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The 'Circles of Doom'.....Open baffleless full range speakers.
The 'Circles of Doom'.....Open baffleless full range speakers.
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Old 23rd July 2021, 05:43 PM   #421
danibosn is offline danibosn  Croatia
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U or H frame actually making transmission line long as frame is. First resonance will show at frequency which 1/4 of lambda (from here quarter wave line name came) is equal to length of frame (from opening to speaker baffle). Because of that H frame is much more convenient as for same separation has half length and resonance will occur at double frequency than U frame.
First resonance cannot be tamed effectively by stuffing because at same time output is taming as well so we on same level. Light stuffing can help with higher modes resonances.
Best option is to use H frame and cross it bellow first peak (what we actually always want, especially for bass) which is always bellow first resonance so we are on safe side

Last edited by danibosn; 23rd July 2021 at 05:56 PM.
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Old Yesterday, 06:43 AM   #422
cowanaudio is offline cowanaudio  Australia
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The 'Circles of Doom'.....Open baffleless full range speakers.
Quote:
Originally Posted by frangus View Post
Could a shallow (say 100mm) U or H frame get close to the same boost but allow crossover at a higher frequency - maybe 500hz?
Quote:
Originally Posted by CharlieLaub View Post
No.

The issue with U- and H-frames that limits their upper range has to do with what is called a "tunnel resonance",
I beg to differ. An H frame for a 15" driver will be 400mm or so across. If that H frame were 100mm deep, each side is only 50mm deep minus timber thickness, so around 38mm deep on each side. You won't measure a tunnel or cavity resonance in an H baffle of those dimensions.

As I showed above, this would behave in a similar manner to a clamshell with around a 200mm spacing.
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Old Yesterday, 01:17 PM   #423
CharlieLaub is offline CharlieLaub  United States
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You call that an H-frame? Hardly! 10cm deep is essentially nothing. My H-frame subwoofer is about 1 meter deep (0.5m per side), and 0.6m wide, loaded with an 18" driver.

But you are correct in that, with your dimensions where width >> depth there will be little to no tunnel resonance and it will be at a very high frequency anyway.
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Old Yesterday, 01:35 PM   #424
gpapag is offline gpapag  Greece
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The 'Circles of Doom'.....Open baffleless full range speakers.
Quote:
Originally Posted by CharlieLaub View Post
It is possible to measure excursion at home, and without any "special" equipment. Here's how:

Here is information about the wedge micrometer from SL's web site, with the following description:
Thanks Charlie for bringing this up.
I have used it, it works fine. Note, the reading on the horizontal scale is peak to peak excursion.

George
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Old Yesterday, 01:50 PM   #425
CharlieLaub is offline CharlieLaub  United States
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Thanks for pointing that out. Good to know.

It would seem that you could simply alter the scale on the lower edge, where you read off the excursion, by a factor of 0.5. This would convert peak-to-peak into on-way excursion and would further magnify the increments, making them easier to read by eye.
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Last edited by CharlieLaub; Yesterday at 01:54 PM.
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Old Yesterday, 09:17 PM   #426
cowanaudio is offline cowanaudio  Australia
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The 'Circles of Doom'.....Open baffleless full range speakers.
Quote:
Originally Posted by CharlieLaub View Post
You call that an H-frame? Hardly! 10cm deep is essentially nothing. My H-frame subwoofer is about 1 meter deep (0.5m per side), and 0.6m wide, loaded with an 18" driver.

But you are correct in that, with your dimensions where width >> depth there will be little to no tunnel resonance and it will be at a very high frequency anyway.
I just answered the question as posted without adding any irrelevant criteria. A 10cm H frame behaves about the same as the clamshell arrangement that is the topic of discussion. I've made several 200mm to 300mm deep H frames and they work well without resonances.
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Old Today, 11:59 AM   #427
LewinskiH01 is online now LewinskiH01  Argentina
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Since you've gone into H frames, and not to derail this very interesting thread, could you please point me to good sources (threads, other) to learn about design of H, M (Linkwitz?) config for OB subs?

Through this great thread I learnt clamshell has the advantage of a compact design, easy to mechanically decouple the vibrations, reaches 6dB gain and could be xo somewhere up to 150Hz (I'm guessing, BM xo at 100Hz). But there is a limit to how large the phantom baffle can be made which is a function of driver diameter.

I'm exploring GR Research 12" servo subs designed for dipole (two drivers per sub), and clamshell is an option yet I'm thinking the smaller size of these drivers might run out of steam in the 20 to 50Hz or so region. Would like to explore other OB sub configurations to compare. Small form factor is always nice in the living room but within certain limits I could extend to a larger size if I can gain additional SPL without distortion and acceptable looks. GR Research shows theirs in an H frame that seems relatively shallow, but show no dimensions. Maybe Linkwitz arrangement in the LX521 is better than H? My subs would be separate from the main speaker. Off the bat it seems U baffle, being inherently asymmetrical for driver load, shouldn't be the best approach, but maybe I'm missing something. Looking forward to learn.

Thanks in advance!
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