, but if you still have bass overhang or too much echo after working through the first two steps, you will know it. We can help you to approach this sensibly, and avoid overdoing it.

We hope that you have found this series on room acoustics to be thought-provoking. We invite your comments and feedback especially if you have followed our recommendations. And again:

To celebrate the completion of this room acoustic writing project, we will have a special 12% discount on ALL acoustic products (whether discussed in this article or not) for the month of February.
For additional information on these offers and frequent updates speak with your Cable Company consultant, or visit our websites: and  For hundreds of specials on used and demo cables, visit

Or call us Mon.- Fri. 10 -6; Sat. 11-5 Eastern time on 800-328-9973 (or 215-862-4870). Fax: 215-862-4871

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