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Novel Open Baffle Construction Techniques
Novel Open Baffle Construction Techniques
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Old 5th October 2017, 07:54 PM   #71
Garytr is offline Garytr  Canada
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Location: Ancaster, ON, Canada
Quote:
Originally Posted by Nelson Pass View Post
The example above was with a 15" Eminence Definimax (my current favorite)
and you can clearly see the peak at around 200 Hz. You definitely want to
be out of there at a frequency lower than that. I cross that system on the
low end at 80 Hz or so at 12 dB/oct, and it meets the Lowther 8" drivers
nicely at about 160 Hz (in phase).
Nelson I know this is old, but I can't find anything different than this slot design, from you. I presently have a PM6A, silver VC, Ticonal magnet in OB with Rythmik OB subs in H frame below. It's good but want to get away from the servo plate amps. Do you have any updates or another OB design? Would an additional 12"-15" on the baffle below the FR driver make any sense for the low mids to mid bass?
Have you done anything with the SAL 08C08 in OB?
Garytr
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Old 5th October 2017, 08:11 PM   #72
Octavia is offline Octavia  United States
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Gary, just curious, why do you want to get away from the servo plate amps?
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Old 5th October 2017, 10:25 PM   #73
CharlieLaub is offline CharlieLaub  United States
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Quote:
Originally Posted by Nelson Pass View Post
The example above was with a 15" Eminence Definimax (my current favorite)
and you can clearly see the peak at around 200 Hz. You definitely want to
be out of there at a frequency lower than that. I cross that system on the
low end at 80 Hz or so at 12 dB/oct, and it meets the Lowther 8" drivers
nicely at about 160 Hz (in phase).

Interesting to see this thread pop up. I've been working up a dipole H-frame subwoofer recently and this seems like a good place to share my thoughts about it.

The problem: I have a dipole speaker that needs bass - low bass. This typically means large cancellation losses unless the front-to-back pathlength can be made pretty long. Long pathlength means a rather large plain baffle, or a folded one like a U-frame, H-frame, etc.

My solution: I decided to make an H-frame because its symmetric radiation pattern matches an OB and found an affordable high-excursion 18" driver. To get a relatively long front-to-back pathlength the H-frame is about 30 inches long, and to accommodate the 18" driver it's about 24" wide. With the driver mounted on a baffle placed about midway front-to-back, the front and rear H-frame tunnels form short transmission lines that have a resonant frequency around 150Hz or so. This resonance is like Nelson's slot loaded 15 Definimax in the OB he posted about (ref: his quote above) but the aspect ratio of the t-line is much lower. This is because he uses a slot, while I use a 24"x24" tunnel. As a result the Q of the resonance in my H-frame is pretty low, so low that I can pretty much ignore it altogether. I have some measurements saved on my other computer, and I will post them shortly.

So, this idea of the tradeoff between H-frame overall length, and the aspect ratio of the transmission line, just recently clicked into place for me. It suggests that if you want a dipole with minimal low frequency losses (e.g. for a subwoofer) you need a "long" pathlength. If you use a large driver so that your t-line is also quite wide, the aspect ratio will be relatively low and the Q of the t-line resonance will be easy to deal with or even inconsequential. This is a nice result because you probably want to use a large driver anyway, to get a low Fs and lots of total displacement. A bigger driver really IS better in this case.

There is one problem, however: vibration. The mass of the large cone is not insignificant. When I was doing some impulse testing of my rather wispy H-frame on a wooden floor (so friction is pretty low and mass is low) the whole assembly would move more than 0.5 inchs at the start of the impulse unless I sat on it. I will definitely need to mass load the H-frame with 100lbs+ of sand or bricks but it does seem to be pretty effective at low frequency reproduction so I am willing to do that.

I will post a pic of the prototype and some measurements soon.
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Old 5th October 2017, 11:21 PM   #74
Garytr is offline Garytr  Canada
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Octavia the amps sound good, are clean powerful, if not somewhat inefficient, but they don't always play nice with other pieces. I finally overcame hum, which I still had after an amp mod from Rythmik, by making diy Hum-X ground filters. The hum I am talking about is thru the speakers, not transformer hum. I continue to get some thumps on shut down with certain preamps.

To be clear the bass system is very good, using a PC with a tone generator is flat to 29hz and plays sub 20hz. It is fast and controlled. Here's a couple pic.'s, just playing with idea of acrylic baffles, so forgive cardboard, I have/will make several shapes/sizes for comparison.
Charlielaub H is 13 5/8 w x 281/4h x 13 3/4 d. Wood is 1 1/8 Beech butcher block, side panels are 1/4" 6061 aluminum to keep things narrow with 2 kinds /layers of damping. Dark grey is acoustic foam. Upper baffle is 13 5/8 - 18 w and 24 3/4 high. Hope this helps
Garytr
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File Type: jpg 20171005_184515.jpg (587.6 KB, 251 views)
File Type: jpg 20171005_184545.jpg (533.0 KB, 245 views)
File Type: jpg 20171005_184610.jpg (561.7 KB, 241 views)
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Old 5th October 2017, 11:32 PM   #75
CharlieLaub is offline CharlieLaub  United States
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Here are a couple of additions pics. First is measurements of front and rear raw (unfiltered) response of the H-frame. Next is a system model, from front, with EQ and low pass filtering applied. Last is the magnitude of the filter functions.
Attached Images
File Type: png SI HT18v2 H-frame nearfield.png (29.3 KB, 241 views)
File Type: png EQd_response.PNG (36.0 KB, 230 views)
File Type: png filters_and_EQ.PNG (30.1 KB, 52 views)
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Old 6th October 2017, 05:46 AM   #76
Ultima Thule is offline Ultima Thule  Europe
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Quote:
Originally Posted by CharlieLaub View Post
Here are a couple of additions pics. First is measurements of front and rear raw (unfiltered) response of the H-frame. Next is a system model, from front, with EQ and low pass filtering applied. Last is the magnitude of the filter functions.
Charlie that looks great me thinks!
The last picture seem to miss the phase graph doesn't it, have you done any measurements of total system phase (w & wo EQ'ing)?

Edit: Forgot to ask, what Qts and VAS etc. did your element have?

Last edited by Ultima Thule; 6th October 2017 at 05:57 AM.
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