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Old 05-02-2007, 11:47 AM   #23
scottdru
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Originally Posted by Bubbagump View Post
I should also mention you want to starddle corners rather than hang them flush to the wall for bass trapping. The air space in the corners is the key here. Without depth behind the trap you don't trap as much bass. (Even better, fill the depth with more insulation, but as I said above, that is just gravy) I could go into 1/4 wave length explanations ad nauseum, but just know, you want to straddle a corner with your traps if at all possible. The next most desireable placement of a trap is 4" from the wall... but a 4" thick trap 4" from the wall loses 8" of space which is a lot... therefore I always recommend to start in the upper corners.
Actually, there is more to it than that, as the behaviour of bass absorbers in corners involves somewhat more complex mechanics than just the 1/4 wavelength issue. Also, the most commonly discussed explanations (e.g., the explanation in Ethan's acoustics FAQ article) with regard to gapping traps from the walls only discuss a single angle of incidence, i.e., a direct wave front (at an angle perpendicular to the face of the trap and the wall), and don't go into detail about waves that enter the trap from other angles of incidence and are reflected back from the wall (and for good reason, because it's fairly complex physics, and beyond the scope of what Ethan's FAQ article is intended for). But that's a whole other 10 posts or so, and I don't want to add to the confusion for some who may be just wrapping their heads around some of this stuff for the first time!

One thing to be aware of is that the absorption curve of these kinds of panels change, depending on whether they are mounted in corners or mounted on a flat wall.

These kinds of panels tend to have an absorption peak right around the 100 Hz range, whereas the absorption peak (or peaks) of panels mounted on a flat wall tend to be in the 200 to 500 Hz range, and the absorption performance tends to roll off at a much higher frequency than for traps mounted in corners.

So, for dealing with room modes in the range of 175 or 200 Hz and lower, you will get the best performance with corner mounted panels. BUT . . . for dealing with room modes in the range of 200 Hz and above, you will tend to get the best performance with these types of panels mounted on a flat wall (or ceiling, etc.) with at least a 3 to 4 inch gap between the panel and the back wall.

If you take a look at the absorption curves on the RealTraps site (http://www.realtraps.com/data.htm), and you compare the absorption curve of corner-mounted MiniTraps to that of wall-mounted MiniTraps, you'll see what I mean.

Those lower room modes are certainly the most difficult (requiring the most mass and coverage) to treat, BUT . . . in many instances with small rooms, there are problematic room modes at more different frequencies, and often with quite a few different room modes (e.g., modes related to any or all of the length, width and height dimensions of the room) that are all stacked up at frequencies very near to each other, which can in effect create wider bands of peaks and nulls than those in the very low range.

Mind you, there is still a good deal of absorption in the 200-500 Hz and above range with corner-mounted panels, but sometimes in various rooms you may need to add wall-mounted panels to effectively address some of the worst problems in a room.

So . . . yes . . . by all means it's really important to treat the corners, and this generally needs to be a priority. But I want to take the explanation of the principles involved here to a more specific level, to emphasize the difference in behaviour of corner-mounted and wall-mounted broadband panels of the type we are talking about here.

Now . . . if you want to figure out at what frequencies the modal problems are most likely to occur in your room, you can use a program like ModeCalc (free download here: http://www.realtraps.com/modecalc.htm) or various online calculators to calculate the likely axial modes based on the dimensions of your room. HOWEVER, these predictions don't take into account the materials or construction methods used in your room that may (or may not) allow for a certain amount of transmission loss in your room. The only REAL way to get a good sense of where the actual problems are in your room is to use a program like ETF or Room EQ Wizard (http://www.hometheatershack.com/roomeq/), or other similar programs, to do a sine wave analysis of your room. Room EQ Wizard is a free download (you have to register as a member of their forum, but registration is free and there is no additional obligation). Actually, there are some discussions about DIY panels at the Home Theater Shack forum too, and Ethan is a moderator there as well.

Most people are utterly shocked when they run this kind of analysis in their room, because they find out that the frequency response of their room looks like the Swiss Alps! But if you never believed you needed serious bass trapping before, you'll certainly believe it after running this kind of acoustics analysis!

Last edited by scottdru; 05-02-2007 at 01:51 PM.
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