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Old 14th May 2012, 03:07 AM   #61
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Originally Posted by soundaatma View Post
... So I fail to understand the concern about matching hugely different driver sizes of different weights.

I dont know if my above understanding is wrong or I am missing something here ?
The issue isn't the woofer specifically, but the cavity resonance inherent in an H frame.

Having built a variation (alpha 15+ FF85) on the MJK passive baffle (a flat baffle) i know well that a big driver can work with a little one.

dave
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Old 14th May 2012, 07:54 AM   #62
T101 is offline T101  Bulgaria
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It is all a matter of achieving flat response. The H frames do it by mirroring their rising response (due to H frame quarter wave resonance governed by the frame depth, it is not a cavity resonance) with the falling electrical response of the crossover.
Thus a flat response is achieved once the quarter wave rise terminate and the FR of the unfiltered system reaches a plato, then we get a crossover point because the crossover continues to reduce the signal.

So it's not cavity resonance, it's rather a quarter wave one governed by the depth.

Matching two drivers is a matter of matching their directivity patterns at crossover (apart from the obvious must for equal level/sensitivity). Dipoles offer us the unique possibility of matching virtually any two drivers. At such low frequencies virtually any driver exhibits pure piston action, so it's infinite baffle radiation diagtam (pattern) would be 180 degrees front and rear hemisphere. And then comes the Dipole which in the same way as colonel Colt from the Navy makes all drivers equal shaping their radiation patterns to the figure of eight.
Having problems with the near rear wall due to the missing shield of the big OB?
- Add absorbing in the rear H frame opening
- Also consider altering the polarity of drivers differently at scenarios close to rear wall and far from it respectively.

Best Regards!

Last edited by T101; 14th May 2012 at 07:59 AM.
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Old 14th May 2012, 08:04 AM   #63
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(due to H frame quarter wave resonance governed by the frame depth, it is not a cavity resonance)
Semantics. The frame depth creates a cavity. There is a resonance related to this depth. Because one end of the (short) pipe is open and the other closed it is 1/4 wave. Since it is undamped you get a ripple train.

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Old 14th May 2012, 08:19 AM   #64
T101 is offline T101  Bulgaria
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It's overall depth actually, the distance traveled by the wave from one side of the membrane to add with wave in some degree of in-phase emitted from the other side. That is why this is a phenomenon exhibited only by dipoles. The same is with OB bump before the +6 db diffraction rise plane in the FR of OB's.
The cavity itself does not induce significant phenomenons at the operating frequencies of the H frame, even standing waves artefacts were not observed in measurements, probably due to the fact that at least 60% of air mass in the cavity couples to the membrane and the vector of the air motion is in perpendicular plane to the cavity boundaries (modes are not excited as pressure is transmitted between the membrane and surrounding atmosphere through coupled air). Further the cavity is considered to be a trapped air mass and an opening (mouth).

Best Regards!

Last edited by T101; 14th May 2012 at 08:22 AM.
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Old 14th May 2012, 08:36 AM   #65
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Describe it however it makes you feel comfortable. It is an airmass in a cavity and it resonates, hence it can be accurately be described as a cavity resonance.

dave
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Old 14th May 2012, 01:27 PM   #66
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When simming an OB you quickly realize that baffle size dictates the low frequency cutoff. Positioning the speaker closer or farther from the walls and floor also changes the response. Once dialed in it's a matter of figuring out bass support.

I'm still loving my OBs with their spacious sound stage. The blend between H-frame and main driver is pretty seamless IMHO.
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Old 14th May 2012, 07:31 PM   #67
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....but then we are talking about a hotel room here. There were a large number of exhibitors in attendance who had ~6.5" w/ Tweeter either in MTM or TMM arrangements that just drove me out of the room with the bass bloom in the lower mid bass. It's difficult to bring a combo that will work well in these rooms....
John,

I hear the same sound from most display rooms. Boom --Tizz. My theory is that these speakers are either voiced for rock or eq's to be dead flat anechoic. Either will accentuate the bass in a small room. My speakers are voiced for moderate size rooms such as the Embassy Suites room at LSAF. Actually, 15'x20' is larger than most people get for a dedicated listening room.

So, my stuff is at an advantage in these venues. Some of the worst offenders are back horns. Most that I have heard at meets sound tubby. I must say that Bob Spence did a great job of setting up his Maxxhorns. The last time I heard them, they sounded horrible. This year they sounded quite nice.

Bob

Last edited by Bob Brines; 14th May 2012 at 07:36 PM.
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Old 15th May 2012, 12:05 PM   #68
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good,
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Old 7th January 2013, 06:14 AM   #69
mortron is offline mortron  Canada
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Bob, where did you XO your H Frames, and what EQ did you apply to get an agreeable performance from them? Thanks
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