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Old 2nd October 2008, 07:26 PM   #1
dscline is offline dscline  United States
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Question How "real" are internal standing waves?

I am going to re-build my sub, which is going to result in a much taller enclosure. It will be a tall slender column with multiple drivers, divided into two sections. This each section will have internal dimensions of approx. 13.5x13.5x39.5". I know to prevent standing waves, you need to stay below the quarter wavelength of the frequencies produced. The quarter wavelength for the 39.5" dimension is about 85hz, and my sub will be crossed over at 80hz. But due to crossover slopes, it will still produce some output well above 85hz.

Would there be a real issue here, or is this so close to the edge that the actual output in frequencies that can cause standing waves in this enclosure will be too low to cause any mentionable problems? FWIW, this will be built using Rythmik servos, wo I suspect the servo system will somewhat attenuate any issues within the box from being reproduced "through" the driver. But I'd still like to hear any thoughts on the issue.

Thanks!
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Old 2nd October 2008, 09:28 PM   #2
Collo is offline Collo  Australia
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Your wall-to-wall resonances are pressure node to pressure node, and are thus half wavelength, not quarter. This doubles the resonant frequency.

Driver-to-wall resonances are velocity node to pressure node and are quarter wave. The most troublesome one is the driver-to-top (or bottom)-wall for tall boxes, or the driver-to-back wall for deep boxes..

Here's yours.....

Click the image to open in full size.

With your sub crossing over at 80hz, I don't think you'll have any dramas. I would add some damping on the faces that are furthest apart to cover harmonics, but thats just me...
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Old 3rd October 2008, 02:11 PM   #3
dscline is offline dscline  United States
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Well ****, typed up a big reply, got an error that my attachment was too big, with no option to recover what I'd typed. Let's try this again:

Thanks for your help, but I inadvertently omitted a crucial detail: my drivers will be mounted at the very far end of the long dimension. My understanding is the driver-wall resonance is still quarter wavelength based. Thanks for the tip on boxnotes... I hadn't seen that software before. I'll skip the screen capture I did of it, but it suggests a resonance at 100hz. I modeled one of my two sections, which has one driver on the front, and one driver on the back, at the very bottom of the enclosure. The other half (separate section) will have a driver at the very top of the back panel, and one on the top panel. So I suspect it will be slightly worse, since middle of the driver in the top panel will be the full 39" away from the bottom panel.

So I guess my best bet is to focus on damping the ends opposite the drivers. I've seen varying opinions of damping the sealed Rythmik subs. Some like no damping at all, some like just a thin lining of the walls (recommended by Brian Ding @ Rythmik), and I think maybe I've seen one person who stuffed it. Brian suggests against using too much damping material, as its non-linear response (relative to air) can be detrimental. So I think my best bet may be to not line all the walls, just focus all the material at the end opposite the drivers. Sound reasonable?
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Old 3rd October 2008, 02:28 PM   #4
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If the file is too big, can't you just push the back button? I just posted an oversize pic like you and it worked here for me.
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Old 3rd October 2008, 02:33 PM   #5
dscline is offline dscline  United States
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No, when I tried to go back, it said the page had expired.
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Old 3rd October 2008, 03:36 PM   #6
GM is online now GM  United States
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Quote:
Originally posted by Collo

Driver-to-wall resonances are velocity node to pressure node and are quarter wave.
Sorry, but these resonances are reflection generated, ergo half WL, same as a plane wave or vent tube.

GM
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Old 3rd October 2008, 05:17 PM   #7
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To expand on what GM is saying.

(assume that one dimension is longer than the rest) If a pipe is open or closed at both ends it will have a half-wave resonance. If it is open at one end it will be a quarter-wave resonance.

The half wave resonance will show up on the impedance plot. The bump just under 100 Hz (i'm guessing the broad bump just above 100 Hz is from not enuff holes in the holey brace). Box is end loaded and about 4.5 x 8 x 35" sealed

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Old 3rd October 2008, 06:27 PM   #8
dscline is offline dscline  United States
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Hmmm, ok, so is Boxnotes wrong? It says: "Standing waves occur between the driver and walls, where the distance represents 1/4 wavelength. You can also get standing waves between opposite walls, where the distance represents 1/2 wavelength" The output values shown in the software also suggest quarter wavelength resonances for the driver, and half wavelength for opposing walls:

Click the image to open in full size.

I guess what I can't wrap my head around is whether the driver would represent an "open" or "closed" end. It's not completely open, but it is essentially just a membrane between the interior and exterior. I would think that would impose an impedance shift, and maybe land somwhere in between depending on the Vas of the driver?

I certainly hope I don't need to worry about quarter wavelength resonances... just trying to make sure before it's built.

Thanks for all the help!
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Old 3rd October 2008, 11:30 PM   #9
Collo is offline Collo  Australia
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Quote:
Originally posted by GM
Sorry, but these resonances are reflection generated, ergo half WL, same as a plane wave or vent tube.GM
The driver acts as an open end and the opposite wall acts as a closed end, so the resonance is quarter wave. As shown in this experiment at Rutgers University (New Jersey)


Damping opposite ends of the longest section of the box is the best approach. Unfortunately at low frequencies, most damping materials don't work very well. Given that the walls are pressure zones rather than velocity zones, any damping material that relies on air movement will not help.

If you look at treatments to be used for room walls, the most effective ones use a membrane that converts pressure into motion, which can then have energy removed by friction. An example of this can be seen with the Modex traps. This approach suggests that some closed cell foam, such as that used in those thin camping beds, would work better than normal stuffing.


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Old 4th October 2008, 12:27 AM   #10
dscline is offline dscline  United States
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Quote:
Originally posted by Collo
When posting a long message, I try to remember to copy the message to the Windows Clipboard before I hit the Preview / Submit button
So do I. Unfortunately, sometimes I fail.

Quote:
Given that the walls are pressure zones rather than velocity zones, any damping material that relies on air movement will not help. If you look at treatments to be used for room walls, the most effective ones use a membrane that converts pressure into motion, which can then have energy removed by friction.
And this is where I'm having difficulty. The problem with membrane type systems is they tend to have their own resonant frequency, and therefore their attenuation tends to be most efficient at a specific frequency. I'm afraid anything like that would introduce it's own coloration, as it would be difficult to devise something that would target the actual resonant frequency in question. The end result could be worse than the problem. I know absobative type bass traps can be made more efficient by moving them further into the room, where they can have an effect where there's still motion. I wonder if I would be best served by a section of stuffing a few inches away from the far end wall?
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