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Selective Damping Using Absorption/Diffusion

Selective Damping Using Absorption/Diffusion
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.





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